Microsoft Windows provides several base mechanisms that kernel-mode components such as
the executive, the kernel, and device drivers use. This chapter explains the following system
mechanisms and describes how they are used:
■ Trap dispatching, including interrupts, deferred procedure calls (DPCs), asynchronous
procedure calls (APCs), exception dispatching, and system service dispatching
■ The executive object manager
■ Synchronization, including spinlocks, kernel dispatcher objects, and how waits are
■ System worker threads
■ Miscellaneous mechanisms such as Windows global flags
■ Local procedure calls (LPCs)
■ Kernel Event Tracing
Interrupts and exceptions are operating system conditions that divert the processor to code outside
the normal flow of control. Either hardware or software can detect them. The term trap
refers to a processor’s mechanism for capturing an executing thread when an exception or an
interrupt occurs and transferring control to a fixed location in the operating system. In Windows,
the processor transfers control to a trap handler, a function specific to a particular interrupt
or exception. Figure 3-1 illustrates some of the conditions that activate trap handlers.
The kernel distinguishes between interrupts and exceptions in the following way. An interrupt
is an asynchronous event (one that can occur at any time) that is unrelated to what the processor
is executing. Interrupts are generated primarily by I/O devices, processor clocks, or
timers, and they can be enabled (turned on) or disabled (turned off). An exception, in contrast,
is a synchronous condition that results from the execution of a particular instruction. Running
a program a second time with the same data under the same conditions can reproduce
exceptions. Examples of exceptions include memory access violations, certain debugger
86 Microsoft Windows Internals, Fourth Edition
instructions, and divide-by-zero errors. The kernel also regards system service calls as exceptions
(although technically they’re system traps).
Figure 3-1 Trap dispatching
Either hardware or software can generate exceptions and interrupts. For example, a bus error
exception is caused by a hardware problem, whereas a divide-by-zero exception is the result of
a software bug. Likewise, an I/O device can generate an interrupt, or the kernel itself can issue
a software interrupt (such as an APC or DPC, described later in this chapter).
When a hardware exception or interrupt is generated, the processor records enough machine
state on the kernel stack of the thread that’s interrupted so that it can return to that point in
the control flow and continue execution as if nothing had happened. If the thread was executing
in user mode, Windows switches to the thread’s kernel-mode stack. Windows then creates
a trap frame on the kernel stack of the interrupted thread into which it stores the
execution state of the thread. The trap frame is a subset of a thread’s complete context, and
you can view its definition by typing dt nt!_ktrap_frame in the kernel debugger. (Thread context
is described in Chapter 6.) The kernel handles software interrupts either as part of hardware
interrupt handling or synchronously when a thread invokes kernel functions related to
the software interrupt.
In most cases, the kernel installs front-end trap handling functions that perform general trap
handling tasks before and after transferring control to other functions that field the trap. For
example, if the condition was a device interrupt, a kernel hardware interrupt trap handler
transfers control to the interrupt service routine (ISR) that the device driver provided for the
interrupting device. If the condition was caused by a call to a system service, the general
System service call
Chapter 3: System Mechanisms 87
system service trap handler transfers control to the specified system service function in the
executive. The kernel also installs trap handlers for traps that it doesn’t expect to see or
doesn’t handle. These trap handlers typically execute the system function KeBugCheckEx,
which halts the computer when the kernel detects problematic or incorrect behavior that, if
left unchecked, could result in data corruption. (For more information on bug checks, see
Chapter 14.) The following sections describe interrupt, exception, and system service dispatching
in greater detail.
Hardware-generated interrupts typically originate from I/O devices that must notify the processor
when they need service. Interrupt-driven devices allow the operating system to get the
maximum use out of the processor by overlapping central processing with I/O operations. A
thread starts an I/O transfer to or from a device and then can execute other useful work while
the device completes the transfer. When the device is finished, it interrupts the processor for
service. Pointing devices, printers, keyboards, disk drives, and network cards are generally
System software can also generate interrupts. For example, the kernel can issue a software
interrupt to initiate thread dispatching and to asynchronously break into the execution of a
thread. The kernel can also disable interrupts so that the processor isn’t interrupted, but it
does so only infrequently—at critical moments while it’s processing an interrupt or dispatching
an exception, for example.
The kernel installs interrupt trap handlers to respond to device interrupts. Interrupt trap
handlers transfer control either to an external routine (the ISR) that handles the interrupt
or to an internal kernel routine that responds to the interrupt. Device drivers supply ISRs to
service device interrupts, and the kernel provides interrupt handling routines for other
types of interrupts.
In the following subsections, you’ll find out how the hardware notifies the processor of device
interrupts, the types of interrupts the kernel supports, the way device drivers interact with the
kernel (as a part of interrupt processing), and the software interrupts the kernel recognizes
(plus the kernel objects that are used to implement them).
Hardware Interrupt Processing
On the hardware platforms supported by Windows, external I/O interrupts come into one of
the lines on an interrupt controller. The controller in turn interrupts the processor on a single
line. Once the processor is interrupted, it queries the controller to get the interrupt request
(IRQ). The interrupt controller translates the IRQ to an interrupt number, uses this number
as an index into a structure called the interrupt dispatch table (IDT), and transfers control to
the appropriate interrupt dispatch routine. At system boot time, Windows fills in the IDT with
pointers to the kernel routines that handle each interrupt and exception.
88 Microsoft Windows Internals, Fourth Edition
EXPERIMENT: Viewing the IDT
You can view the contents of the IDT, including information on what trap handlers Windows
has assigned to interrupts (including exceptions and IRQs), using the !idt kernel
debugger command. The !idt command with no flags shows vectors that map to
addresses in modules other than Ntoskrnl.exe.
The following example shows what the output of the !idt command looks like:
30: 806b14c0 hal!HalpClockInterrupt
31: 8a39dc3c i8042prt!I8042KeyboardInterruptService (KINTERRUPT 8a39dc00)
34: 8a436dd4 serial!SerialCIsrSw (KINTERRUPT 8a436d98)
35: 8a44ed74 NDIS!ndisMIsr (KINTERRUPT 8a44ed38)
portcls!CInterruptSync::Release+0x10 (KINTERRUPT 899c44a0)
38: 806abe80 hal!HalpProfileInterrupt
39: 8a4a8abc ACPI!ACPIInterruptServiceRoutine (KINTERRUPT 8a4a8a80)
3b: 8a48d8c4 pcmcia!PcmciaInterrupt (KINTERRUPT 8a48d888)
ohci1394!OhciIsr (KINTERRUPT 8a41da18)
VIDEOPRT!pVideoPortInterrupt (KINTERRUPT 8a1bc2c0)
USBPORT!USBPORT_InterruptService (KINTERRUPT 8a2302b8)
USBPORT!USBPORT_InterruptService (KINTERRUPT 8a0b8008)
USBPORT!USBPORT_InterruptService (KINTERRUPT 8a170008)
USBPORT!USBPORT_InterruptService (KINTERRUPT 8a258380)
NDIS!ndisMIsr (KINTERRUPT 8a0e0430)
3c: 8a39d3ec i8042prt!I8042MouseInterruptService (KINTERRUPT 8a39d3b0)
3e: 8a47264c atapi!IdePortInterrupt (KINTERRUPT 8a472610)
3f: 8a489b3c atapi!IdePortInterrupt (KINTERRUPT 8a489b00)
On the system used to provide the output for this experiment, the keyboard device
driver’s (I8042prt.sys) keyboard ISR is at interrupt number 0x3C and several devices—
including the video adapter, PCMCIA bus, USB and IEEE 1394 ports, and network
adapter—share interrupt 0x3B.
Windows maps hardware IRQs to interrupt numbers in the IDT, and the system also uses the
IDT to configure trap handlers for exceptions. For example, the x86 and x64 exception number
for a page fault (an exception that occurs when a thread attempts to access a page of virtual
memory that isn’t defined or present) is 0xe. Thus, entry 0xe in the IDT points to the
system’s page fault handler. Although the architectures supported by Windows allow up to
256 IDT entries, the number of IRQs a particular machine can support is determined by the
design of the interrupt controller the machine uses.
Chapter 3: System Mechanisms 89
Each processor has a separate IDT so that different processors can run different ISRs, if appropriate.
For example, in a multiprocessor system, each processor receives the clock interrupt,
but only one processor updates the system clock in response to this interrupt. All the processors,
however, use the interrupt to measure thread quantum and to initiate rescheduling when
a thread’s quantum ends. Similarly, some system configurations might require that a particular
processor handle certain device interrupts.
x86 Interrupt Controllers
Most x86 systems rely on either the i8259A Programmable Interrupt Controller (PIC) or a
variant of the i82489 Advanced Programmable Interrupt Controller (APIC); the majority of
new computers include an APIC. The PIC standard originates with the original IBM PC. PICs
work only with uniprocessor systems and have 15 interrupt lines. APICs and SAPICs (discussed
shortly) work with multiprocessor systems and have 256 interrupt lines. Intel and
other companies have defined the Multiprocessor Specification (MP Specification), a design
standard for x86 multiprocessor systems that centers on the use of APIC. To provide compatibility
with uniprocessor operating systems and boot code that starts a multiprocessor system
in uniprocessor mode, APICs support a PIC compatibility mode with 15 interrupts and delivery
of interrupts to only the primary processor. Figure 3-2 depicts the APIC architecture. The
APIC actually consists of several components: an I/O APIC that receives interrupts from
devices, local APICs that receive interrupts from the I/O APIC on a private APIC bus and that
interrupt the CPU they are associated with, and an i8259A-compatible interrupt controller
that translates APIC input into PIC-equivalent signals. The I/O APIC is responsible for implementing
interrupt routing algorithms—which are software-selectable (the hardware abstraction
layer, or HAL, makes the selection on Windows)—that both balance the device interrupt
load across processors and attempt to take advantage of locality, delivering device interrupts
to the same processor that has just fielded a previous interrupt of the same type.
Figure 3-2 x86 APIC architecture
90 Microsoft Windows Internals, Fourth Edition
x64 Interrupt Controllers
Because the x64 architecture is compatible with x86 operating systems, x64 systems must
provide the same interrupt controllers as does the x86. A significant difference, however, is
that the x64 versions of Windows will not run on systems that do not have an APIC and they
use the APIC for interrupt control.
IA64 Interrupt Controllers
The IA64 architecture relies on the Streamlined Advanced Programmable Interrupt Controller
(SAPIC), which is an evolution of the APIC. A major difference between the APIC and SAPIC
architectures is that the I/O APICs on an APIC system deliver interrupts to local APICs over a
private APIC bus, whereas on a SAPIC system interrupts traverse the I/O and system bus for
faster delivery. Another difference is that interrupt routing and load balancing is handled by
the APIC bus on an APIC system, but a SAPIC system, which doesn’t have a private APIC bus,
requires that the support be programmed into the firmware. Even if load balancing and routing
are present in the firmware, Windows does not take advantage of it; instead, it statically
assigns interrupts to processors in a round-robin manner.
EXPERIMENT: Viewing the PIC and APIC
You can view the configuration of the PIC on a uniprocessor and the APIC on a multiprocessor
by using the !pic and !apic kernel debugger commands, respectively. (You
can’t use LiveKd for this experiment because LiveKd can’t access hardware.) Here’s the
output of the !pic command on a uniprocessor. (Note that the !pic command doesn’t
work if your system is using an APIC HAL.)
----- IRQ Number ----- 00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F
Physically in service: . . . . . . . . . . . . . . . .
Physically masked: . . . Y . . Y Y . . Y . . Y . .
Physically requested: . . . . . . . . . . . . . . . .
Level Triggered: . . . . . Y . . . Y . Y . . . .
Here’s the output of the !apic command on a system running with the MPS HAL. The
“0:” prefix for the debugger prompt indicates that commands are running on processor
0, so this is the I/O APIC for processor 0:
Apic @ fffe0000 ID:0 (40010) LogDesc:01000000 DestFmt:ffffffff TPR 20
TimeCnt: 0bebc200clk SpurVec:3f FaultVec:e3 error:0
Ipi Cmd: 0004001f Vec:1F FixedDel Dest=Self edg high
Timer..: 000300fd Vec:FD FixedDel Dest=Self edg high masked
Linti0.: 0001003f Vec:3F FixedDel Dest=Self edg high masked
Linti1.: 000184ff Vec:FF NMI Dest=Self lvl high masked
TMR: 61, 82, 91-92, B1
Chapter 3: System Mechanisms 91
The following output is for the !ioapic command, which displays the configuration of the
I/O APIC, the interrupt controller component connected to devices:
0: kd> !ioapic
IoApic @ ffd02000 ID:8 (11) Arb:0
Inti00.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti01.: 00000962 Vec:62 LowestDl Lg:03000000 edg
Inti02.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti03.: 00000971 Vec:71 LowestDl Lg:03000000 edg
Inti04.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti05.: 00000961 Vec:61 LowestDl Lg:03000000 edg
Inti06.: 00010982 Vec:82 LowestDl Lg:02000000 edg masked
Inti07.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti08.: 000008d1 Vec:D1 FixedDel Lg:01000000 edg
Inti09.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti0A.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti0B.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti0C.: 00000972 Vec:72 LowestDl Lg:03000000 edg
Inti0D.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti0E.: 00000992 Vec:92 LowestDl Lg:03000000 edg
Inti0F.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti10.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti11.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti12.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti13.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti14.: 0000a9a3 Vec:A3 LowestDl Lg:03000000 lvl
Inti15.: 0000a993 Vec:93 LowestDl Lg:03000000 lvl
Inti16.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Inti17.: 000100ff Vec:FF FixedDel PhysDest:00 edg masked
Software Interrupt Request Levels (IRQLs)
Although interrupt controllers perform a level of interrupt prioritization, Windows imposes
its own interrupt priority scheme known as interrupt request levels (IRQLs). The kernel represents
IRQLs internally as a number from 0 through 31 on x86 and from 0 to 15 on x64 and
IA64, with higher numbers representing higher-priority interrupts. Although the kernel
defines the standard set of IRQLs for software interrupts, the HAL maps hardware-interrupt
numbers to the IRQLs. Figure 3-3 shows IRQLs defined for the x86 architecture, and Figure
3-4 shows IRQLs for the x64 and IA64 architectures.
Note SYNCH_LEVEL, which multiprocessor versions of the kernel use to protect access to
per-processor processor control blocks (PRCB), is not shown in the charts because its value varies
across different versions of Windows. See Chapter 6 for a description of SYNCH_LEVEL and
its possible values.
92 Microsoft Windows Internals, Fourth Edition
Figure 3-3 x86 interrupt request levels (IRQLs)
Figure 3-4 x64 and IA64 interrupt request levels (IRQLs)
Interrupts are serviced in priority order, and a higher-priority interrupt preempts the servicing
of a lower-priority interrupt. When a high-priority interrupt occurs, the processor saves the
interrupted thread’s state and invokes the trap dispatchers associated with the interrupt. The
trap dispatcher raises the IRQL and calls the interrupt’s service routine. After the service routine
executes, the interrupt dispatcher lowers the processor’s IRQL to where it was before the
interrupt occurred and then loads the saved machine state. The interrupted thread resumes
executing where it left off. When the kernel lowers the IRQL, lower-priority interrupts that
were masked might materialize. If this happens, the kernel repeats the process to handle the
IRQL priority levels have a completely different meaning than thread-scheduling priorities
(which are described in Chapter 6). A scheduling priority is an attribute of a thread, whereas
• • •
Normal thread execution
Synch (Srv 2003)
Correctable Machine Check
Synch (MP only)
Dispatch/DPC & Synch (UP only)
Chapter 3: System Mechanisms 93
an IRQL is an attribute of an interrupt source, such as a keyboard or a mouse. In addition,
each processor has an IRQL setting that changes as operating system code executes.
