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Revision as of 16:34, 3 November 2019 by Hbelusca (talk | contribs) (Command Reference)
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This page was a mirror of the KDBG page (rev 72) on Blight's Wiki


KDB, the Kernel DeBugger of ReactOS, is built into ntoskrnl. For each exception which is handled by the kernel, KDB gets up to two chances to handle it. The first chance occurs before the kernel tries to find and execute an exception handler. The last chance occurs after the kernel fails to find an exception handler or the handler did not handle the exception.

For each type of exception known by KDB, you can set the condition when KDB should be entered individually for first and last chance. The possible settings for the conditions are never, umode, kmode and always. never means that KDB will never be entered when the exception is raised, umode means that it will be entered when the exception was raised in user mode, kmode means to enter KDB when the exception was raised in kernel mode, and always means to always enter KDB.

Enabling KDBG before build

KDBG is automatically enabled or disabled by the configuration options script (cmake/config.cmake).

KDBG is by default configured to be enabled for the bootloader entries "ReactOS (Debug)" and "ReactOS (RosDbg)" for all ISO images. See also Debugging#Breaking into the built-in kernel debugger.


As soon as possible KDB loads and interprets the KDBinit file (\SystemRoot\system32\drivers\etc\KDBinit)

The KDBinit file can contain all commands available in the command line interface of KDB plus the break command. When a break is read from the KDBinit file KDB is entered – you can use this to automatically enter KDB when booting.

Here's the default KDBinit file (the break is commented out):

# Example KDBinit file

# Set the disassembly flavor to "intel" (default is "at&t")
set syntax intel

# Change the condition to enter KDB on INT3 to "always" (default is "kmode")
#set condition INT3 first always

# This is a special command available only in the KDBinit file - it breaks into
# KDB when it is interpreting the init file at startup.

Command Reference


Syntax: ? expression

Evaluate expression. The result is displayed in hexadecimal and unsigned decimal format. If it is < 0 it is also displayed in signed decimal format.

kdb:> ? eip
0xc00a1535  3221886261  -1073081035

The following operands are supported: +,-,*,/,%,==,!=,>,<,>=,<=
Braces are supported.
Brackets can be used to dereference memory. The memory size (byte, word, dword or qword) can be specified as a prefix before the opening bracket. The default is the size of a pointer (dword for IA32)

kdb:> ? 1+2*3
0x00000007           7
kdb:> ? (1+2)*3
0x00000009           9
kdb:> ? [eip]
0xffbe16e8  4290647784    -4319512
kdb:> ? word[eip]
0x000016e8        5864
kdb:> ? byte[eip]
0x000000e8         232         -24
kdb:> ? byte[eip+1]
0x00000016          22
kdb:> ? [0]
        ^ Couldn't access memory at 0x0


Syntax: bc [breakpoint]

Clear breakpoint.


Syntax: bd [breakpoint]

Disable breakpoint.


Syntax: be [breakpoint]

Enable breakpoint.


Syntax: bl

List breakpoints.


Syntax: bpm [r|w|rw|x] [byte|word|dword] [address]

Set memory breakpoint at address.


Syntax: bpx [address]

Set software execution breakpoint at address.

Note: Don't forget to type 'set condition INT3 first always' when you set a breakpoint in usermode or KDB will not be entered on the breakpoint.


Syntax: bt [*frameaddr]

Print current backtrace or from given frame address.

kdb:> bt


Syntax: bugcheck

Crash the system.


Syntax: cont

Continue execution (leave debugger).


