The document provides an introduction to the Windows kernel and how system calls work. It discusses how system calls allow entering kernel mode using interrupts, how the system service dispatch table maps system calls to kernel routines, and how parameters are passed. It also describes I/O request packets (IRPs) which encapsulate I/O requests as they pass through different drivers in the kernel. Finally, it covers interrupt request levels (IRQLs) and deferred procedure calls (DPCs) which handle interrupts and deferred processing.

1. Introduction to Windows Kernel
Sisimon Soman

2. How system call works
• Cannot directly enter kernel space using jmp or a call instruction.
• When make a system call (like CreateFile, ReadFile) OS enter kernel mode
(Ring 0) using instruction int 2E (it is called interrupt gate).
• Code segment descriptor contain information about the ‘Ring’ at which
the code can run. For kernel mode modules it will be always Ring 0. If a
user mode program try to do ‘jmp <kernel mode address>’ it will cause
access violation, because of the segment descriptor flag says processor
should be in Ring 0.
• The frequency of entering kernel mode is high (most of the Windows API
call cause to enter kernel mode) sysenter is the new optimized instruction
to enter kernel mode.

3. Rings continued..

4. System Call continued..
• Windows maintains a system service dispatch table which is
similar to the IDT. Each entry in system service table point to
kernel mode system call routine.
• The int 2E probe and copy parameters from user mode stack
to thread’s kernel mode stack and fetch and execute the
correct system call procedure from the system service table.
• There are multiple system service tables. One table for NT
Native APIs, one table for IIS and GDI etc.

5. Lets try it in WinDBG..
• NtWriteFile:
mov eax, 0x0E ; build 2195 system service number for NtWriteFile
mov ebx, esp ; point to parameters
int 0x2E ; execute system service trap
ret 0x2C ; pop parameters off stack and return to caller

6. App issue ReadFile
NtReadFile
User Land
Kernel Land
IO Manager
IO Mgr create IRP Packet,
IRP send to driver stack
File System
Volume Manager
Disk Class Driver
Hardware Driver

7. What is IO Request Packet (IRP)
• IO Operation passes thru,
– Different stages.
– Different threads.
– Different drivers.
• IRP Encapsulate the IO request.
• IRP is thread independent.

8. IRP Continued..
• Compare IRP with Windows Messages -MSG
structure.
• Each driver in the stack do its own task, finally
forward the IRP to the lower driver in the
stack.
• IRP can be processed synchronously or
asynchronously.

9. IRP Continued..
• Usually lower level hardware driver takes more time.
H/W driver can mark the IRP for pending and return.
• When H/W finish IO, H/W driver complete the IRP by
calling IoCompleteRequest().
• IoCompleteRequest() call IO completion routine set
by drivers in stack and complete the IO.

10. Structure of IRP
• Fixed IRP Header IRP Header
• Variable Stack locations –
– One sub stack per driver Stack Location 1
Stack Location 2
Stack Location 3
Stack Location N

11. Flow of IRP
IRP for Storage Stack
Storage Stack IRP Header
File System Stack Location 1
Volume Manager Stack Location 2
Disk Class Driver Stack Location 3
Hardware Driver Stack Location 4
Forward IRP to lower
driver in the stack

12. Flow of IRP Completion
IRP for Storage Stack
Storage Stack IRP Header
File System –
Stack Location 1
Completion Routine
Volume Manager –
Stack Location 2
Completion Routine
Disk Class Driver –
Stack Location 3
Completion Routine
Hardware Driver –
Stack Location 4
Complete the IRP
Call the completion routine while
completing the IRP

13. IRP Header
• IO buffer Information.
• Flags
– Page IO Flag
– No Caching IO flag
• IO Status – On Completion set this to IO
Completed.
• IRP cancel routine

14. IRP Stack Location
• IO Manager get the driver count in the stack
from the top device in the stack.
• While creating IRP, IO manager allocate the IO
stack locations equal to the device count from
the top device object.

15. Contents of IO Stack Location
• IO Completion routine specific to the driver.
• File object specific to the request.

16. Software Interrupt Request Levels (IRQLs)
• Windows has its own interrupt priority schemes know as
IRQL.
• IRQL levels from 0 to 31, the higher the number means higher
priority interrupt level.
• HAL map hardware interrupts to IRQL 3 (Device 1) to IRQL 31
(High)
• When higher priority interrupt occur, it mask the all lower
interrupts and execute the ISR for the higher interrupt.
• After executing the ISR, kernel lower the interrupt levels and
execute the lower interrupt ISR.
• ISR routine should do minimal work and it should defer the
major chunk of work to Deferred Procedure Call (DPC) which
run at lower IRQL 2.

17. Software Interrupt Request Levels (IRQLs)

18. IRQL and DPC
• DPC concept is similar to other OS, in Linux it
is called bottom half.
• DPC is per processor, means a duel processor
SMP box contains two DPC Qs.
• The ISR routine generally fetch data from
hardware and queue a DPC for further
processing.
• IRQL priority is different from thread
scheduling priority.

19. IRQL and DPC
• The scheduler (dispatcher) also runs at IRQL 2.
• So a code that execute on or above IRQL 2(dispatch
level) cannot preempt.
• From the Diagram, see only hardware interrupts and
some higher priority interrupts like clock, power fail
are above IRQL 2.
• Most of the time OS will be in IRQL 0(Passive level)
• All user programs and most of the kernel code
execute on Passive level only.

20. IRQL continued..
• Scheduler runs at IRQL 2, so what happen if my driver try to wait on or
above dispatch level ?.
• Simple system will crash with ‘Blue Screen’, usually with the bug check ID
IRQL_NOT_LESSTHAN_EQUAL.
• Because if wait above dispatch level, no one there to come and switch the
thread.
• What happen if try to access a PagedPool in above dispatch level ?.
• If the pages are on disk, then a page fault exception will happen, the
current thread need to wait and page fault handler will read the pages
from page file to page frames in memory.
• If page fault happen above the dispatch level, no one there to stop the
current thread and schedule the page fault handler. Thus cannot access
PagedPool on or above dispatch level.

21. IRQL 1 - APCs
• Asynchronous Procedure Call (APC) run at IRQL 1.
• The main duty of APC is to send the data to user thread
context.
• APC Q is thread specific, each thread has its own APC Q.
• User space thread initiate the read operation from a device
and either it wait to finish it or continue with another job.
• The IO may finish sometime later, now the buffer need to
send to the calling thread’s process context. It is the duty of
APC.