Each processor’s IRQL setting determines which interrupts that processor can receive. IRQLs
are also used to synchronize access to kernel-mode data structures. (You’ll find out more
about synchronization later in this chapter.) As a kernel-mode thread runs, it raises or lowers
the processor’s IRQL either directly by calling KeRaiseIrql and KeLowerIrql or, more commonly,
indirectly via calls to functions that acquire kernel synchronization objects. As Figure
3-5 illustrates, interrupts from a source with an IRQL above the current level interrupt the
processor, whereas interrupts from sources with IRQLs equal to or below the current level are
masked until an executing thread lowers the IRQL.
Figure 3-5 Masking interrupts
Because accessing a PIC is a relatively slow operation, HALs that use a PIC implement a performance
optimization, called lazy IRQL, that avoids PIC accesses. When the IRQL is raised,
the HAL notes the new IRQL internally instead of changing the interrupt mask. If a lower-priority
interrupt subsequently occurs, the HAL sets the interrupt mask to the settings appropriate
for the first interrupt and postpones the lower-priority interrupt until the IRQL is lowered.
Thus, if no lower-priority interrupts occur while the IRQL is raised, the HAL doesn’t need to
modify the PIC.
A kernel-mode thread raises and lowers the IRQL of the processor on which it’s running,
depending on what it’s trying to do. For example, when an interrupt occurs, the trap handler
(or perhaps the processor) raises the processor’s IRQL to the assigned IRQL of the interrupt
source. This elevation masks all interrupts at and below that IRQL (on that processor only),
which ensures that the processor servicing the interrupt isn’t waylaid by an interrupt at the
same or a lower level. The masked interrupts are either handled by another processor or held
• • •
Interrupts masked on
IRQL = DPC/dispatch
IRQL = Clock
Interrupts masked on
94 Microsoft Windows Internals, Fourth Edition
back until the IRQL drops. Therefore, all components of the system, including the kernel and
device drivers, attempt to keep the IRQL at passive level (sometimes called low level). They do
this because device drivers can respond to hardware interrupts in a timelier manner if the
IRQL isn’t kept unnecessarily elevated for long periods.
Note An exception to the rule that raising the IRQL blocks interrupts of that level and lower
relates to APC_LEVEL interrupts. If a thread raises the IRQL to APC_LEVEL and then is rescheduled
because of a DISPATCH_LEVEL interrupt, the system might deliver an APC_LEVEL interrupt
to the newly scheduled thread. Thus, APC_LEVEL can be considered a thread-local rather than
EXPERIMENT: Viewing the IRQL
If you are running the kernel debugger on Windows Server 2003, you can view a processor’s
IRQL with the !irql debugger command:
Debugger saved IRQL for processor 0x0 -- 0 (LOW_LEVEL)
Note that there is a field called IRQL in a data structure called the processor control region
(PCR) and its extension the processor control block (PRCB), which contain information
about the state of each processor in the system, such as the current IRQL, a pointer to
the hardware IDT, the currently running thread, and the next thread selected to run.
The kernel and the HAL use this information to perform architecture-specific and
machine-specific actions. Portions of the PCR and PRCB structures are defined publicly
in the Windows Device Driver Kit (DDK) header file Ntddk.h, so examine that file if you
want a complete definition of these structures.
You can view the contents of the PCR with the kernel debugger by using the !pcr command:
PCR Processor 0 @ffdff000
Chapter 3: System Mechanisms 95
Unfortunately, Windows does not maintain the Irql field on systems that do not use lazy
IRQL, so on most systems the field will always be 0.
Because changing a processor’s IRQL has such a significant effect on system operation, the
change can be made only in kernel mode—user-mode threads can’t change the processor’s
IRQL. This means that a processor’s IRQL is always at passive level when it’s executing usermode
code. Only when the processor is executing kernel-mode code can the IRQL be higher.
Each interrupt level has a specific purpose. For example, the kernel issues an interprocessor
interrupt (IPI) to request that another processor perform an action, such as dispatching a particular
thread for execution or updating its translation look-aside buffer cache. The system
clock generates an interrupt at regular intervals, and the kernel responds by updating the
clock and measuring thread execution time. If a hardware platform supports two clocks, the
kernel adds another clock interrupt level to measure performance. The HAL provides a number
of interrupt levels for use by interrupt-driven devices; the exact number varies with the processor
and system configuration. The kernel uses software interrupts (described later in this chapter)
to initiate thread scheduling and to asynchronously break into a thread’s execution.
Mapping Interrupts to IRQLs IRQL levels aren’t the same as the interrupt requests (IRQs)
defined by interrupt controllers—the architectures on which Windows runs don’t implement
the concept of IRQLs in hardware. So how does Windows determine what IRQL to assign to
an interrupt? The answer lies in the HAL. In Windows, a type of device driver called a bus
driver determines the presence of devices on its bus (PCI, USB, and so on) and what interrupts
can be assigned to a device. The bus driver reports this information to the Plug and Play
manager, which decides, after taking into account the acceptable interrupt assignments for all
other devices, which interrupt will be assigned to each device. Then it calls the HAL function
HalpGetSystemInterruptVector, which maps interrupts to IRQLs.
96 Microsoft Windows Internals, Fourth Edition
The algorithm for assignment differs for the various HALs that Windows includes. On uniprocessor
x86 systems, the HAL performs a straightforward translation: the IRQL of a given interrupt
vector is calculated by subtracting the interrupt vector from 27. Thus, if a device uses
interrupt vector 5, its ISR executes at IRQL 22. On an x86 multiprocessor system, the mapping
isn’t as simple. APICs support over 200 interrupt vectors, so there aren’t enough IRQLs
for a one-to-one correspondence. The multiprocessor HAL therefore assigns IRQLs to interrupt
vectors in a round-robin manner, cycling through the device IRQL (DIRQL) range. As a
result, on an x86 multiprocessor system there’s no easy way for you to predict or to know
what IRQL Windows assigns to APIC IRQs. Finally, on x64 and IA64 systems, the HAL computes
the IRQL for a given IRQ by dividing the interrupt vector assigned to the IRQ by 16.
Predefined IRQLs Let’s take a closer look at the use of the predefined IRQLs, starting from
the highest level shown in Figure 3-5:
■ The kernel uses high level only when it’s halting the system in KeBugCheckEx and masking
out all interrupts.
■ Power fail level originated in the original Microsoft Windows NT design documents,
which specified the behavior of system power failure code, but this IRQL has never been
■ Inter-processor interrupt level is used to request another processor to perform an action,
such as queue a DISPATCH_LEVEL interrupt to schedule a particular thread for execution,
updating the processor’s translation look-aside buffer (TLB) cache, system shutdown,
or system crash.
■ Clock level is used for the system’s clock, which the kernel uses to track the time of day
as well as to measure and allot CPU time to threads.
■ The system’s real-time clock uses profile level when kernel profiling, a performance measurement
mechanism, is enabled. When kernel profiling is active, the kernel’s profiling
trap handler records the address of the code that was executing when the interrupt
occurred. A table of address samples is constructed over time that tools can extract and
analyze. You can download Kernrate, a kernel profiling tool that you can use to configure
and view profiling-generated statistics, from http://www.microsoft.com/whdc/system/
sysperf/krview.mspx. See the Kernrate experiment for more information on using
■ The device IRQLs are used to prioritize device interrupts. (See the previous section for
how hardware interrupt levels are mapped to IRQLs.)
■ DPC/dispatch-level and APC-level interrupts are software interrupts that the kernel and
device drivers generate. (DPCs and APCs are explained in more detail later in this chapter.)
■ The lowest IRQL, passive level, isn’t really an interrupt level at all; it’s the setting at which
normal thread execution takes place and all interrupts are allowed to occur.
Chapter 3: System Mechanisms 97
EXPERIMENT: Using Kernel Profiler to Profile Execution
You can use the Kernel Profiler tool to enable the system profiling timer, collect samples
of the code that is executing when the timer fires, and display a summary showing the
frequency distribution across image files and functions. It can be used to track CPU
usage consumed by individual processes and/or time spent in kernel mode independent
of processes (for example, interrupt service routines). Kernel profiling is useful
when you want to obtain a breakdown of where the system is spending time.
In its simplest form, Kernrate samples where time has been spent in each kernel module
(for example, Ntoskrnl, drivers, and so on). For example, after installing the Krview
package referred to previously, try performing the following steps:
1. Open a command prompt.
2. Type cd c:\program files\krview\kernrates.
3. Type dir. (You will see kernrate images for each platform.)
4. Run the image that matches your platform (with no arguments or switches). For
example, Kernrate_i386_XP.exe is the image for Windows XP running on an x86
5. While Kernrate is running, go perform some other activity on the system. For example,
run Windows Media Player and play some music, run a graphics-intensive game,
or perform network activity such as doing a directory of a remote network share.
6. Press Ctrl+C to stop Kernrate. This causes Kernrate to display the statistics from
the sampling period.
In the sample partial output from Kernrate, Windows Media Player was running, playing
a track from a CD.
< KERNRATE LOG >
Date: 2004/05/13 Time: 9:48:28
Machine Name: BIGDAVID
Number of Processors: 1
Kernrate User-Specified Command Line:
***> Press ctrl-c to finish collecting profile data
===> Finished Collecting Data, Starting to Process Results
P0 K 0:00:03.234 (11.7%) U 0:00:08.352 (30.2%) I 0:00:16.093 (58.1%)
DPC 0:00:01.772 ( 6.4%) Interrupt 0:00:00.350 ( 1.3%)
98 Microsoft Windows Internals, Fourth Edition
Interrupts= 52899, Interrupt Rate= 1911/
sec.Time 7315 hits, 19531 events per hit --------
Module Hits msec %Total Events/Sec
gv3 4735 27679 64 % 3341135
smwdm 872 27679 11 % 615305
win32k 764 27679 10 % 539097
ntoskrnl 739 27679 10 % 521457
hal 124 27679 1 % 87497
The overall summary shows that the system spent 11.7 percent of the time in kernel
mode, 30.2 percent in user mode, 58.1 percent idle, 6.4 percent at DPC level, and 1.3
percent at interrupt level. The module with the highest hit rate was GV3.SYS, the processor
driver for the Pentium M Geyserville family. It is used for performance collection,
which is why it is first. The module with the second highest hit rate was Smwdm.sys, the
audio driver for the sound card on the machine used for the test. This makes sense
because the major activity going on in the system was Windows Media Player sending
sound I/O to the sound driver.
If you have symbols available, you can zoom in on individual modules and see the time
spent by function name. For example, profiling the system while dragging a window
around the screen rapidly resulted in the following (partial) output:
C:\Program Files\KrView\Kernrates>Kernrate_i386_XP.exe -z ntoskrnl -z win32k
< KERNRATE LOG >
Date: 2004/05/13 Time: 10:26:55
Time 4087 hits, 19531 events per hit --------
Module Hits msec %Total Events/Sec
win32k 1649 10424 40 % 3089660
ati2dvag 1269 10424 31 % 2377670
ntoskrnl 794 10424 19 % 1487683
gv3 162 10424 3 % 303532
----- Zoomed module win32k.sys (Bucket size = 16 bytes, Rounding Down) -------
Module Hits msec %Total Events/Sec
EngPaint 328 10424 19 % 614559
EngLpkInstalled 302 10424 18 % 565844
----- Zoomed module ntoskrnl.exe (Bucket size = 16 bytes, Rounding Down) -----
Module Hits msec %Total Events/Sec
KiDispatchInterrupt 243 10424 26 % 455298
ZwYieldExecution 50 10424 5 % 93682
InterlockedDecrement 39 10424 4 % 73072
The module with the highest hit rate was Win32k.sys, the windowing system driver.
Second on the list was the video driver. These results make sense because the main
activity in the system was drawing on the screen. Note in the zoomed display for
Win32k.sys, the function with the highest hit was EngPaint, the main GDI function to
paint on the screen.
Chapter 3: System Mechanisms 99
One important restriction on code running at DPC/dispatch level or above is that it can’t wait
for an object if doing so would necessitate the scheduler to select another thread to execute,
which is an illegal operation because the scheduler synchronizes its data structures at DPC/
dispatch level and cannot therefore be invoked to perform a reschedule. Another restriction is
that only nonpaged memory can be accessed at IRQL DPC/dispatch level or higher. This rule
is actually a side-effect of the first restriction because attempting to access memory that isn’t
resident results in a page fault. When a page fault occurs, the memory manager initiates a disk
I/O and then needs to wait for the file system driver to read the page in from disk. This wait
would in turn require the scheduler to perform a context switch (perhaps to the idle thread if
no user thread is waiting to run), thus violating the rule that the scheduler can’t be invoked
(because the IRQL is still DPC/dispatch level or higher at the time of the disk read). If either
of these two restrictions is violated, the system crashes with an
IRQL_NOT_LESS_OR_EQUAL crash code. (See Chapter 4 for a thorough discussion of system
crashes.) Violating these restrictions is a common bug in device drivers. The Windows
Driver Verifier, explained in the section “Driver Verifier” in Chapter 7, has an option you can
set to assist in finding this particular type of bug.
Interrupt Objects The kernel provides a portable mechanism—a kernel control object
called an interrupt object—that allows device drivers to register ISRs for their devices. An interrupt
object contains all the information the kernel needs to associate a device ISR with a particular
level of interrupt, including the address of the ISR, the IRQL at which the device
interrupts, and the entry in the kernel’s IDT with which the ISR should be associated. When
an interrupt object is initialized, a few instructions of assembly language code, called the dispatch
code, are copied from an interrupt handling template, KiInterruptTemplate, and stored
in the object. When an interrupt occurs, this code is executed.
This interrupt-object resident code calls the real interrupt dispatcher, which is typically either
the kernel’s KiInterruptDispatch or KiChainedDispatch routine, passing it a pointer to the interrupt
object. KiInterruptDispatch is the routine used for interrupt vectors for which only one
interrupt object is registered, and KiChainedDispatch is for vectors shared among multiple
interrupt objects. The interrupt object contains information this second dispatcher routine
needs to locate and properly call the ISR the device driver provides. The interrupt object also
stores the IRQL associated with the interrupt so that KiInterruptDispatch or KiChainedDispatch
can raise the IRQL to the correct level before calling the ISR and then lower the IRQL
after the ISR has returned. This two-step process is required because there’s no way to pass a
pointer to the interrupt object (or any other argument for that matter) on the initial dispatch
because the initial dispatch is done by hardware. On a multiprocessor system, the kernel allocates
and initializes an interrupt object for each CPU, enabling the local APIC on that CPU to
accept the particular interrupt. Figure 3-6 shows typical interrupt control flow for interrupts
associated with interrupt objects.
100 Microsoft Windows Internals, Fourth Edition
Figure 3-6 Typical interrupt control flow
EXPERIMENT: Examining Interrupt Internals
Using the kernel debugger, you can view details of an interrupt object, including its
IRQL, ISR address, and custom interrupt dispatching code. First, execute the !idt command
and locate the entry that includes a reference to I8042KeyboardInterruptService,
the ISR routine for the PS2 keyboard device:
31: 8a39dc3c i8042prt!I8042KeyboardInterruptService (KINTERRUPT 8a39dc00)
To view the contents of the interrupt object associated with the interrupt, execute dt
nt!_kinterrupt with the address following KINTERRUPT:
kd> dt nt!_kinterrupt 8a39dc00
+0x000 Type : 22
+0x002 Size : 484
+0x004 InterruptListEntry : _LIST_ENTRY [ 0x8a39dc04 - 0x8a39dc04 ]
+0x00c ServiceRoutine : 0xba7e74a2 i8042prt!I8042KeyboardInterruptService+0
+0x010 ServiceContext : 0x8a067898
+0x014 SpinLock : 0
+0x018 TickCount : 0xffffffff
+0x01c ActualLock : 0x8a067958 -> 0
+0x020 DispatchAddress : 0x80531140 nt!KiInterruptDispatch+0
+0x024 Vector : 0x31
+0x028 Irql : 0x1a ’’
+0x029 SynchronizeIrql : 0x1a ’’
+0x02a FloatingSave : 0 ’’
KiInterruptDispatch Driver ISR
Read from device
Chapter 3: System Mechanisms 101
+0x02b Connected : 0x1 ’’
+0x02c Number : 0 ’’
+0x02d ShareVector : 0 ’’
+0x030 Mode : 1 ( Latched )
+0x034 ServiceCount : 0
+0x038 DispatchCount : 0xffffffff
+0x03c DispatchCode :  0x56535554
In this example, the IRQL Windows assigned to the interrupt is 0x1a (which is 26 in
decimal). Because this output is from a uniprocessor x86 system, we calculate that the
IRQ is 1, because IRQLs on x86 uniprocessors are calculated by subtracting the IRQ
from 27. We can verify this by opening the Device Manager (on the Hardware tab in the
System applet in the Control Panel), locating the PS/2 keyboard device, and viewing its
resource assignments, as shown in the following figure.