Syntax: cregs

Display control registers.

kdb:> cregs
CR0  0x8001003b  PE MP TS ET NE WP PG
CR2  0xccc91000
CR3  0x002f9000  Pagedir-Base 0x002f9000
GDTR  Base 0xc00bb100  Size 0x0058
LDTR  Base 0x00000000  Size 0x0000
IDTR  Base 0xc0122ac0  Size 0x0800


Syntax: disasm [address] [L count]

Disassemble count instructions at address.

kdb:> disasm
<c00a1535>: call   <0xC009D350>
<c00a153a>: call   <0xC009D110>
<c00a153f>: call   <ntoskrnl.exe:2e990>
<c00a1544>: call   <0xC009DDD0>
<c00a1549>: mov    0xc0104d40,%eax
<c00a154e>: cmpb   $0x0,(%eax)
<c00a1551>: je     <0xC00A1597>
<c00a1553>: lea    0x0(%esi),%esi
<c00a1559>: lea    0x0(%edi,1),%edi
<c00a1560>: sub    $0x8,%esp

Intel syntax is also supported:

kdb:> set syntax intel
kdb:> disasm
<c00a1535>: call   <0xC009D350>
<c00a153a>: call   <0xC009D110>
<c00a153f>: call   <ntoskrnl.exe:2e990>
<c00a1544>: call   <0xC009DDD0>
<c00a1549>: mov    eax,ds:0xc0104d40
<c00a154e>: cmp    BYTE PTR [eax],0x0
<c00a1551>: je     <0xC00A1597>
<c00a1553>: lea    esi,[esi]
<c00a1559>: lea    edi,[edi*1]
<c00a1560>: sub    esp,0x8


Syntax: dmesg

Display debug messages on the screen (with pagination).


Syntax: dregs

Display debug registers.

kdb:> dregs
DR0  0x00000000
DR1  0x00000000
DR2  0x00000000
DR3  0x00000000
DR6  0xffff0ff0
DR7  0x00000400


Syntax: dt [mod] [type] [addr]

Print a struct. The address is optional.


Syntax: filter [error|warning|trace|info|level]+|-[componentname|default]

Enable/disable debug channels.


kdb:> filter trace+i8042prt


kdb:> filter +serial


Syntax: gdt

Display the global descriptor table.

kdb:> gdt
GDT Base: 0x806c9000  Limit: 0x03ff
  Idx  Sel.    Type         Base        Limit       DPL  Attribs
  001  0x0008  CODE32       0x00000000  0x0fffffff  00   R/X A
  002  0x0010  DATA32       0x00000000  0x0fffffff  00   R/W A
  003  0x001b  CODE32       0x00000000  0x0fffffff  03   R/X A
  004  0x0023  DATA32       0x00000000  0x0fffffff  03   R/W A
  005  0x0028  TSS32(Busy)  0x8009c000  0x000020ac  00
  006  0x0030  DATA32       0xffdff000  0x00001fff  00   R/W A
  007  0x003b  DATA32       0x00000000  0x00001000  03   R/W A
  008  0x0043  DATA16       0x00000400  0x00010000  03   R/W
  009  0x0048  LDT          0x00000000  0x00000001  00
  010  0x0050  TSS32(Avl)   0x80a10ca0  0x00000069  00
  011  0x0058  TSS32(Avl)   0x80a10c20  0x00000069  00
  012  0x0060  DATA16       0x00020000  0x00010000  00   R/W
  013  0x0068  DATA16       0x000b8000  0x00004000  00   R/W
  014  0x0070  DATA16       0xffff7000  0x00000400  00   R/W
  015  0x0078  UNKNOWN      [NP]        [NP]        00   NP


Syntax: help

Display help screen.

kdb:> help
Kernel debugger commands:
* Data:
  ? expression         - Evaluate expression.
  disasm [address] [L count] - Disassemble count instructions at address.
  x [address] [L count] - Display count dwords, starting at addr.
  regs                 - Display general purpose registers.
  cregs                - Display control registers.
  sregs                - Display status registers.
  dregs                - Display debug registers.
  bt [*frameaddr|thread id] - Prints current backtrace or from given frame addr

* Flow control:
  cont                 - Continue execution (leave debugger)
  step [count]         - Execute single instructions, stepping into interrupts.
  next [count]         - Execute single instructions, skipping calls and reps.
  bl                   - List breakpoints.
  be [breakpoint]      - Enable breakpoint.
  bd [breakpoint]      - Disable breakpoint.
  bc [breakpoint]      - Clear breakpoint.
  bpx [address] [IF condition] - Set software execution breakpoint at address.
  bpm [r|w|rw|x] [byte|word|dword] [address] [IF condition] - Set memory breakpoint at address.