On a multiprocessor x86, the IRQ will be essentially randomly assigned, and on an x64
or IA64 system you will see that the IRQ is the interrupt vector number (0x31—49 decimal—
in this example) divided by 16.
The ISR’s address for the interrupt object is stored in the ServiceRoutine field (which is
what !idt displays in its output), and the interrupt code that actually executes when an
interrupt occurs is stored in the DispatchCode array at the end of the interrupt object.
The interrupt code stored there is programmed to build the trap frame on the stack and
then call the function stored in the DispatchAddress field (KiInterruptDispatch in the
example), passing it a pointer to the interrupt object.
102 Microsoft Windows Internals, Fourth Edition
Windows and Real-Time Processing
Deadline requirements, either hard or soft, characterize real-time environments. Hard
real-time systems (for example, a nuclear power plant control system) have deadlines
that the system must meet to avoid catastrophic failures such as loss of equipment or
life. Soft real-time systems (for example, a car’s fuel-economy optimization system) have
deadlines that the system can miss, but timeliness is still a desirable trait. In real-time
systems, computers have sensor input devices and control output devices. The designer
of a real-time computer system must know worst-case delays between the time an input
device generates an interrupt and the time the device’s driver can control the output
device to respond. This worst-case analysis must take into account the delays the operating
system introduces as well as the delays the application and device drivers impose.
Because Windows doesn’t prioritize device IRQs in any controllable way and user-level
applications execute only when a processor’s IRQL is at passive level, Windows isn’t
always suitable as a real-time operating system. The system’s devices and device drivers—
not Windows—ultimately determine the worst-case delay. This factor becomes a
problem when the real-time system’s designer uses off-the-shelf hardware. The designer
can have difficulty determining how long every off-the-shelf device’s ISR or DPC might
take in the worst case. Even after testing, the designer can’t guarantee that a special case
in a live system won’t cause the system to miss an important deadline. Furthermore, the
sum of all the delays a system’s DPCs and ISRs can introduce usually far exceeds the tolerance
of a time-sensitive system.
Although many types of embedded systems (for example, printers and automotive computers)
have real-time requirements, Windows XP Embedded doesn’t have real-time
characteristics. It is simply a version of Windows XP that makes it possible, using system
designer technology that Microsoft licensed from VenturCom, to produce small-footprint
versions of Windows XP suitable for running on devices with limited resources.
For example, a device that has no networking capability would omit all the Windows XP
components related to networking, including network management tools and adapter
and protocol stack device drivers.
Still, there are third-party vendors that supply real-time kernels for Windows. The
approach these vendors take is to embed their real-time kernel in a custom HAL and to
have Windows run as a task in the real-time operating system. The task running Windows
serves as the user interface to the system and has a lower priority than the tasks
responsible for managing the device. See VenturCom’s Web site, www.venturcom.com, for
an example of a third-party real-time kernel extension for Windows.
Associating an ISR with a particular level of interrupt is called connecting an interrupt object,
and dissociating an ISR from an IDT entry is called disconnecting an interrupt object. These
Chapter 3: System Mechanisms 103
operations, accomplished by calling the kernel functions IoConnectInterrupt and IoDisconnectInterrupt,
allow a device driver to “turn on” an ISR when the driver is loaded into the system
and to “turn off” the ISR if the driver is unloaded.
Using the interrupt object to register an ISR prevents device drivers from fiddling directly
with interrupt hardware (which differs among processor architectures) and from needing to
know any details about the IDT. This kernel feature aids in creating portable device drivers
because it eliminates the need to code in assembly language or to reflect processor differences
in device drivers.
Interrupt objects provide other benefits as well. By using the interrupt object, the kernel
can synchronize the execution of the ISR with other parts of a device driver that might share
data with the ISR. (See Chapter 9 for more information about how device drivers respond to
Furthermore, interrupt objects allow the kernel to easily call more than one ISR for any interrupt
level. If multiple device drivers create interrupt objects and connect them to the same
IDT entry, the interrupt dispatcher calls each routine when an interrupt occurs at the specified
interrupt line. This capability allows the kernel to easily support “daisy-chain” configurations,
in which several devices share the same interrupt line. The chain breaks when one of
the ISRs claims ownership for the interrupt by returning a status to the interrupt dispatcher.
If multiple devices sharing the same interrupt require service at the same time, devices not
acknowledged by their ISRs will interrupt the system again once the interrupt dispatcher has
lowered the IRQL. Chaining is permitted only if all the device drivers wanting to use the same
interrupt indicate to the kernel that they can share the interrupt; if they can’t, the Plug and
Play manager reorganizes their interrupt assignments to ensure that it honors the sharing
requirements of each. If the interrupt vector is shared, the interrupt object invokes KiChained-
Dispatch, which will invoke the ISRs of each registered interrupt object in turn until one of
them claims the interrupt or all have been executed. In the earlier sample !idt output, vector
0x3b is connected to several chained interrupt objects.
Although hardware generates most interrupts, the Windows kernel also generates software
interrupts for a variety of tasks, including these:
■ Initiating thread dispatching
■ Non-time-critical interrupt processing
■ Handling timer expiration
■ Asynchronously executing a procedure in the context of a particular thread
■ Supporting asynchronous I/O operations
These tasks are described in the following subsections.
104 Microsoft Windows Internals, Fourth Edition
Dispatch or Deferred Procedure Call (DPC) Interrupts When a thread can no longer continue
executing, perhaps because it has terminated or because it voluntarily enters a wait state,
the kernel calls the dispatcher directly to effect an immediate context switch. Sometimes, however,
the kernel detects that rescheduling should occur when it is deep within many layers of
code. In this situation, the kernel requests dispatching but defers its occurrence until it completes
its current activity. Using a DPC software interrupt is a convenient way to achieve this
The kernel always raises the processor’s IRQL to DPC/dispatch level or above when it needs
to synchronize access to shared kernel structures. This disables additional software interrupts
and thread dispatching. When the kernel detects that dispatching should occur, it requests a
DPC/dispatch-level interrupt; but because the IRQL is at or above that level, the processor
holds the interrupt in check. When the kernel completes its current activity, it sees that it’s
going to lower the IRQL below DPC/dispatch level and checks to see whether any dispatch
interrupts are pending. If there are, the IRQL drops to DPC/dispatch level and the dispatch
interrupts are processed. Activating the thread dispatcher by using a software interrupt is a
way to defer dispatching until conditions are right. However, Windows uses software interrupts
to defer other types of processing as well.
In addition to thread dispatching, the kernel also processes deferred procedure calls (DPCs)
at this IRQL. A DPC is a function that performs a system task—a task that is less time-critical
than the current one. The functions are called deferred because they might not execute immediately.
DPCs provide the operating system with the capability to generate an interrupt and execute a
system function in kernel mode. The kernel uses DPCs to process timer expiration (and
release threads waiting for the timers) and to reschedule the processor after a thread’s quantum
expires. Device drivers use DPCs to complete I/O requests. To provide timely service for
hardware interrupts, Windows—with the cooperation of device drivers—attempts to keep the
IRQL below device IRQL levels. One way that this goal is achieved is for device driver ISRs to
perform the minimal work necessary to acknowledge their device, save volatile interrupt state,
and defer data transfer or other less time-critical interrupt processing activity for execution in
a DPC at DPC/dispatch IRQL. (See Chapter 9 for more information on DPCs and the I/O system.)
A DPC is represented by a DPC object, a kernel control object that is not visible to user-mode
programs but is visible to device drivers and other system code. The most important piece of
information the DPC object contains is the address of the system function that the kernel will
call when it processes the DPC interrupt. DPC routines that are waiting to execute are stored
in kernel-managed queues, one per processor, called DPC queues. To request a DPC, system
code calls the kernel to initialize a DPC object and then places it in a DPC queue.
By default, the kernel places DPC objects at the end of the DPC queue of the processor on
which the DPC was requested (typically the processor on which the ISR executed). A device
Chapter 3: System Mechanisms 105
driver can override this behavior, however, by specifying a DPC priority (low, medium, or
high, where medium is the default) and by targeting the DPC at a particular processor. A DPC
aimed at a specific CPU is known as a targeted DPC. If the DPC has a low or medium priority,
the kernel places the DPC object at the end of the queue; if the DPC has a high priority, the
kernel inserts the DPC object at the front of the queue.
When the processor’s IRQL is about to drop from an IRQL of DPC/dispatch level or higher to
a lower IRQL (APC or passive level), the kernel processes DPCs. Windows ensures that the
IRQL remains at DPC/dispatch level and pulls DPC objects off the current processor’s queue
until the queue is empty (that is, the kernel “drains” the queue), calling each DPC function in
turn. Only when the queue is empty will the kernel let the IRQL drop below DPC/dispatch
level and let regular thread execution continue. DPC processing is depicted in Figure 3-7.
Figure 3-7 Delivering a DPC
DPC priorities can affect system behavior another way. The kernel usually initiates DPC queue
draining with a DPC/dispatch-level interrupt. The kernel generates such an interrupt only if
the DPC is directed at the processor the ISR is requested on and the DPC has a high or
medium priority. If the DPC has a low priority, the kernel requests the interrupt only if the
number of outstanding DPC requests for the processor rises above a threshold or if the number
of DPCs requested on the processor within a time window is low. If a DPC is targeted at a
CPU different from the one on which the ISR is running and the DPC’s priority is high, the
kernel immediately signals the target CPU (by sending it a dispatch IPI) to drain its DPC
queue. If the priority is medium or low, the number of DPCs queued on the target processor
must exceed a threshold for the kernel to trigger a DPC/dispatch interrupt. The system idle
thread also drains the DPC queue for the processor it runs on. Although DPC targeting and
The dispatcher executes each DPC routine
in the DPC queue, emptying the queue as
it proceeds. If required, the dispatcher also
reschedules the processor.
After the DPC interrupt,
control transfers to the
When the IRQL drops below
DPC/dispatch level, a DPC
A timer expires, and the kernel
queues a DPC that will release
any threads waiting on the
timer. The kernel then
requests a software interrupt.
• • •
106 Microsoft Windows Internals, Fourth Edition
priority levels are flexible, device drivers rarely need to change the default behavior of their
DPC objects. Table 3-1 summarizes the situations that initiate DPC queue draining.
Because user-mode threads execute at low IRQL, the chances are good that a DPC will interrupt
the execution of an ordinary user’s thread. DPC routines execute without regard to what
thread is running, meaning that when a DPC routine runs, it can’t assume what process
address space is currently mapped. DPC routines can call kernel functions, but they can’t call
system services, generate page faults, or create or wait for dispatcher objects (explained later
in this chapter). They can, however, access nonpaged system memory addresses, because system
address space is always mapped regardless of what the current process is.
DPCs are provided primarily for device drivers, but the kernel uses them too. The kernel most
frequently uses a DPC to handle quantum expiration. At every tick of the system clock, an
interrupt occurs at clock IRQL. The clock interrupt handler (running at clock IRQL) updates
the system time and then decrements a counter that tracks how long the current thread has
run. When the counter reaches 0, the thread’s time quantum has expired and the kernel
might need to reschedule the processor, a lower-priority task that should be done at DPC/dispatch
IRQL. The clock interrupt handler queues a DPC to initiate thread dispatching and then
finishes its work and lowers the processor’s IRQL. Because the DPC interrupt has a lower priority
than do device interrupts, any pending device interrupts that surface before the clock
interrupt completes are handled before the DPC interrupt occurs.
EXPERIMENT: Monitoring Interrupt and DPC Activity
You can use Process Explorer to monitor interrupt and DPC activity by adding the
Context Switch Delta column and watching the Interrupt and DPC processes. These
are not real processes, but they are shown as processes for convenience and therefore
do not incur context switches. Process Explorer’s context switch count for these
pseudo processes reflects the number of occurrences of each within the previous
refresh interval. You can stimulate interrupt and DPC activity by moving the mouse
quickly around the screen.
Table 3-1 DPC Interrupt Generation Rules
DPC Priority DPC Targeted at ISR’s Processor DPC Targeted at Another Processor
Low DPC queue length exceeds maximum
DPC queue length or DPC
request rate is less than minimum
DPC request rate
DPC queue length exceeds maximum
DPC queue length or System is idle
Medium Always DPC queue length exceeds maximum
DPC queue length or System is idle
High Always Always
Chapter 3: System Mechanisms 107
You can also trace the execution of specific interrupt service routines and deferred procedure
calls with the built-in event tracing support (described later in this chapter) in
Windows XP Service Pack 2 and Windows Server 2003 Service Pack 1 and later.
1. Start capturing events by typing the following command:
tracelog -start -f kernel.etl -b 64 -UsePerfCounter -
eflag 8 0x307 0x4084 0 0 0 0 0 0
2. Stop capturing events by typing:
tracelog -stop to stop logging.
3. Generate reports for the event capture by typing:
tracerpt kernel.etl -df -o -report
This will generate two files: workload.txt and dumpfile.csv.
4. Open “workload.txt” and you will see summaries of the time spent in ISRs and
DPCs by each driver type.
5. Open the file “dumpfile.csv” created in step 4; search for lines with “DPC” or “ISR”
in the second value. For example, the following three lines from a dumpfile.csv
generated using the above commands show a timer DPC, a DPC, and an ISR:
PerfInfo, TimerDPC, 0xFFFFFFFF, 127383953645422825, 0,
0, 127383953645421500, 0xFB03A385, 0, 0
PerfInfo, DPC, 0xFFFFFFFF, 127383953645424040, 0,
0, 127383953645421394, 0x804DC87D, 0, 0
PerfInfo, ISR, 0xFFFFFFFF, 127383953645470903, 0,
0, 127383953645468696, 0xFB48D5E0, 0, 0, 0
Doing an “ln” command in the kernel debugger on the start address in each event
record (the eighth value on each line) shows the name of the function that executed the
DPC or ISR:
lkd> ln 0xFB03A385
(fb03a385) rdbss!RxTimerDispatch | (fb03a41e) rdbss!RxpWorkerThreadDispatcher
lkd> ln 0x804DC87D
(804dc87d) nt!KiTimerExpiration | (804dc93b) nt!KeSetTimerEx
lkd> ln 0xFB48D5E0
(fb48d5e0) atapi!IdePortInterrupt | (fb48d622) atapi!IdeCheckEmptyChannel
The first is a DPC for a timer expiration for a timer queued by the file system redirector
client driver. The second is a DPC for a generic timer expiration. The third address is the
address of the ISR for the ATAPI port driver. For more information, see http://
108 Microsoft Windows Internals, Fourth Edition
Asynchronous Procedure Call (APC) Interrupts Asynchronous procedure calls (APCs)
provide a way for user programs and system code to execute in the context of a particular user
thread (and hence a particular process address space). Because APCs are queued to execute in
the context of a particular thread and run at an IRQL less than DPC/dispatch level, they don’t
operate under the same restrictions as a DPC. An APC routine can acquire resources (objects),
wait for object handles, incur page faults, and call system services.
APCs are described by a kernel control object, called an APC object. APCs waiting to execute
reside in a kernel-managed APC queue. Unlike the DPC queue, which is systemwide, the APC
queue is thread-specific—each thread has its own APC queue. When asked to queue an APC,
the kernel inserts it into the queue belonging to the thread that will execute the APC routine.
The kernel, in turn, requests a software interrupt at APC level, and when the thread eventually
begins running, it executes the APC.
There are two kinds of APCs: kernel mode and user mode. Kernel-mode APCs don’t require
“permission” from a target thread to run in that thread’s context, while user-mode APCs do.
Kernel-mode APCs interrupt a thread and execute a procedure without the thread’s intervention
or consent. There are also two types of kernel-mode APCs: normal and special. A thread
can disable both types by raising the IRQL to APC_LEVEL or by calling KeEnterGuardedRegion,
which was introduced in Windows Server 2003. KeEnterGuardedRegionThread disables
APC delivery by setting the SpecialApcDisable field in the calling thread’s KTHREAD structure
(described further in Chapter 6). A thread can disable normal APCs only by calling KeEnterCriticalRegion,
which sets the KernelApcDisable field in the thread’s KTHREAD structure.
The executive uses kernel-mode APCs to perform operating system work that must be completed
within the address space (in the context) of a particular thread. It can use special kernel-
mode APCs to direct a thread to stop executing an interruptible system service, for
example, or to record the results of an asynchronous I/O operation in a thread’s address
space. Environment subsystems use special kernel-mode APCs to make a thread suspend or
terminate itself or to get or set its user-mode execution context. The POSIX subsystem uses
kernel-mode APCs to emulate the delivery of POSIX signals to POSIX processes.
Device drivers also use kernel-mode APCs. For example, if an I/O operation is initiated and a
thread goes into a wait state, another thread in another process can be scheduled to run.
When the device finishes transferring data, the I/O system must somehow get back into the
context of the thread that initiated the I/O so that it can copy the results of the I/O operation
to the buffer in the address space of the process containing that thread. The I/O system uses
a special kernel-mode APC to perform this action. (The use of APCs in the I/O system is discussed
in more detail in Chapter 9.)
Chapter 3: System Mechanisms 109
Several Windows APIs, such as ReadFileEx, WriteFileEx, and QueueUserAPC, use user-mode
APCs. For example, the ReadFileEx and WriteFileEx functions allow the caller to specify a completion
routine to be called when the I/O operation finishes. The I/O completion is implemented
by queueing an APC to the thread that issued the I/O. However, the callback to the
completion routine doesn’t necessarily take place when the APC is queued because user-mode
APCs are delivered to a thread only when it’s in an alertable wait state. A thread can enter a wait
state either by waiting for an object handle and specifying that its wait is alertable (with the
Windows WaitForMultipleObjectsEx function) or by testing directly whether it has a pending
APC (using SleepEx). In both cases, if a user-mode APC is pending, the kernel interrupts
(alerts) the thread, transfers control to the APC routine, and resumes the thread’s execution
when the APC routine completes. Unlike kernel-mode APCs, which execute at APC level, usermode
APCs execute at passive level.
APC delivery can reorder the wait queues—the lists of which threads are waiting for what, and
in what order they are waiting. (Wait resolution is described in the section “Low-IRQL Synchronization”
later in this chapter.) If the thread is in a wait state when an APC is delivered,
after the APC routine completes, the wait is reissued or reexecuted. If the wait still isn’t
resolved, the thread returns to the wait state, but now it will be at the end of the list of objects
it’s waiting for. For example, because APCs are used to suspend a thread from execution, if the
thread is waiting for any objects, its wait will be removed until the thread is resumed, after
which that thread will be at the end of the list of threads waiting to access the objects it was
In contrast to interrupts, which can occur at any time, exceptions are conditions that result
directly from the execution of the program that is running. Windows introduced a facility
known as structured exception handling, which allows applications to gain control when exceptions
occur. The application can then fix the condition and return to the place the exception
occurred, unwind the stack (thus terminating execution of the subroutine that raised the
exception), or declare back to the system that the exception isn’t recognized and the system
should continue searching for an exception handler that might process the exception. This
section assumes you’re familiar with the basic concepts behind Windows structured exception
handling—if you’re not, you should read the overview in the Windows API reference documentation
on the Platform SDK or chapters 23 through 25 in Jeffrey Richter’s book
Programming Applications for Microsoft Windows (Fourth Edition, Microsoft Press, 2000) before
proceeding. Keep in mind that although exception handling is made accessible through language
extensions (for example, the __try construct in Microsoft Visual C++), it is a system
mechanism and hence isn’t language-specific. Other examples of consumers of Windows
exception handling include C++ and Java exceptions.
110 Microsoft Windows Internals, Fourth Edition
On the x86, all exceptions have predefined interrupt numbers that directly correspond to the
entry in the IDT that points to the trap handler for a particular exception. Table 3-2 shows
x86-defined exceptions and their assigned interrupt numbers. Because the first entries of the
IDT are used for exceptions, hardware interrupts are assigned entries later in the table, as
All exceptions, except those simple enough to be resolved by the trap handler, are serviced by
a kernel module called the exception dispatcher. The exception dispatcher’s job is to find an
exception handler that can “dispose of” the exception. Examples of architecture-independent
exceptions that the kernel defines include memory access violations, integer divide-by-zero,
integer overflow, floating-point exceptions, and debugger breakpoints. For a complete list of
architecture-independent exceptions, consult the Windows API reference documentation.
The kernel traps and handles some of these exceptions transparently to user programs. For
example, encountering a breakpoint while executing a program being debugged generates an
exception, which the kernel handles by calling the debugger. The kernel handles certain other
exceptions by returning an unsuccessful status code to the caller.
A few exceptions are allowed to filter back, untouched, to user mode. For example, a memory
access violation or an arithmetic overflow generates an exception that the operating system
doesn’t handle. An environment subsystem can establish frame-based exception handlers to
Table 3-2 x86 Exceptions and Their Interrupt Numbers
Interrupt Number Exception
0 Divide Error
1 DEBUG TRAP
2 NMI/NPX Error
5 BOUND/Print Screen
6 Invalid Opcode
7 NPX Not Available
8 Double Exception
9 NPX Segment Overrun
A Invalid Task State Segment (TSS)
B Segment Not Present
C Stack Fault
D General Protection
E Page Fault
F Intel Reserved
10 Floating Point
11 Alignment Check
Chapter 3: System Mechanisms 111
deal with these exceptions. The term frame-based refers to an exception handler’s association
with a particular procedure activation. When a procedure is invoked, a stack frame representing
that activation of the procedure is pushed onto the stack. A stack frame can have one or
more exception handlers associated with it, each of which protects a particular block of code
in the source program. When an exception occurs, the kernel searches for an exception handler
associated with the current stack frame. If none exists, the kernel searches for an exception
handler associated with the previous stack frame, and so on, until it finds a frame-based
exception handler. If no exception handler is found, the kernel calls its own default exception
When an exception occurs, whether it is explicitly raised by software or implicitly raised by
hardware, a chain of events begins in the kernel. The CPU hardware transfers control to the
kernel trap handler, which creates a trap frame (as it does when an interrupt occurs). The trap
frame allows the system to resume where it left off if the exception is resolved. The trap handler
also creates an exception record that contains the reason for the exception and other pertinent
If the exception occurred in kernel mode, the exception dispatcher simply calls a routine to
locate a frame-based exception handler that will handle the exception. Because unhandled
kernel-mode exceptions are considered fatal operating system errors, you can assume that the
dispatcher always finds an exception handler.
If the exception occurred in user mode, the exception dispatcher does something more elaborate.
As you’ll see in Chapter 6, the Windows subsystem has a debugger port and an exception
port to receive notification of user-mode exceptions in Windows processes. The kernel
uses these in its default exception handling, as illustrated in Figure 3-8.
Figure 3-8 Dispatching an exception
112 Microsoft Windows Internals, Fourth Edition
Debugger breakpoints are common sources of exceptions. Therefore, the first action the
exception dispatcher takes is to see whether the process that incurred the exception has an
associated debugger process. If it does and the system is Windows 2000, the exception dispatcher
sends the first-chance debug message via an LPC to the debugger port associated with
the process that incurred the exception. The LPC message is sent to the session manager process,
which then dispatches it to the appropriate debugger process. On Windows XP and Windows
Server 2003, the exception dispatcher sends a debugger object message to the debug
object associated with the process (which internally the system refers to as a port).
If the process has no debugger process attached, or if the debugger doesn’t handle the exception,
the exception dispatcher switches into user mode, copies the trap frame to the user stack
formatted as a CONTEXT data structure (documented in the Platform SDK), and calls a routine
to find a frame-based exception handler. If none is found, or if none handles the exception,
the exception dispatcher switches back into kernel mode and calls the debugger again to
allow the user to do more debugging. (This is called the second-chance notification.)
If the debugger isn’t running and no frame-based handlers are found, the kernel sends a message
to the exception port associated with the thread’s process. This exception port, if one
exists, was registered by the environment subsystem that controls this thread. The exception
port gives the environment subsystem, which presumably is listening at the port, the opportunity
to translate the exception into an environment-specific signal or exception. CSRSS (Client/
Server Run-Time Subsystem) simply presents a message box notifying the user of the fault
and terminates the process, and when POSIX gets a message from the kernel that one of its
threads generated an exception, the POSIX subsystem sends a POSIX-style signal to the
thread that caused the exception. However, if the kernel progresses this far in processing the
exception and the subsystem doesn’t handle the exception, the kernel executes a default
exception handler that simply terminates the process whose thread caused the exception.
All Windows threads have an exception handler declared at the top of the stack that processes
unhandled exceptions. This exception handler is declared in the internal Windows start-ofprocess
or start-of-thread function. The start-of-process function runs when the first thread in a
process begins execution. It calls the main entry point in the image. The start-of-thread function
runs when a user creates additional threads. It calls the user-supplied thread start routine
specified in the CreateThread call.
Chapter 3: System Mechanisms 113
EXPERIMENT: Viewing the Real User Start Address for Windows
The fact that each Windows thread begins execution in a system-supplied function (and
not the user-supplied function) explains why the start address for thread 0 is the same
for every Windows process in the system (and why the start addresses for secondary
threads are also the same). The start address for thread 0 in Windows processes is the
Windows start-of-process function; the start address for any other threads would be the
Windows start-of-thread function. To see the user-supplied function address, use the
Tlist utility in the Windows Support Tools. Type tlist process-name or tlist process-id
to get the detailed process output that includes this information. For example, compare
the thread start addresses for the Windows Explorer process as reported by Pstat (in the
Platform SDK) and Tlist:
pid:3f8 pri: 8 Hnd: 329 Pf: 80043 Ws: 4620K explorer.exe
tid pri Ctx Swtch StrtAddr User Time Kernel Time State
7c 9 16442 77E878C1 0:00:01.241 0:00:01.251 Wait:UserRequest
42c 11 157888 77E92C50 0:00:07.110 0:00:34.309 Wait:UserRequest
44c 8 6357 77E92C50 0:00:00.070 0:00:00.140 Wait:UserRequest
1cc 8 3318 77E92C50 0:00:00.030 0:00:00.070 Wait:DelayExecution
C:\> tlist explorer
1016 explorer.exe Program Manager
VirtualSize: 25348 KB PeakVirtualSize: 31052 KB
WorkingSetSize: 1804 KB PeakWorkingSetSize: 3276 KB
149 Win32StartAddr:0x01009dbd LastErr:0x0000007e State:Waiting
86 Win32StartAddr:0x77c5d4a5 LastErr:0x00000000 State:Waiting
62 Win32StartAddr:0x00000977 LastErr:0x00000000 State:Waiting
179 Win32StartAddr:0x0100d8d4 LastErr:0x00000002 State:Waiting
The start address of thread 0 reported by Pstat is the internal Windows start-of-process
function; the start addresses for threads 1 through 3 are the internal Windows start-ofthread
functions. Tlist, on the other hand, shows the user-supplied Windows start
address (the user function called by the internal Windows start function).
114 Microsoft Windows Internals, Fourth Edition
Because most threads in Windows processes start at one of the system-supplied wrapper
functions, Process Explorer, when displaying the start address of threads in a process,
skips the initial call frame that represents the wrapper function and instead shows
the second frame on the stack. For example, notice the thread start address of a process
Process Explorer does display the complete call hierarchy when it displays the call stack.
Notice the following results when the Stack button is clicked:
Line 12 in the preceding figure is the first frame on the stack—the start of the process
wrapper. The second frame (line 11) is the main entry point into Notepad.exe.
Chapter 3: System Mechanisms 115
The generic code for these internal start functions is shown here:
DWORD dwThreadExitCode = lpStartAddr(lpvThreadParm);
Notice that the Windows unhandled exception filter is called if the thread has an exception
that it doesn’t handle. The purpose of this function is to provide the system-defined behavior
for what to do when an exception is not handled, which is based on the contents of the
HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\AeDebug registry key. There
are two important values: Auto and Debugger. Auto tells the unhandled exception filter
whether to automatically run the debugger or ask the user what to do. By default, it is set to 1,
which means that it will launch the debugger automatically. However, installing development
tools such as Visual Studio changes this to 0. The Debugger value is a string that points to the
path of the debugger executable to run in the case of an unhandled exception.
The default debugger is \Windows\System32\Drwtsn32.exe (Dr. Watson), which isn’t really
a debugger but rather a postmortem tool that captures the state of the application “crash” and
records it in a log file (Drwtsn32.log) and a process crash dump file (User.dmp), both found
by default in the \Documents And Settings\All Users\Documents\DrWatson folder. To see
(or modify) the configuration for Dr. Watson, run it interactively—it displays a window with
the current settings, as shown in Figure 3-9.
Figure 3-9 Windows 2000 Dr. Watson default settings
116 Microsoft Windows Internals, Fourth Edition
The log file contains basic information such as the exception code, the name of the image that
failed, a list of loaded DLLs, and a stack and instruction trace for the thread that incurred the
exception. For a detailed description of the contents of the log file, run Dr. Watson and click
the Help button shown in Figure 3-9.
The crash dump file contains the private pages in the process at the time of the exception.
(The file doesn’t include code pages from EXEs or DLLs.) This file can be opened by WinDbg,
the Windows debugger that comes with the Debugging Tools package, or by Visual Studio
2003 and later. Because the User.dmp file is overwritten each time a process crashes, unless
you rename or copy the file after each process crash, you’ll have only the latest one on your
On Windows 2000 Professional systems, visual notification is turned on by default. The message
box shown in Figure 3-10 is displayed by Dr. Watson after it generates the crash dump
and records information in its log file.
Figure 3-10 Windows 2000 Dr. Watson error message
The Dr. Watson process remains until the message box is dismissed, which is why on Windows
2000 Server systems visual notification is turned off by default. This default is used
because if a server application fails, there is usually nobody at the console to see it and dismiss
the message box. Instead, server applications should log errors to the Windows event log.
On Windows 2000, if the Auto value is set to zero, the message box shown in Figure 3-11 is
Figure 3-11 Windows 2000 Unhandled exception message
If the OK button is clicked, the process exits. If Cancel is clicked, the system defined debugger
process (specified by the Debugger’s value in the registry path referred to earlier) is launched.
Chapter 3: System Mechanisms 117
EXPERIMENT: Unhandled Exceptions
To see a sample Dr. Watson log file, download and run the program Accvio.exe, which
you can download from www.sysinternals.com/windowsinternals.shtml. This program
generates a memory access violation by attempting to write to address 0, which is always
an invalid address in Windows processes. (See Table 7-6 in Chapter 7.)
1. Run the Registry Editor, and locate HKLM\SOFTWARE\ Microsoft\Windows
2. If the Debugger value is “drwtsn32 -p %ld -e %ld –g”, your system is set up to run
Dr. Watson as the default debugger. Proceed to step 4.
3. If the value of Debugger was not set up to run Drwtsn32.exe, you can still test
Dr. Watson by temporarily installing it and then restoring your previous debugger
a. Save the current value somewhere (for example, in a Notepad file or in the
current paste buffer).
b. Select Run from the taskbar Start menu, and then type drwtsn32 –i. (This
initializes the Debugger field to run Dr. Watson.)