* Process/Thread:
  thread [list[ pid]|[attach ]tid] - List threads in current or specified process, display thread with given id or attach to thread.
  proc [list|[attach ]pid] - List processes, display process with given id or attach to process.

* System info:
  mod [address]        - List all modules or the one containing address.
  gdt                  - Display global descriptor table.
  ldt                  - Display local descriptor table.
  idt                  - Display interrupt descriptor table.
  pcr                  - Display processor control region.
  tss                  - Display task state segment.

* Others:
  bugcheck             - Bugchecks the system.
  set [var] [value]    - Sets var to value or displays value of var.
  help                 - Display help screen.


Syntax: idt

Display the interrupt descriptor table.

kdb:> idt
IDT Base: 0x8012d1d0  Limit: 0x0800
  Idx  Type        Seg. Sel.  Offset      DPL
  000  TRAPGATE32  0x0008     0x800039bf  00
  001  TRAPGATE32  0x0008     0x800039ce  00
  002  TRAPGATE32  0x0008     0x800039dd  00
  003  TRAPGATE32  0x0008     0x800039ec  03
  004  TRAPGATE32  0x0008     0x800039fb  00
  005  TRAPGATE32  0x0008     0x80003a0a  00
  006  TRAPGATE32  0x0008     0x80003a19  00
  007  TRAPGATE32  0x0008     0x80003a28  00
  008  TASKGATE    0x0050                 00
  009  TRAPGATE32  0x0008     0x80003a3d  00
  010  TRAPGATE32  0x0008     0x80003a4c  00
  011  TRAPGATE32  0x0008     0x80003a59  00
  012  TRAPGATE32  0x0008     0x80003a66  00
  013  TRAPGATE32  0x0008     0x80003a73  00
  014  INTGATE32   0x0008     0x80003a80  00
  015  TRAPGATE32  0x0008     0x80003a8d  00
  016  TRAPGATE32  0x0008     0x80003a9c  00
  017  TRAPGATE32  0x0008     0x80003aab  00
  018  TRAPGATE32  0x0008     0x80003aba  00
  019  TRAPGATE32  0x0008     0x80003ac9  00
  020  TRAPGATE32  0x0008     0x80003ad8  00
  044  TRAPGATE32  0x0008     0x80003ad8  00
  045  TRAPGATE32  0x0008     0x8006ef9f  03
  046  TRAPGATE32  0x0008     0x80003b79  03
  047  TRAPGATE32  0x0008     0x80003ad8  00


Syntax: kmsg

Alias for dmesg.


Syntax: ldt

Display the local descriptor table.

kdb:> ldt
Local descriptor table is empty.


Syntax: mod [address]

List all modules or the one containing address.

kdb:> mod
  Base      Size      Name
  c0000000  00096ca0  ntoskrnl.exe
  c039c000  00007de0  hal.dll


Syntax: next [count]

Execute single instructions, skipping calls and reps.


Syntax: pcr

Display the processor control region.

kdb:> pcr
Current PCR is at 0xFFDFF000.
  Tib.ExceptionList:         0xffffffff
  Tib.StackBase:             0x00000000
  Tib.StackLimit:            0x00000000
  Tib.SubSystemTib:          0x8009c000
  Tib.FiberData/Version:     0x000282ed
  Tib.ArbitraryUserPointer:  0x00000001
  Tib.Self:                  0x00000000
  SelfPcr:                   0xffdff000
  PCRCB:                     0xffdff120
  Irql:                      0x00
  IRR:                       0x00000000
  IrrActive:                 0x00000000
  IDR:                       0xffff20b8
  KdVersionBlock:            0x00000000
  IDT:                       0x806c9400
  GDT:                       0x806c9000
  TSS:                       0x8009c000
  MajorVersion:              0x0001
  MinorVersion:              0x0001
  SetMember:                 0x00000001
  StallScaleFactor:          0x00000473
  Number:                    0x00
  L2CacheAssociativity:      0x08
  VdmAlert:                  0x00000000
  L2CacheSize:               0x00040000
  InterruptMode:             0x00000000