3. Run the test program Accvio.exe.
4. You should see one of the message boxes described earlier (depending on which
version of Windows you are running).
5. If you have the default Dr. Watson settings, you should now be able to examine the
log file and dump file in the dump file directory. To see the configuration settings
for Dr. Watson, run drwtsn32 with no additional arguments. (Select Run from the
Start menu, and then type drwtsn32.)
6. Alternatively, in the list of Application Errors displayed by Dr. Watson, click on the
last entry and then click the View button—the portion of the Dr. Watson log file
containing the details of the access violation from Accvio.exe will be displayed.
(For details on the log file format, open the help in Dr. Watson and select Dr. Watson
Log File Overview.)
7. If the original value of Debugger wasn’t the default Dr. Watson settings, restore the
saved value from step 1.
As another experiment, try changing the value of Debugger to another program, such as
Notepad.exe (Notepad editor) or Sol.exe (Solitaire). Rerun Accvio.exe, and notice that
whatever program is specified in the Debugger value is run—that is, there’s no validation
that the program defined in Debugger is actually a debugger. Make sure you restore your
registry settings. (As noted in step 3b, to reset to the system default Dr. Watson settings,
type drwtsn32 –i in the Run dialog box or at a command prompt.)
118 Microsoft Windows Internals, Fourth Edition
Windows Error Reporting
Windows XP and Windows Server 2003 have a new, more sophisticated error-reporting
mechanism called Windows Error Reporting that automates the submission of both usermode
process crashes as well as kernel-mode system crashes. (For a description of how this
applies to system crashes, see Chapter 14).
Windows Error Reporting can be configured by going to My Computer, selecting Properties,
Advanced, and then Error Reporting (which brings up the dialog box shown in Figure 3-12)
or by local or domain group policy settings under System, Error Reporting. These settings are
stored in the registry under the key HKLM\Software\Microsoft\PCHealth\ErrorReporting.
Figure 3-12 Error Reporting Configuration dialog box
When an unhandled exception is caught by the unhandled exception filter (described in the
previous section), an initial check is made to see whether or not to initiate Windows Error
Reporting. If the registry value HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\
AeDebug\Auto is set to zero or the Debugger string contains the text “Drwtsn32”, the
unhandled exception filter loads \Windows\System32\Faultrep.dll into the failing process
and calls its ReportFault function. ReportFault then checks the error-reporting configuration
stored under HKLM\Software\Microsoft\PCHealth\ErrorReporting to see whether this process
crash should be reported, and if so, how. In the normal case, ReportFault creates a process
running \Windows\System32\Dwwin.exe, which displays a message box announcing the process
crash along with an option to submit the error report to Microsoft as seen in Figure 3-13.
Chapter 3: System Mechanisms 119
Figure 3-13 Windows Error Reporting dialog box
If the Send Error Report button is clicked, the error report (a minidump and a text file with
details on the DLL version numbers loaded in the process) is sent to Microsoft’s online crash
analysis server, Watson.Microsoft.com. (Unlike kernel mode system crashes, in this situation
there is no way to find out whether a solution is available at the time of the report submission.)
Then the unhandled exception filter creates a process to run the system-defined debugger (normally
Drwtsn32.exe), which by default creates its own dump file and log entry. Unlike Windows
2000, the dump file is a minidump, not a full dump. So, in the case where a full process memory
dump is needed to debug a failing application, you can change the configuration of Dr. Watson
by running it with no command-line arguments as described in the previous section.
In environments where systems are not connected to the Internet or where the administrator
wants to control which error reports are submitted to Microsoft, the destination for the error
report can be configured to be an internal file server. Microsoft provides to qualified customers
a tool set called Corporate Error Reporting that understands the directory structure created
by Windows Error Reporting and provides the administrator with the option to take
selective error reports and submit them to Microsoft. (For more information, see http://
System Service Dispatching
As Figure 3-1 illustrated, the kernel’s trap handlers dispatch interrupts, exceptions, and system
service calls. In the preceding sections, you’ve seen how interrupt and exception handling
work; in this section, you’ll learn about system services. A system service dispatch is triggered
as a result of executing an instruction assigned to system service dispatching. The instruction
that Windows uses for system service dispatching depends on the processor on which it’s executing.
32-Bit System Service Dispatching
On x86 processors prior to the Pentium II, Windows uses the int 0x2e instruction (46) decimal,
which results in a trap. Windows fills in entry 46 in the IDT to point to the system service
120 Microsoft Windows Internals, Fourth Edition
dispatcher. (Refer to Table 3-1.) The trap causes the executing thread to transition into kernel
mode and enter the system service dispatcher. A numeric argument passed in the EAX processor
register indicates the system service number being requested. The EBX register points to
the list of parameters the caller passes to the system service.
On x86 Pentium II processors and higher, Windows uses the special sysenter instruction,
which Intel defined specifically for fast system service dispatches. To support the instruction,
Windows stores at boot time the address of the kernel’s system service dispatcher routine in
a register associated with the instruction. The execution of the instruction causes the change
to kernel-mode and execution of the system service dispatcher. The system service number is
passed in the EAX processor register, and the EDX register points to the list of caller arguments.
To return to user-mode, the system service dispatcher usually executes the sysexit
instruction. (In some cases, like when the single-step flag is enabled on the processor, the system
service dispatcher uses the iretd instruction instead.)
On K6 and higher 32-bit AMD processors, Windows uses the special syscall instruction, which
functions similar to the x86 sysenter instruction, with Windows configuring a syscall-associated
processor register with the address of the kernel’s system service dispatcher. The system
call number is passed in the EAX register, and the stack stores the caller arguments. After completing
the dispatch, the kernel executes the sysret instruction.
At boot time, Windows detects the type of processor on which it’s executing and sets up the
appropriate system call code to be used. The system service code for NtReadFile in user mode
looks like this:
77f5bfa8 b8b7000000 mov eax,0xb7
77f5bfad ba0003fe7f mov edx,0x7ffe0300
77f5bfb2 ffd2 call edx
77f5bfb4 c22400 ret 0x24
The system service number is 0xb7 (183 in decimal) and the call instruction executes the system
service dispatch code set up by the kernel, which in this example is at address
0x7ffe0300. Because this was taken from a Pentium M, it uses sysenter:
7ffe0300 8bd4 mov edx,esp
7ffe0302 0f34 sysenter
7ffe0304 c3 ret
64-Bit System Service Dispatching
On the x64 architecture, Windows uses the syscall instruction, which functions like the AMD
K6’s syscall instruction, for system service dispatching, passing the system call number in the
EAX register, the first four parameters in registers, and any parameters beyond those four on
Chapter 3: System Mechanisms 121
00000000`77f9fc60 4c8bd1 mov r10,rcx
00000000`77f9fc63 b8bf000000 mov eax,0xbf
00000000`77f9fc68 0f05 syscall
00000000`77f9fc6a c3 ret
On the IA64 architecture, Windows uses the epc (Enter Privileged Mode) instruction. The first
eight system call arguments are passed in registers, and the rest are passed on the stack.
Kernel-Mode System Service Dispatching
As Figure 3-14 illustrates, the kernel uses this argument to locate the system service information
in the system service dispatch table. This table is similar to the interrupt dispatch table
described earlier in the chapter except that each entry contains a pointer to a system service
rather than to an interrupt handling routine.
Note System service numbers can change between service packs—Microsoft occasionally
adds or removes system services, and the system service numbers are generated automatically
as part of a kernel compile.
Figure 3-14 System service exceptions
The system service dispatcher, KiSystemService, copies the caller’s arguments from the thread’s
user-mode stack to its kernel-mode stack (so that the user can’t change the arguments as the
kernel is accessing them), and then executes the system service. If the arguments passed to a
system service point to buffers in user space, these buffers must be probed for accessibility
before kernel-mode code can copy data to or from them.
As you’ll see in Chapter 6, each thread has a pointer to its system service table. Windows has
two built-in system service tables, but up to four are supported. The system service dispatcher
determines which table contains the requested service by interpreting a 2-bit field in the 32-
dispatcher System service 2
• • •
122 Microsoft Windows Internals, Fourth Edition
bit system service number as a table index. The low 12 bits of the system service number serve
as the index into the table specified by the table index. The fields are shown in Figure 3-15.
Figure 3-15 System service number to system service translation
Service Descriptor Tables
A primary default array table, KeServiceDescriptorTable, defines the core executive system services
implemented in Ntosrknl.exe. The other table array, KeServiceDescriptorTableShadow,
includes the Windows USER and GDI services implemented in the kernel-mode part of the
Windows subsystem, Win32k.sys. The first time a Windows thread calls a Windows USER or
GDI service, the address of the thread’s system service table is changed to point to a table that
includes the Windows USER and GDI services. The KeAddSystemServiceTable function allows
Win32k.sys and other device drivers to add system service tables. If you install Internet Information
Services (IIS) on Windows 2000, its support driver (Spud.sys) upon loading defines
an additional service table, leaving only one left for definition by third parties. With the exception
of the Win32k.sys service table, a service table added with KeAddSystemServiceTable is
copied into both the KeServiceDescriptorTable array and the KeServiceDescriptorTableShadow
array. Windows supports the addition of only two system service tables beyond the core and
Index into table System service number
31 13 11 0
IIS Spud Driver
IIS Spud Driver
Chapter 3: System Mechanisms 123
Note Windows Server 2003 service pack 1 and higher does not support adding additional
system service tables beyond that added by Win32k.sys, so adding system service tables is not
a way to extend the functionality of those systems.
The system service dispatch instructions for Windows executive services exist in the system
library Ntdll.dll. Subsystem DLLs call functions in Ntdll to implement their documented
functions. The exception is Windows USER and GDI functions, in which the system service
dispatch instructions are implemented directly in User32.dll and Gdi32.dll—there is no
Ntdll.dll involved. These two cases are shown in Figure 3-16.
Figure 3-16 System service dispatching
As shown in Figure 3-16, the Windows WriteFile function in Kernel32.dll calls the NtWriteFile
function in Ntdll.dll, which in turn executes the appropriate instruction to cause a system service
trap, passing the system service number representing NtWriteFile. The system service dispatcher
(function KiSystemService in Ntoskrnl.exe) then calls the real NtWriteFile to process
Windows kernel APIs
Windows USER and
Used by all
Service entry point
Software interrupt Software interrupt
Call USER or
Return to caller
Do the operation
Return to caller
Return to caller
Return to caller
Do the operation
Return to caller
124 Microsoft Windows Internals, Fourth Edition
the I/O request. For Windows USER and GDI functions, the system service dispatch calls
functions in the loadable kernel-mode part of the Windows subsystem, Win32k.sys.
EXPERIMENT: Viewing System Service Activity
You can monitor system service activity by watching the System Calls/Sec performance
counter in the System object. Run the Performance tool, and in chart view, click the Add
button to add a counter to the chart; select the System object, select the System Calls/
Sec counter, and then click the Add button to add the counter to the chart.
As mentioned in Chapter 2, Windows implements an object model to provide consistent and
secure access to the various internal services implemented in the executive. This section
describes the Windows object manager, the executive component responsible for creating,
deleting, protecting, and tracking objects. The object manager centralizes resource control
operations that otherwise would be scattered throughout the operating system. It was
designed to meet the goals listed on later in the chapter.
EXPERIMENT: Exploring the Object Manager
Throughout this section, you’ll find experiments that show you how to peer into the
object manager database. These experiments use the following tools, which you should
become familiar with if you aren’t already:
■ Winobj (available from www.sysinternals.com) displays the internal object manager’s
namespace. There is also a version of Winobj in the Platform SDK (in \Program
Files\Microsoft Platform SDK\Bin\Winnt\Winobj.exe), but the Winobj
from www.sysinternals.com displays more accurate information about objects (such
as the reference count, the number of open handles, security descriptors, and so
■ Process Explorer and Handle from www.sysinternals.com (introduced in
Chapter 1) displays the open handles for a process.
■ Oh.exe (available in Windows resource kits) displays the open handles for a process,
but it requires a global flag to be set in order to operate.
■ The Openfiles /query command (in Windows XP and Windows Server 2003) displays
the open handles for a process, but it requires a global flag to be set in order
■ The kernel debugger !handle command displays the open handles for a process.
Chapter 3: System Mechanisms 125
The object viewer provides a way to traverse the namespace that the object manager
maintains. (As we’ll explain later, not all objects have names.) Try running the WinObj
object manager utility from www.sysinternals.com and examining the layout, shown here:
As noted previously, both the OH utility and the Openfiles /query command require that
a Windows global flag called maintain objects list be enabled. (See the “Windows Global
Flags” section later in this chapter for more details about global flags.) OH will set the
flag if it is not set. If you type Openfiles /Local, it will tell you whether the flag is
enabled. You can enable it with the Openfiles /Local ON command. In either case, you
must reboot the system for the setting to take effect. Neither Process Explorer nor Handle
from www.sysinternals.com require object tracking to be turned on because they use
a device driver to obtain the information.
The object manager was designed to meet the following goals:
■ Provide a common, uniform mechanism for using system resources
■ Isolate object protection to one location in the operating system so that C2 security compliance
can be achieved
■ Provide a mechanism to charge processes for their use of objects so that limits can be
placed on the usage of system resources
■ Establish an object-naming scheme that can readily incorporate existing objects, such as
the devices, files, and directories of a file system, or other independent collections of
126 Microsoft Windows Internals, Fourth Edition
■ Support the requirements of various operating system environments, such as the ability
of a process to inherit resources from a parent process (needed by Windows and POSIX)
and the ability to create case-sensitive filenames (needed by POSIX)
■ Establish uniform rules for object retention (that is, for keeping an object available until
all processes have finished using it)
Internally, Windows has two kinds of objects: executive objects and kernel objects. Executive
objects are objects implemented by various components of the executive (such as the process
manager, memory manager, I/O subsystem, and so on). Kernel objects are a more primitive
set of objects implemented by the Windows kernel. These objects are not visible to user-mode
code but are created and used only within the executive. Kernel objects provide fundamental
capabilities, such as synchronization, on which executive objects are built. Thus, many executive
objects contain (encapsulate) one or more kernel objects, as shown in Figure 3-17.
Details about the structure of kernel objects and how they are used to implement synchronization
are given later in this chapter. In the remainder of this section, we’ll focus on how the
object manager works and on the structure of executive objects, handles, and handle tables.
Here we’ll just briefly describe how objects are involved in implementing Windows security
access checking; we’ll cover this topic thoroughly in Chapter 8.
Figure 3-17 Executive objects that contain kernel objects
Each Windows environment subsystem projects to its applications a different image of the
operating system. The executive objects and object services are primitives that the environment
subsystems use to construct their own versions of objects and other resources.
Owned by the Executive object
Owned by the
Owned by the
Chapter 3: System Mechanisms 127
Executive objects are typically created either by an environment subsystem on behalf of a user
application or by various components of the operating system as part of their normal operation.
For example, to create a file, a Windows application calls the Windows CreateFile function, implemented
in the Windows subsystem DLL Kernel32.dll. After some validation and initialization,
CreateFile in turn calls the native Windows service NtCreateFile to create an executive file object.
The set of objects an environment subsystem supplies to its applications might be larger or
smaller than the set the executive provides. The Windows subsystem uses executive objects to
export its own set of objects, many of which correspond directly to executive objects. For
example, the Windows mutexes and semaphores are directly based on executive objects
(which are in turn based on corresponding kernel objects). In addition, the Windows subsystem
supplies named pipes and mailslots, resources that are based on executive file objects.
Some subsystems, such as POSIX, don’t support objects as objects at all. The POSIX subsystem
uses executive objects and services as the basis for presenting POSIX-style processes,
pipes, and other resources to its applications.
Table 3-3 lists the primary objects the executive provides and briefly describes what they represent.
You can find further details on executive objects in the chapters that describe the
related executive components (or in the case of executive objects directly exported to Windows,
in the Windows API reference documentation).
Note The executive implements a total of 27 object types in Windows 2000 and 29 on Windows
XP and Windows Server 2003. (These newer Windows versions add the DebugObject and
KeyedEvent objects.) Many of these objects are for use only by the executive component that
defines them and are not directly accessible by Windows APIs. Examples of these objects
include Driver, Device, and EventPair.
Table 3-3 Executive Objects Exposed to the Windows API
Object Type Represents
Symbolic link A mechanism for referring to an object name indirectly.
Process The virtual address space and control information necessary for the execution
of a set of thread objects.
Thread An executable entity within a process.
Job A collection of processes manageable as a single entity through the job.
Section A region of shared memory (known as a file mapping object in Windows).
File An instance of an opened file or an I/O device.
Access token The security profile (security ID, user rights, and so on) of a process or a thread.
Event An object with a persistent state (signaled or not signaled) that can be used for
synchronization or notification.
Semaphore A counter that provides a resource gate by allowing some maximum number of
threads to access the resources protected by the semaphore.
Mutex* A synchronization mechanism used to serialize access to a resource.
Timer A mechanism to notify a thread when a fixed period of time elapses.
128 Microsoft Windows Internals, Fourth Edition
Note Externally in the Windows API, mutants are called mutexes. Internally, the kernel
object that underlies mutexes is called a mutant.
As shown in Figure 3-18, each object has an object header and an object body. The object manager
controls the object headers, and the owning executive components control the object
bodies of the object types they create. In addition, each object header points to the list of processes
that have the object open and to a special object called the type object that contains
information common to each instance of the object.
Figure 3-18 Structure of an object
IoCompletion A method for threads to enqueue and dequeue notifications of the completion
of I/O operations (known as an I/O completion port in the Windows API).
Key A mechanism to refer to data in the registry. Although keys appear in the
object manager namespace, they are managed by the configuration manager,
in a way similar to that in which file objects are managed by file system drivers.
Zero or more key values are associated with a key object; key values contain
data about the key.
WindowStation An object that contains a clipboard, a set of global atoms, and a group of desktop
Desktop An object contained within a window station. A desktop has a logical display
surface and contains windows, menus, and hooks.
Table 3-3 Executive Objects Exposed to the Windows API
Object Type Represents
Open handle count
Open handles list
Default quota charges
Generic access rights mapping
Open, close, delete,
Chapter 3: System Mechanisms 129
Object Headers and Bodies
The object manager uses the data stored in an object’s header to manage objects without
regard to their type. Table 3-4 briefly describes the object header attributes.
In addition to an object header, each object has an object body whose format and contents are
unique to its object type; all objects of the same type share the same object body format. By
creating an object type and supplying services for it, an executive component can control the
manipulation of data in all object bodies of that type.
The object manager provides a small set of generic services that operate on the attributes
stored in an object’s header and can be used on objects of any type (although some generic
services don’t make sense for certain objects). These generic services, some of which the Windows
subsystem makes available to Windows applications, are listed in Table 3-5.
Although these generic object services are supported for all object types, each object has its
own create, open, and query services. For example, the I/O system implements a create file
service for its file objects, and the process manager implements a create process service for its
process objects. Although a single create object service could have been implemented, such a
routine would have been quite complicated, because the set of parameters required to initialize
a file object, for example, differs markedly from that required to initialize a process object.
Also, the object manager would have incurred additional processing overhead each time a
thread called an object service to determine the type of object the handle referred to and to
call the appropriate version of the service. For these reasons and others, the create, open, and
query services are implemented separately for each object type.
Table 3-4 Standard Object Header Attributes
Object name Makes an object visible to other processes for sharing
Object directory Provides a hierarchical structure in which to store object names
Security descriptor Determines who can use the object and what they can do with it (Note: it
might be null for objects without a name.)
Quota charges Lists the resource charges levied against a process when it opens a handle
to the object
Open handle count Counts the number of times a handle has been opened to the object
Open handles list Points to the list of processes that have opened handles to the object (not
present for all objects)
Object type Points to a type object that contains attributes common to objects of this
Reference count Counts the number of times a kernel-mode component has referenced
the address of the object
130 Microsoft Windows Internals, Fourth Edition
Object headers contain data that is common to all objects but that can take on different values
for each instance of an object. For example, each object has a unique name and can have a
unique security descriptor. However, objects also contain some data that remains constant for
all objects of a particular type. For example, you can select from a set of access rights specific
to a type of object when you open a handle to objects of that type. The executive supplies terminate
and suspend access (among others) for thread objects and read, write, append, and
delete access (among others) for file objects. Another example of an object-type-specific
attribute is synchronization, which is described shortly.
To conserve memory, the object manager stores these static, object-type-specific attributes
once when creating a new object type. It uses an object of its own, a type object, to record this
data. As Figure 3-19 illustrates, if the object-tracking debug flag (described in the “Windows
Global Flags” section later in this chapter) is set, a type object also links together all objects of
the same type (in this case the Process type), allowing the object manager to find and enumerate
them, if necessary.
Figure 3-19 Process objects and the process type object
Table 3-5 Generic Object Services
Close Closes a handle to an object
Duplicate Shares an object by duplicating a handle and giving it to another
Query object Gets information about an object’s standard attributes
Query security Gets an object’s security descriptor
Set security Changes the protection on an object
Wait for a single object Synchronizes a thread’s execution with one object
Wait for multiple objects Synchronizes a thread’s execution with multiple objects
Chapter 3: System Mechanisms 131
EXPERIMENT: Viewing Object Headers and Type Objects
You can see the list of type objects declared to the object manager with the Winobj tool
from www.sysinternals.com. After running Winobj, open the \ObjectTypes directory, as
You can look at the process object type data structure in the kernel debugger by first
identifying a process object with the !process command:
kd> !process 0 0
**** NT ACTIVE PROCESS DUMP ****
PROCESS 8a4ce668 SessionId: none Cid: 0004 Peb: 00000000 ParentCid: 0000
DirBase: 00039000 ObjectTable: e1001c88 HandleCount: 474.
Then execute the !object command with the process object address as the argument:
kd> !object 8a4ce668
Object: 8a4ce668 Type: (8a4ceca0) Process
HandleCount: 2 PointerCount: 89
Notice that the object header starts 0x18 (24 decimal) bytes prior to the start of the
object body. You can view the object header with this command:
kd> dt _object_header 8a4ce650
+0x000 PointerCount : 79
+0x004 HandleCount : 2
+0x004 NextToFree : 0x00000002
+0x008 Type : 0x8a4ceca0
+0x00c NameInfoOffset : 0 ’’
+0x00d HandleInfoOffset : 0 ’’
+0x00e QuotaInfoOffset : 0 ’’
132 Microsoft Windows Internals, Fourth Edition
+0x00f Flags : 0x22 ’"‘
+0x010 ObjectCreateInfo : 0x80545620
+0x010 QuotaBlockCharged : 0x80545620
+0x014 SecurityDescriptor : 0xe10001dc
+0x018 Body : _QUAD
Now look at the object type data structure by obtaining its address from the Type field
of the object header data structure:
kd> dt _object_type 8a4ceca0
+0x000 Mutex : _ERESOURCE
+0x038 TypeList : _LIST_ENTRY [ 0x8a4cecd8 - 0x8a4cecd8 ]
+0x040 Name : _UNICODE_STRING “Process"
+0x048 DefaultObject : (null)
+0x04c Index : 5
+0x050 TotalNumberOfObjects : 0x30
+0x054 TotalNumberOfHandles : 0x1b4
+0x058 HighWaterNumberOfObjects : 0x3f
+0x05c HighWaterNumberOfHandles : 0x1b8
+0x060 TypeInfo : _OBJECT_TYPE_INITIALIZER
+0x0ac Key : 0x636f7250
+0x0b0 ObjectLocks :  _ERESOURCE
The output shows that the object type structure includes the name of the object type,
tracks the total number of active objects of that type, and tracks the peak number of
handles and objects of that type. The TypeInfo field stores the pointer to the data structure
that stores attributes common to all objects of the object type as well as pointers to
the object type’s methods:
kd> dt _object_type_initializer 8a4ceca0+60
+0x000 Length : 0x4c
+0x002 UseDefaultObject : 0 ’’
+0x003 CaseInsensitive : 0 ’’
+0x004 InvalidAttributes : 0xb0
+0x008 GenericMapping : _GENERIC_MAPPING
+0x018 ValidAccessMask : 0x1f0fff
+0x01c SecurityRequired : 0x1 ’’
+0x01d MaintainHandleCount : 0 ’’
+0x01e MaintainTypeList : 0 ’’
+0x020 PoolType : 0 ( NonPagedPool )
+0x024 DefaultPagedPoolCharge : 0x1000
+0x028 DefaultNonPagedPoolCharge : 0x288
+0x02c DumpProcedure : (null)
+0x030 OpenProcedure : (null)
+0x034 CloseProcedure : (null)
+0x038 DeleteProcedure : 0x805abe6e nt!PspProcessDelete+0
+0x03c ParseProcedure : (null)
+0x040 SecurityProcedure : 0x805cf682 nt!SeDefaultObjectMethod+0
+0x044 QueryNameProcedure : (null)
+0x048 OkayToCloseProcedure : (null)
Chapter 3: System Mechanisms 133
Type objects can’t be manipulated from user mode because the object manager supplies no
services for them. However, some of the attributes they define are visible through certain
native services and through Windows API routines. The attributes stored in the type objects
are described in Table 3-6.
Synchronization, one of the attributes visible to Windows applications, refers to a thread’s
ability to synchronize its execution by waiting for an object to change from one state to
another. A thread can synchronize with executive job, process, thread, file, event, semaphore,
mutex, and timer objects. Other executive objects don’t support synchronization. An object’s
ability to support synchronization is based on whether the object contains an embedded dispatcher
object, a kernel object that is covered in the section “Low-IRQL Synchronization” later
in this chapter.
The last attribute in Table 3-6, methods, comprises a set of internal routines that are similar to
C++ constructors and destructors—that is, routines that are automatically called when an
object is created or destroyed. The object manager extends this idea by calling an object
method in other situations as well, such as when someone opens or closes a handle to an
object or when someone attempts to change the protection on an object. Some object types
specify methods, whereas others don’t, depending on how the object type is to be used.
When an executive component creates a new object type, it can register one or more methods
with the object manager. Thereafter, the object manager calls the methods at well-defined
points in the lifetime of objects of that type, usually when an object is created, deleted, or modified
in some way. The methods that the object manager supports are listed in Table 3-7.
Table 3-6 Type Object Attributes
Type name The name for objects of this type (“process,” “event,” “port,”
and so on)
Pool type Indicates whether objects of this type should be allocated from
paged or nonpaged memory
Default quota charges Default paged and nonpaged pool values to charge to process
Access types The types of access a thread can request when opening a handle
to an object of this type (“read,” “write,” “terminate,” “suspend,”
and so on)
Generic access rights mapping A mapping between the four generic access rights (read, write,
execute, and all) to the type-specific access rights
Synchronization Indicates whether a thread can wait for objects of this type
Methods One or more routines that the object manager calls automatically
at certain points in an object’s lifetime
134 Microsoft Windows Internals, Fourth Edition
The object manager calls the open method whenever it creates a handle to an object, which it
does when an object is created or opened. However, only one object type, the Windowstation,
defines an open method. The Windowstation object type requires an open method so that
Win32k.sys can share a piece of memory with the process that serves as a desktop-related
An example of the use of a close method occurs in the I/O system. The I/O manager registers
a close method for the file object type, and the object manager calls the close method each
time it closes a file object handle. This close method checks whether the process that is closing
the file handle owns any outstanding locks on the file and, if so, removes them. Checking
for file locks isn’t something the object manager itself could or should do.
The object manager calls a delete method, if one is registered, before it deletes a temporary
object from memory. The memory manager, for example, registers a delete method for the section
object type that frees the physical pages being used by the section. It also verifies that any
internal data structures the memory manager has allocated for a section are deleted before the
section object is deleted. Once again, the object manager can’t do this work because it knows
nothing about the internal workings of the memory manager. Delete methods for other types
of objects perform similar functions.
The parse method (and similarly, the query name method) allows the object manager to relinquish
control of finding an object to a secondary object manager if it finds an object that exists
outside the object manager namespace. When the object manager looks up an object name, it
suspends its search when it encounters an object in the path that has an associated parse
method. The object manager calls the parse method, passing to it the remainder of the object
name it is looking for. There are two namespaces in Windows in addition to the object manager’s:
the registry namespace, which the configuration manager implements, and the file system
namespace, which the I/O manager implements with the aid of file system drivers. (See
Chapter 5 for more information on the configuration manager and Chapter 9 for more about
the I/O manager and file system drivers.)
Table 3-7 Object Methods
Method When Method Is Called
Open When an object handle is opened
Close When an object handle is closed
Delete Before the object manager deletes an object
Query name When a thread requests the name of an object, such as a file, that exists in a
secondary object namespace
Parse When the object manager is searching for an object name that exists in a
secondary object namespace
Security When a process reads or changes the protection of an object, such as a file, that
exists in a secondary object namespace
Chapter 3: System Mechanisms 135
For example, when a process opens a handle to the object named \Device\Floppy0\
docs\resume.doc, the object manager traverses its name tree until it reaches the device object
named Floppy0. It sees that a parse method is associated with this object, and it calls the
method, passing to it the rest of the object name it was searching for—in this case, the string
\docs\resume.doc. The parse method for device objects is an I/O routine because the I/O
manager defines the device object type and registers a parse method for it. The I/O manager’s
parse routine takes the name string and passes it to the appropriate file system, which finds
the file on the disk and opens it.
The security method, which the I/O system also uses, is similar to the parse method. It is
called whenever a thread tries to query or change the security information protecting a file.
This information is different for files than for other objects because security information is
stored in the file itself rather than in memory. The I/O system, therefore, must be called to
find the security information and read or change it.
Object Handles and the Process Handle Table
When a process creates or opens an object by name, it receives a handle that represents its
access to the object. Referring to an object by its handle is faster than using its name because
the object manager can skip the name lookup and find the object directly. Processes can also
acquire handles to objects by inheriting handles at process creation time (if the creator specifies
the inherit handle flag on the CreateProcess call and the handle was marked as inheritable,
either at the time it was created or afterward by using the Windows SetHandleInformation
function) or by receiving a duplicated handle from another process. (See the Windows
All user-mode processes must own a handle to an object before their threads can use the
object. Using handles to manipulate system resources isn’t a new idea. C and Pascal (and
other language) run-time libraries, for example, return handles to opened files. Handles serve
as indirect pointers to system resources; this indirection keeps application programs from fiddling
directly with system data structures.
Note Executive components and device drivers can access objects directly because they are
running in kernel mode and therefore have access to the object structures in system memory.
However, they must declare their usage of the object by incrementing the reference count so
that the object won’t be deallocated while it’s still being used. (See the section “Object Retention”
later in this chapter for more details.)
Object handles provide additional benefits. First, except for what they refer to, there is no difference
between a file handle, an event handle, and a process handle. This similarity provides a consistent
interface to reference objects, regardless of their type. Second, the object manager has the
exclusive right to create handles and to locate an object that a handle refers to. This means that
the object manager can scrutinize every user-mode action that affects an object to see whether
the security profile of the caller allows the operation requested on the object in question.
136 Microsoft Windows Internals, Fourth Edition
EXPERIMENT: Viewing Open Handles
Run Process Explorer, and make sure the lower pane is enabled and configured to show
open handles. (Click on View, Lower Pane View, and then Handles). Then open a command
prompt and view the handle table for the new Cmd.exe process. You should see
an open file handle to the current directory. For example, assuming the current directory
is C:\, Process Explorer shows the following:
If you then change the current directory with the CD command, you will see in Process
Explorer that the handle to the previous current directory is closed and a new handle is
opened to the new current directory. The previous handle is highlighted briefly in red,
and the new handle is highlighted in green. The duration of the highlight can be
adjusted by clicking Options and then Difference Highlight Duration.