Syntax: proc [list|[attach ]pid]

List processes, display process with given id or attach to process.

kdb:> proc
Current process:
  PID:             0x00000004
  State:           Unknown (0x0)
  Image Filename:  System
kdb:> proc list
  PID         State       Filename
 *0x00000004  Unknown     System
  0x00000058  Active      smss.exe
  0x0000007c  Active      csrss.exe
  0x00000094  Active      winlogon.exe
  0x000000b0  Active      setup.exe


Syntax: regs

Display general purpose registers.

kdb:> regs
CS:EIP  0x0008:0xc00b880a
SS:ESP  0xd754:0x00000000
   EAX  0x01c634d3   EBX  0x0000940e
   ECX  0x00000000   EDX  0x00000000
   ESI  0x002ff000   EDI  0x00000000
   EBP  0xc0103c70
EFLAGS  0x00200202  IF IOPL0 ID


Syntax: set [var] [value]

Sets var to value or displays value of var.

kdb:> set
Available settings:
  syntax [intel|at&t]
  condition [exception|*] [first|last] [never|always|kmode|umode]
kdb:> set syntax
syntax = at&t
kdb:> set condition
Conditions:                 (First)  (Last)
  #00  ZERODEVIDE           never    kmode
  #01  DEBUGTRAP            always   never
  #02  NMI                  never    always
  #03  INT3                 kmode    never
  #04  OVERFLOW             never    kmode
  #05  BOUND                never    kmode
  #06  INVALIDOP            never    kmode
  #07  NOMATHCOP            never    kmode
  #08  DOUBLEFAULT          always   always
  #09  RESERVED(9)          always   always
  #10  INVALIDTSS           never    kmode
  #11  SEGMENTNOTPRESENT    never    kmode
  #12  STACKFAULT           never    kmode
  #13  GPF                  never    kmode
  #14  PAGEFAULT            never    kmode
  #15  RESERVED(15)         always   always
  #16  MATHFAULT            never    kmode
  #17  ALIGNMENTCHECK       never    kmode
  #18  MACHINECHECK         never    kmode
  #19  SIMDFAULT            never    kmode
       OTHERS               never    kmode

Example: Enabling usermode software breakpoints.

kdb:> set condition int3 first always
kdb:> set condition int3
Condition for exception #03 (INT3): FirstChance always  LastChance never

Example: Catching all exceptions.

kdb:> set condition * first always


Syntax: sregs

Display segment/selector registers.

kdb:> sregs
CS  0x0008  Index 0x0001  GDT RPL0
DS  0x0010  Index 0x0002  GDT RPL0
ES  0x0010  Index 0x0002  GDT RPL0
FS  0x0030  Index 0x0006  GDT RPL0
GS  0x0010  Index 0x0002  GDT RPL0
SS  0xd754  Index 0x1aea  LDT RPL0


Syntax: step [count]

Execute single instructions, stepping into interrupts (i.e. when EIP points to int 0x2e KDB gets the address of the interrupt handler, sets a INT3 there and continues execution)


Syntax: thread [list[ pid]|[attach ]tid]

List threads in current or specified process, display thread with given id or attach to thread.