Process Explorer’s differences highlighting feature makes it easy to see changes in the
handle table. For example, if a process is leaking handles, viewing the handle table with
Process Explorer can quickly show what handle or handles are being opened but not
closed. This information can assist the programmer to find the handle leak.
Chapter 3: System Mechanisms 137
You can also display the open handle table by using the command line Handle tool from
www.sysinternals.com. For example, note the following partial output of Handle examining
the handle table for a Cmd.exe process before and after changing the directory:
C:\>handle -p cmd.exe
Copyright (C) 1997-2004 Mark Russinovich
Sysinternals - www.sysinternals.com
cmd.exe pid: 3184 BIGDAVID\dsolomon
b0: File C:\
C:\WINDOWS>handle -p cmd.exe
cmd.exe pid: 3184 BIGDAVID\dsolomon
b4: File C:\WINDOWS
An object handle is an index into a process-specific handle table, pointed to by the executive
process (EPROCESS) block (described in Chapter 6). The first handle index is 4, the second
8, and so on. A process’s handle table contains pointers to all the objects that the process has
opened a handle to. Handle tables are implemented as a three-level scheme, similar to the way
that the x86 memory management unit implements virtual-to-physical address translation,
giving a maximum of more than 16,000,000 handles per process. (See Chapter 7 for details
about memory management in x86 systems.)
In Windows 2000, when a process is created, the object manager allocates the top level of the
handle table, which contains pointers to the middle-level tables; the middle level, which contains
the first array of pointers to subhandle tables; and the lowest level, which contains the
first subhandle table. Figure 3-20 illustrates the Windows 2000 handle table architecture. In
Windows 2000, the object manager treats the low 24 bits of an object handle’s value as three
8-bit fields that index into each of the three levels in the handle table. In Windows XP and
Windows Server 2003, only the lowest level handle table is allocated on process creation—the
other levels are created as needed. In Windows 2000, the subhandle table consists of 255
usable entries. In Windows XP and Windows Server 2003, the subhandle table consists of as
many entries as will fit in a page minus one entry that is used for handle auditing. For example,
for x86 systems a page is 4096 bytes, divided by the size of a handle table entry (8 bytes),
which is 512, minus 1, which is a total of 511 entries in the lowest level handle table. In Windows
XP and Windows Server 2003, the mid-level handle table contains a full page of pointers
to subhandle tables, so the number of subhandle tables depends on the size of the page and
the size of a pointer for the platform.
138 Microsoft Windows Internals, Fourth Edition
Figure 3-20 Windows 2000 process handle table architecture
EXPERIMENT: Creating the Maximum Number of Handles
The test program Testlimit from www.sysinternals.com/windowsinternals.shtml has an
option to open handles to an object until it cannot open any more handles. You can use
this to see how many handles can be created in a single process on your system. Because
handle tables are allocated from paged pool, you might run out of paged pool before you
hit the maximum number of handles that can be created in a single process. To see how
many handles you can create on your system, follow these steps:
1. Download the Testlimit zip file from the link just mentioned, and unzip it into a
2. Run Process Explorer, and click View and then System Information. Notice the
current and maximum size of paged pool. (To display the maximum pool size values,
Process Explorer must be configured properly to access the symbols for the
kernel image, Ntoskrnl.exe.) Leave this system information display running so
that you can see pool utilization when you run the Testlimit program.
3. Open a command prompt.
4. Run the Testlimit program with the “-h” switch (do this by typing testlimit –h).
When Testlimit fails to open a new handle, it will display the total number of handles
it was able to create. If the number is less than approximately 16 million, you
are probably running out of paged pool before hitting the theoretical per-process
5. Close the command-prompt window; doing this will kill the Testlimit process,
thus closing all the open handles.
Chapter 3: System Mechanisms 139
As shown in Figure 3-21, on x86 systems, each handle entry consists of a structure with two
32-bit members: a pointer to the object (with flags), and the granted access mask. On 64-bit
systems, a handle table entry is 12 bytes long: a 64-bit pointer to the object header and a 32-
bit access mask. (Access masks are described in Chapter 8.)
On Windows 2000, the first 32-bit member contains both a pointer to the object header and
four flags. Because object headers are always 8-byte aligned, the low-order 3 bits of this field
are free for use as flags. An entry’s high bit is used as a lock. When the object manager translates
a handle to an object pointer, it locks the handle entry while the translation is in
progress. Because all objects are located in the system address space, the high bit of the object
pointer is set. (The addresses are guaranteed to be higher than 0x80000000 even on systems
with the /3GB boot switch.) Thus, the object manager can keep the high bit clear when a handle
table entry is unlocked and, in the process of locking the entry, set the bit and obtain the
object’s correct pointer value. The object manager needs to lock a process’s entire handle
table, using a handle table lock associated with each process, only when the process creates a
new handle or closes an existing handle. In Windows XP and Windows Server 2003, the lock
bit is the low-order bit of the object pointer. The flag that was stored in this low-order bit in
Windows 2000 is now stored in an unused bit in the access mask.
Figure 3-21 Structure of a handle table entry
The first flag indicates whether the caller is allowed to close this handle. The second flag is the
inheritance designation—that is, it indicates whether processes created by this process will get
a copy of this handle in their handle tables. As already noted, handle inheritance can be specified
on handle creation or later with the SetHandleInformation function. (This flag can also be
specified with the Windows SetHandleInformation function.) The third flag indicates whether
closing the object should generate an audit message. (This flag isn’t exposed to Windows—the
object manager uses it internally.)
System components and device drivers often need to open handles to objects that user-mode
applications shouldn’t have access to. This is done by creating handles in the kernel handle
table (referenced internally with the name ObpKernelHandleTable). The handles in this table
are accessible only from kernel mode and in any process context. This means that a kernelmode
function can reference the handle in any process context with no performance impact.
The object manager recognizes references to handles from the kernel handle table when the
Audit on close
Protect from close
Pointer to object header A I P
140 Microsoft Windows Internals, Fourth Edition
high bit of the handle is set—that is, when references to kernel-handle-table handles have values
greater than 0x80000000. On Windows 2000, the kernel-handle table is an independent
handle table, but on Windows XP and Windows Server 2003 the kernel-handle table also
serves as the handle table for the System process.
EXPERIMENT: Viewing the Handle Table with the Kernel
The !handle command in the kernel debugger takes three arguments:
!handle <handle index> <flags> <processid>
The handle index identifies the handle entry in the handle table. (Zero means display all
handles.) The first handle is index 4, the second 8, and so on. For example, typing !handle
4 will show the first handle for the current process.
The flags you can specify are a bitmask, where bit 0 means display only the information
in the handle entry, bit 1 means display free handles (not just used handles), and bit 2
means display information about the object that the handle refers to. The following
command displays full details about the handle table for process ID 0x408:
kd> !handle 0 7 408
processor number 0
Searching for Process with Cid == 408
PROCESS 865f0790 SessionId: 0 Cid: 0408 Peb: 7ffdf000 ParentCid: 01dc
DirBase: 04fd3000 ObjectTable: 856ca888 TableSize: 21.
Handle Table at e2125000 with 21 Entries in use
0000: free handle
0004: Object: e20da2e0 GrantedAccess: 000f001f
Object: e20da2e0 Type: (81491b80) Section
HandleCount: 1 PointerCount: 1
0008: Object: 80b13330 GrantedAccess: 00100003
Object: 80b13330 Type: (81495100) Event
HandleCount: 1 PointerCount: 1
When you open a file, you must specify whether you intend to read or to write. If you try to
write to a file that is opened for read access, you get an error. Likewise, in the executive, when
a process creates an object or opens a handle to an existing object, the process must specify a
set of desired access rights—that is, what it wants to do with the object. It can request either a set
of standard access rights (such as read, write, and execute) that apply to all object types or
Chapter 3: System Mechanisms 141
specific access rights that vary depending on the object type. For example, the process can
request delete access or append access to a file object. Similarly, it might require the ability to
suspend or terminate a thread object.
When a process opens a handle to an object, the object manager calls the security reference
monitor, the kernel-mode portion of the security system, sending it the process’s set of desired
access rights. The security reference monitor checks whether the object’s security descriptor
permits the type of access the process is requesting. If it does, the reference monitor returns a
set of granted access rights that the process is allowed, and the object manager stores them in
the object handle it creates. How the security system determines who gets access to which
objects is explored in Chapter 8.
Thereafter, whenever the process’s threads use the handle, the object manager can quickly
check whether the set of granted access rights stored in the handle corresponds to the usage
implied by the object service the threads have called. For example, if the caller asked for read
access to a section object but then calls a service to write to it, the service fails.
There are two types of objects: temporary and permanent. Most objects are temporary—that is,
they remain while they are in use and are freed when they are no longer needed. Permanent
objects remain until they are explicitly freed. Because most objects are temporary, the rest of this
section describes how the object manager implements object retention—that is, retaining temporary
objects only as long as they are in use and then deleting them. Because all user-mode processes
that access an object must first open a handle to it, the object manager can easily track
how many of these processes, and even which ones, are using an object. Tracking these handles
represents one part in implementing retention. The object manager implements object retention
in two phases. The first phase is called name retention, and it is controlled by the number of open
handles to an object that exist. Every time a process opens a handle to an object, the object manager
increments the open handle counter in the object’s header. As processes finish using the
object and close their handles to it, the object manager decrements the open handle counter.
When the counter drops to 0, the object manager deletes the object’s name from its global
namespace. This deletion prevents new processes from opening a handle to the object.
The second phase of object retention is to stop retaining the objects themselves (that is, to
delete them) when they are no longer in use. Because operating system code usually accesses
objects by using pointers instead of handles, the object manager must also record how many
object pointers it has dispensed to operating system processes. It increments a reference count
for an object each time it gives out a pointer to the object; when kernel-mode components finish
using the pointer, they call the object manager to decrement the object’s reference count.
The system also increments the reference count when it increments the handle count, and
likewise decrements the reference count when the handle count decrements, because a handle
is also a reference to the object that must be tracked. (For further details on object retention,
see the DDK documentation on the functions ObReferenceObjectByPointer and
142 Microsoft Windows Internals, Fourth Edition
Figure 3-22 illustrates two event objects that are in use. Process A has the first event open. Process
B has both events open. In addition, the first event is being referenced by some kernelmode
structure; thus, the reference count is 3. So even if processes A and B closed their handles
to the first event object, it would continue to exist because its reference count is 1. However,
when process B closes its handle to the second event object, the object would be
Figure 3-22 Handles and reference counts
So even after an object’s open handle counter reaches 0, the object’s reference count might
remain positive, indicating that the operating system is still using the object. Ultimately, when
the reference count drops to 0, the object manager deletes the object from memory.
Because of the way object retention works, an application can ensure that an object and its
name remain in memory simply by keeping a handle open to the object. Programmers who
write applications that contain two or more cooperating processes need not be concerned
that one process might delete an object before the other process has finished using it. In addition,
closing an application’s object handles won’t cause an object to be deleted if the operating
system is still using it. For example, one process might create a second process to execute
a program in the background; it then immediately closes its handle to the process. Because
the operating system needs the second process to run the program, it maintains a reference to
its process object. Only when the background program finishes executing does the object
manager decrement the second process’s reference count and then delete it.
Process A System space
Chapter 3: System Mechanisms 143
Resource accounting, like object retention, is closely related to the use of object handles. A
positive open handle count indicates that some process is using that resource. It also indicates
that some process is being charged for the memory the object occupies. When an object’s handle
count and reference count drop to 0, the process that was using the object should no
longer be charged for it.
Many operating systems use a quota system to limit processes’ access to system resources.
However, the types of quotas imposed on processes are sometimes diverse and complicated,
and the code to track the quotas is spread throughout the operating system. For example, in
some operating systems, an I/O component might record and limit the number of files a process
can open, whereas a memory component might impose a limit on the amount of memory
a process’s threads can allocate. A process component might limit users to some maximum
number of new processes they can create or a maximum number of threads within a process.
Each of these limits is tracked and enforced in different parts of the operating system.
In contrast, the Windows object manager provides a central facility for resource accounting.
Each object header contains an attribute called quota charges that records how much the
object manager subtracts from a process’s allotted paged and/or nonpaged pool quota when
a thread in the process opens a handle to the object.
Each process on Windows points to a quota structure that records the limits and current values
for nonpaged pool, paged pool, and page file usage. (Type dt nt!_EPROCESS_QUOTA_ENTRY
in the kernel debugger to see the format of this structure.) These quotas default to 0 (no limit)
but can be specified by modifying registry values. (See NonPagedPoolQuota, PagedPoolQuota, and
PagingFileQuota under HKLM\System\CurrentControlSet\Session Manager\Memory Management.)
Note that all the processes in an interactive session share the same quota block (and
there’s no documented way to create processes with their own quota blocks).
An important consideration in creating a multitude of objects is the need to devise a successful
system for keeping track of them. The object manager requires the following information
to help you do so:
■ A way to distinguish one object from another
■ A method for finding and retrieving a particular object
The first requirement is served by allowing names to be assigned to objects. This is an extension
of what most operating systems provide—the ability to name selected resources, files,
pipes, or a block of shared memory, for example. The executive, in contrast, allows any
resource represented by an object to have a name. The second requirement, finding and
retrieving an object, is also satisfied by object names. If the object manager stores objects by
name, it can find an object by looking up its name.
144 Microsoft Windows Internals, Fourth Edition
Object names also satisfy a third requirement, which is to allow processes to share objects.
The executive’s object namespace is a global one, visible to all processes in the system. One
process can create an object and place its name in the global namespace, and a second process
can open a handle to the object by specifying the object’s name. If an object isn’t meant to be
shared in this way, its creator doesn’t need to give it a name.
To increase efficiency, the object manager doesn’t look up an object’s name each time someone
uses the object. Instead, it looks up a name under only two circumstances. The first is
when a process creates a named object: the object manager looks up the name to verify that it
doesn’t already exist before storing the new name in the global namespace. The second is
when a process opens a handle to a named object: the object manager looks up the name,
finds the object, and then returns an object handle to the caller; thereafter, the caller uses the
handle to refer to the object. When looking up a name, the object manager allows the caller to
select either a case-sensitive or a case-insensitive search, a feature that supports POSIX and
other environments that use case-sensitive filenames.
Where the names of objects are stored depends on the object type. Table 3-8 lists the standard
object directories found on all Windows systems and what types of objects have their names
stored there. Of the directories listed, only \BaseNamedObjects and \GLOBAL?? (\?? on Windows
2000) are visible to user programs (see the Session Namespace section later in this
chapter for more information).
Because the base kernel objects such as mutexes, events, semaphores, waitable timers, and
sections have their names stored in a single object directory, no two of these objects can have
the same name, even if they are of a different type. This restriction emphasizes the need to
choose names carefully so that they don’t collide with other names (for example, prefix
names with your company and product name).
Table 3-8 Standard Object Directories
Directory Types of Object Names Stored
\GLOBAL?? (\?? in Windows
MS-DOS device names (\DosDevices is a symbolic link to this
\BaseNamedObjects Mutexes, events, semaphores, waitable timers, and section
\Callback Callback objects
\Device Device objects
\Driver Driver objects
\FileSystem File system driver objects and file system recognizer device
\KnownDlls Section names and path for known DLLs (DLLs mapped by the
system at startup time)
\Nls Section names for mapped national language support tables
\ObjectTypes Names of types of objects
\RPC Control Port objects used by remote procedure calls (RPCs)
Chapter 3: System Mechanisms 145
Object names are global to a single computer (or to all processors on a multiprocessor computer),
but they’re not visible across a network. However, the object manager’s parse method
makes it possible to access named objects that exist on other computers. For example, the
I/O manager, which supplies file object services, extends the functions of the object manager
to remote files. When asked to open a remote file object, the object manager calls a
parse method, which allows the I/O manager to intercept the request and deliver it to a network
redirector, a driver that accesses files across the network. Server code on the remote
Windows system calls the object manager and the I/O manager on that system to find the file
object and return the information back across the network.
EXPERIMENT: Looking at the Base Named Objects
You can see the list of base objects that have names with the Winobj tool from www.sysinternals.
com. Run Winobj.exe and click on \BaseNamedObjects, as shown here:
The named objects are shown on the right. The icons indicate the object type.
■ Mutexes are indicated with a stop sign.
■ Sections (Windows file mapping objects) are shown as memory chips.
■ Events are shown as exclamation points.
■ Semaphores are indicated with an icon that resembles a traffic signal.
■ Symbolic links have icons that are curved arrows.
\Security Names of objects specific to the security subsystem
\Windows Windows subsystem ports and window stations
Table 3-8 Standard Object Directories
Directory Types of Object Names Stored
146 Microsoft Windows Internals, Fourth Edition
Object directories The object directory object is the object manager’s means for supporting
this hierarchical naming structure. This object is analogous to a file system directory and
contains the names of other objects, possibly even other object directories. The object directory
object maintains enough information to translate these object names into pointers to the
objects themselves. The object manager uses the pointers to construct the object handles that
it returns to user-mode callers. Both kernel-mode code (including executive components and
device drivers) and user-mode code (such as subsystems) can create object directories in
which to store objects. For example, the I/O manager creates an object directory named
\Device, which contains the names of objects representing I/O devices.
Symbolic links In certain file systems (on NTFS and some UNIX systems, for example), a
symbolic link lets a user create a filename or a directory name that, when used, is translated by
the operating system into a different file or directory name. Using a symbolic link is a simple
method for allowing users to indirectly share a file or the contents of a directory, creating a
cross-link between different directories in the ordinarily hierarchical directory structure.
The object manager implements an object called a symbolic link object, which performs a similar
function for object names in its object namespace. A symbolic link can occur anywhere
within an object name string. When a caller refers to a symbolic link object’s name, the object
manager traverses its object namespace until it reaches the symbolic link object. It looks
inside the symbolic link and finds a string that it substitutes for the symbolic link name. It
then restarts its name lookup.
One place in which the executive uses symbolic link objects is in translating MS-DOS-style
device names into Windows internal device names. In Windows, a user refers to floppy and
hard disk drives using the names A:, B:, C:, and so on and serial ports as COM1, COM2, and
so on. The Windows subsystem makes these symbolic link objects protected, global data by
placing them in the object manager namespace under the \?? object directory on Windows
2000 and the \Global?? directory on Windows XP and Windows Server 2003.
Windows NT was originally written with the assumption that only one user would log on to
the system interactively and that the system would run only one instance of any interactive
application. The addition of Windows Terminal Services in Windows 2000 Server and fast
user switching in Windows XP changed these assumptions, thus requiring changes to the
object manager namespace model to support multiple users. (For a basic description of terminal
services and sessions, see Chapter 1.)
Chapter 3: System Mechanisms 147
A user logged on to the console session has access to the global namespace, a namespace that
serves as the first instance of the namespace. Additional sessions are given a session-private
view of the namespace known as a local namespace. The parts of the namespace that are localized
for each session include \DosDevices, \Windows, and \BaseNamedObjects. Making separate
copies of the same parts of the namespace is known as instancing the namespace.
Instancing \DosDevices makes it possible for each user to have different network drive letters
and Windows objects such as serial ports. On Windows 2000, the global \DosDevices directory
is named \?? and is the directory to which the \DosDevices symbolic link points, and
local \DosDevices directories are identified by the session id for the terminal server session.
On Windows XP and later, the global \DosDevices directory is named \Global?? and is the
directory to which \DosDevices points, and local \DosDevices directories are identified by
the logon session ID.
The \Windows directory is where Win32k.sys creates the interactive window station,
\WinSta0. A Terminal Services environment can support multiple interactive users, but each
user needs an individual version of WinSta0 to preserve the illusion that he or she is accessing
the predefined interactive window station in Windows. Finally, applications and the system
create shared objects in \BaseNamedObjects, including events, mutexes, and memory sections.
If two users are running an application that creates a named object, each user session
must have a private version of the object so that the two instances of the application don’t
interfere with one another by accessing the same object.
The object manager implements a local namespace by creating the private versions of the
three directories mentioned under a directory associated with the user’s session under \Sessions\
X (where X is the session identifier). When a Windows application in remote session
two creates a named event, for example, the object manager transparently redirects the
object’s name from \BaseNamedObjects to \Sessions\2\BaseNamedObjects.
All object manager functions related to namespace management are aware of the instanced
directories and participate in providing the illusion that nonconsole sessions use the same
namespace as the console session. Windows subsystem DLLs prefix names passed by Windows
applications that reference objects in \DosDevices with \?? (for example, C:\Windows
becomes \??\C:\Windows). When the object manager sees the special \?? prefix, the steps it
takes depends on the version of Windows, but it always relies on a field named DeviceMap in
the executive process object (EPROCESS, which is described further in Chapter 6) that points
to a data structure shared by other processes in the same session. The DosDevicesDirectory
field of the DeviceMap structure points at the object manager directory that represents the
process’s local \DosDevices. The target directory varies depending on the system:
■ If the system is Windows 2000 and Terminal Services are not installed, the DosDevices-
Directory field of the DeviceMap structure of the process points at the \?? directory
because there are no local namespaces.
148 Microsoft Windows Internals, Fourth Edition
■ If the system is Windows 2000 and Terminal Services are installed, when a new session
becomes active the system copies all the objects from the global \?? directory into the
session’s local \Devices directory and the DosDevicesDirectory field of the DeviceMap
structure points at the local directory.
■ On Windows XP and Windows Server 2003, the system does not make copies of global
objects in the local DosDevices directories. When the object manager sees a reference to
\??, it locates the process’s local \DosDevices by using the DosDevicesDirectory field of
the DeviceMap. If the object manager doesn’t find the object in that directory, it checks
the DeviceMap field of the directory object, and if it’s valid it looks for the object in the
directory pointed to by the GlobalDosDevicesDirectory field of the DeviceMap structure,
which is always \Global??.
Under certain circumstances, applications that are Terminal Services–aware need to access
objects in the console session even if the application is running in a remote session. The application
might want to do this to synchronize with instances of itself running in other remote
sessions or with the console session. For these cases, the object manager provides the special
override “\Global” that an application can prefix to any object name to access the global
namespace. For example, an application in session two opening an object named \Global\
ApplicationInitialized is directed to \BasedNamedObjects\ApplicationInitialized instead
On Windows XP and Windows Server 2003, an application that wants to access an object in
the global \DosDevices directory does not need to use the \Global prefix as long as the object
doesn’t exist in its local \DosDevices directory. This is because the object manager will automatically
look in the global directory for the object if it doesn’t find it in the local directory.
However, an application running on Windows 2000 with Terminal Services must always specify
the \Global prefix to access objects in the global \DosDevices directory.
EXPERIMENT: Viewing Namespace Instancing
You can see the object manager instance of the namespace by creating a session other
than the console session and then viewing the handle table for a process in that session.
On Windows XP Home Edition or on a Windows XP Professional system that is not a
member of a domain, disconnect the console session (by clicking Start, clicking Log Off,
and choosing Disconnect and Switch User, or by pressing the Windows key + L) and
logging in to a new account. If you have a Windows 2000 Server, Advanced Server, or
Datacenter Server system, run the Terminal Services client, connect to the server, and
Chapter 3: System Mechanisms 149
Once you are logged in to the new session, run Winobj.exe from www.sysinternals.com
and click on the \Sessions directory. You’ll see a subdirectory with a numeric name for
each active remote session. If you open one of these directories, you’ll see subdirectories
named \DosDevices, \Windows, and \BaseNamedObjects, which are the local
namespace subdirectories of the session. The following screen shot shows a local
Next run Process Explorer and select a process in the new session (such as
Explorer.exe), and then view the handle table (by clicking View, Lower Pane View, and
then Handles). You should see a handle to \Windows\Windowstations\WinSta0
underneath \Sessions\n, where n is the session id. Objects with global names will
appear under \Sessions\n\BaseNamedObjects.
The concept of mutual exclusion is a crucial one in operating systems development. It refers to
the guarantee that one, and only one, thread can access a particular resource at a time. Mutual
exclusion is necessary when a resource doesn’t lend itself to shared access or when sharing
would result in an unpredictable outcome. For example, if two threads copy a file to a printer
port at the same time, their output could be interspersed. Similarly, if one thread reads a memory
location while another one writes to it, the first thread will receive unpredictable data. In
general, writable resources can’t be shared without restrictions, whereas resources that aren’t
subject to modification can be shared. Figure 3-23 illustrates what happens when two threads
running on different processors both write data to a circular queue.
150 Microsoft Windows Internals, Fourth Edition
Figure 3-23 Incorrect sharing of memory
Because the second thread got the value of the queue tail pointer before the first thread had
finished updating it, the second thread inserted its data into the same location that the first
thread had used, overwriting data and leaving one queue location empty. Even though this
figure illustrates what could happen on a multiprocessor system, the same error could occur
on a single-processor system if the operating system were to perform a context switch to the
second thread before the first thread updated the queue tail pointer.
Sections of code that access a nonshareable resource are called critical sections. To ensure correct
code, only one thread at a time can execute in a critical section. While one thread is writing
to a file, updating a database, or modifying a shared variable, no other thread can be
allowed to access the same resource. The pseudocode shown in Figure 3-23 is a critical section
that incorrectly accesses a shared data structure without mutual exclusion.
The issue of mutual exclusion, although important for all operating systems, is especially
important (and intricate) for a tightly coupled, symmetric multiprocessing (SMP) operating system
such as Windows, in which the same system code runs simultaneously on more than one
processor, sharing certain data structures stored in global memory. In Windows, it is the kernel’s
job to provide mechanisms that system code can use to prevent two threads from modifying
the same structure at the same time. The kernel provides mutual-exclusion primitives
that it and the rest of the executive use to synchronize their access to global data structures.
Because the scheduler synchronizes access to its data structures at DPC/Dispatch level IRQL,
the kernel and executive cannot rely on synchronization mechanisms that would result in a
page fault or reschedule operation to synchronize access to data structures when the IRQL is
DPC/Dispatch level or higher (levels known as an elevated or high IRQL). In the following sections,
you’ll find out how the kernel and executive uses mutual exclusion to protect its global
Get queue tail
Insert data at current location
• • •
• • • • • • • • •
• • •
Increment tail pointer
Processor A Processor B
Get queue tail
Insert data at current location /*ERROR*/
Increment tail pointer
Chapter 3: System Mechanisms 151
data structures when the IRQL is high and what mutual-exclusion and synchronization mechanisms
the kernel and executive use when the IRQL is low (below DPC/Dispatch level).
At various stages during its execution, the kernel must guarantee that one, and only one, processor
at a time is executing within a critical section. Kernel critical sections are the code segments
that modify a global data structure such as the kernel’s dispatcher database or its DPC
queue. The operating system can’t function correctly unless the kernel can guarantee that
threads access these data structures in a mutually exclusive manner.
The biggest area of concern is interrupts. For example, the kernel might be updating a global
data structure when an interrupt occurs whose interrupt-handling routine also modifies the
structure. Simple single-processor operating systems sometimes prevent such a scenario by
disabling all interrupts each time they access global data, but the Windows kernel has a more
sophisticated solution. Before using a global resource, the kernel temporarily masks those
interrupts whose interrupt handlers also use the resource. It does so by raising the processor’s
IRQL to the highest level used by any potential interrupt source that accesses the global
data. For example, an interrupt at DPC/dispatch level causes the dispatcher, which uses the
dispatcher database, to run. Therefore, any other part of the kernel that uses the dispatcher
database raises the IRQL to DPC/dispatch level, masking DPC/dispatch-level interrupts
before using the dispatcher database.
This strategy is fine for a single-processor system, but it’s inadequate for a multiprocessor configuration.
Raising the IRQL on one processor doesn’t prevent an interrupt from occurring on
another processor. The kernel also needs to guarantee mutually exclusive access across several
The simplest form of synchronization mechanisms rely on hardware support for multiprocessor-
safe manipulating integer values and for performing comparisons. They include functions
such as InterlockedIncrement, InterlockedDecrement, InterlockedExchange, and Interlocked-
CompareExchange. The InterlockedDecrement function, for example, uses the x86 lock instruction
prefix (for example, lock xadd) to lock the multiprocessor bus during the subtraction
operation so that another processor that’s also modifying the memory location being decremented
won’t be able to modify between the decrement’s read of the original value and write of
the decremented value. This form of basic synchronization is used by the kernel and drivers.
152 Microsoft Windows Internals, Fourth Edition
The mechanism the kernel uses to achieve multiprocessor mutual exclusion is called a spinlock.
A spinlock is a locking primitive associated with a global data structure, such as the DPC
queue shown in Figure 3-24.
Figure 3-24 Using a spinlock
Before entering either critical section shown in the figure, the kernel must acquire the spinlock
associated with the protected DPC queue. If the spinlock isn’t free, the kernel keeps trying
to acquire the lock until it succeeds. The spinlock gets its name from the fact that the
kernel (and thus, the processor) is held in limbo, “spinning,” until it gets the lock.
Spinlocks, like the data structures they protect, reside in global memory. The code to acquire
and release a spinlock is written in assembly language for speed and to exploit whatever locking
mechanism the underlying processor architecture provides. On many architectures, spinlocks
are implemented with a hardware-supported test-and-set operation, which tests the
value of a lock variable and acquires the lock in one atomic instruction. Testing and acquiring
the lock in one instruction prevents a second thread from grabbing the lock between the time
when the first thread tests the variable and the time when it acquires the lock.
All kernel-mode spinlocks in Windows have an associated IRQL that is always at DPC/dispatch
level or higher. Thus, when a thread is trying to acquire a spinlock, all other activity at
the spinlock’s IRQL or lower ceases on that processor. Because thread dispatching happens at
DPC/dispatch level, a thread that holds a spinlock is never preempted because the IRQL
masks the dispatching mechanisms. This masking allows code executing a critical section protected
by a spinlock to continue executing so that it will release the lock quickly. The kernel
uses spinlocks with great care, minimizing the number of instructions it executes while it
holds a spinlock.
Add DPC from queue
Release DPC queue spinlock Release DPC queue spinlock
Remove DPC from queue
• • •
• • •
Try to acquire
Try to acquire
Processor A Processor B
Chapter 3: System Mechanisms 153
Note Because the IRQL is an effective synchronization mechanism on uniprocessors, the
spinlock acquisition and release functions of uniprocessor HALs don’t implement spinlocks—
they simply raise and lower the IRQL.
The kernel makes spinlocks available to other parts of the executive through a set of kernel
functions, including KeAcquireSpinlock and KeReleaseSpinlock. Device drivers, for example,
require spinlocks in order to guarantee that device registers and other global data structures
are accessed by only one part of a device driver (and from only one processor) at a time. Spinlocks
are not for use by user programs—user programs should use the objects described in the
Kernel spinlocks carry with them restrictions for code that uses them. Because spinlocks
always have an IRQL of DPC/dispatch level or higher, as explained earlier, code holding a
spinlock will crash the system if it attempts to make the scheduler perform a dispatch operation
or if it causes a page fault.
A special type of spinlock called a queued spinlock is used in some circumstances instead of a
standard spinlock. A queued spinlock is a form of spinlock that scales better on multiprocessors
than a standard spinlock. In general, Windows will use only standard spinlocks when it
expects there to be low contention for the lock.
A queued spinlock work like this: When a processor wants to acquire a queued spinlock that
is currently held, it places its identifier in a queue associated with the spinlock. When the processor
that’s holding the spinlock releases it, it hands the lock over to the first processor identified
in the queue. In the meantime, a processor waiting for a busy spinlock checks the status
not of the spinlock itself but of a per-processor flag that the processor ahead of it in the queue
sets to indicate that the waiting processor’s turn has arrived.
The fact that queued spinlocks result in spinning on per-processor flags rather than global
spinlocks has two effects. The first is that the multiprocessor’s bus isn’t as heavily trafficked by