kdb:> thread
Current Thread:
  TID:            0x00000000
  State:          Running (0x2)
  Priority:       8
  Affinity:       0x00000001
  Initial Stack:  0x8011f5a0
  Stack Limit:    0x8011c5a0
  Stack Base:     0x8011f5a0
  Kernel Stack:   0x8011f5a0
  Trap Frame:     0x00000000
  NPX State:      Invalid (0x1)
kdb:> thread list
  TID         State        Prior.  Affinity    EBP         EIP
 *0x00000000  Running        8     0x00000001  0x00000000  0x00000000
  0x00000008  Blocked       16     0xffffffff  0x9cdcbd38  0x8001009e
  0x0000000c  Blocked       16     0xffffffff  0x9cdced38  0x8001009e
  0x00000010  Blocked       16     0xffffffff  0x9cdd1d38  0x8001009e
  0x00000014  Blocked       16     0xffffffff  0x9cdd4d38  0x8001009e
  0x00000018  Blocked       16     0xffffffff  0x9cdd7d38  0x8001009e
  0x0000001c  Ready          0     0xffffffff  0x00000000  0x00000000
  0x00000020  Ready          0     0xffffffff  0x00000000  0x00000000
  0x00000024  Ready          0     0xffffffff  0x00000000  0x00000000
  0x00000028  Ready          0     0xffffffff  0x00000000  0x00000000
  0x0000002c  Ready          0     0xffffffff  0x00000000  0x00000000
  0x00000030  Blocked       31     0xffffffff  0x9cde9d38  0x8001009e
  0x00000034  Blocked       31     0xffffffff  0x9cdecd38  0x8001009e
  0x00000038  Blocked       31     0xffffffff  0x9cdefd38  0x8001009e
  0x0000003c  Blocked       31     0xffffffff  0x9cdf2d38  0x8001009e
  0x00000040  Blocked       31     0xffffffff  0x9cdf5d38  0x8001009e
  0x00000044  Blocked        8     0xffffffff  0x9cdf8d28  0x8001009e
  0x00000048  Ready          0     0xffffffff  0x00000000  0x00000000
  0x0000004c  Blocked       17     0xffffffff  0x9cdfed08  0x8001009e
  0x00000050  Ready          1     0xffffffff  0x00000000  0x00000000
  0x00000054  Blocked       16     0xffffffff  0x9ce04d28  0x8001009e


Syntax: tss

Display task state segment.

kdb:> tss
Current TSS is at 0x8012d9f0.
  PreviousTask:  0x00000000
  Ss0:Esp0:      0x0010:0x80137df0
  Ss1:Esp1:      0x0000:0x00000000
  Ss2:Esp2:      0x0000:0x00000000
  Cr3:           0x00000000
  Eip:           0x00000000
  Eflags:        0x00000000
  Eax:           0x00000000
  Ecx:           0x00000000
  Edx:           0x00000000
  Ebx:           0x00000000
  Esp:           0x00000000
  Ebp:           0x00000000
  Esi:           0x00000000
  Edi:           0x00000000
  Es:            0x0000
  Cs:            0x0000
  Ss:            0x0000
  Ds:            0x0000
  Fs:            0x0000
  Gs:            0x0000
  Ldt:           0x0048
  Trap:          0x0000
  IoMapBase:     0xffff


Syntax: x [address] [L count]

Display count dwords, starting at address.

kdb:> x ebp
<c0103c70>: c0103c80 c000fbec 00000000 012ff000
<c0103c80>: c0103de0 c00b91e6 00000001 c011bc90
<c0103c90>: 00000014 c0103dc4 00000000 00000000
<c0103ca0>: 00000000 c03c6000 c0397000 c0125000


Syntax: !pool [Address [Flags]]

Display information about pool allocations.


Syntax: !poolused [flags] [tag]

Display current pool usage (paged and non paged) for all tags.

It is possible to show only a tag, or a subset of tags, thanks to the wildcard '?' (replaces a char).

kdb:> !poolused
Pool Used:
                NonPaged                        Paged
Tag             Allocs          Used            Allocs          Used
'ObCI'          3               192             0               0
'VPB '          8               768             0               0
'ObSc'          0               0               21              2840
'Rqrv'          0               0               121             664
'USTR'          0               0               365             6528
'SCSI'          51              163984          20              384
'RSYM'          6               2271840         0               0
'TOKu'          0               0               2               192


Syntax: !poolfind Tag [Pool]

Search for pool tag allocations.


Syntax: !filecache

Display cache usage.


Syntax: !defwrites

Display cache write values.


Syntax: !irpfind [Pool [startaddress [criteria data]]]

Lists IRPs potentially matching criteria.


Syntax: !handle [Handle]

Displays info about handles.