8.11. Using Bochs internal debugger

Note, if you are looking for a graphical front-end for the bochs debugger, you may want to check out BFE. This is a package written by a Bochs user which can interface with the text based Bochs debugger. No linking is necessary. It's not part of Bochs, but you may find it useful.

You can now conditionally compile in a GDB like command line debugger, that allows you to set breakpoints, step through instructions, and other useful functions. If there isn't a command for something you believe is generally useful for the debugger, let me know and I'll implement it if possible.

To use the debugger, you must configure Bochs with the --enable-debugger and --enable-disasm flags. For example:

  ./configure --enable-debugger --enable-disasm

Note: You must use flex version 2.5.4 or greater. I have heard that version 2.5.2 will not work.

When you first start up Bochs, you will see the command line prompt

From here, you may use the following commands:

8.11.1. Execution Control

  c                           continue executing

  s     [count]               execute count instructions, default is 1
  step  [count]

  s     [cpu] [count]         for SMP simulation, execute count instructions on cpu, default is 1
  step  [cpu] [count]

  s     all [count]           for SMP simulation, execute count instructions on all cpus
  step  all [count]

  Ctrl-C                      stop execution, and return to command line prompt
  Ctrl-D                      if at empty line on command line, exit

  q                           quit debugger and execution

8.11.2. BreakPoints

  NOTE: The format of 'seg', 'off', and 'addr' in these descriptions,
        are as follows.  I don't have any way to set the current radix.

        hexidecimal:    0xcdef0123
        decimal:        123456789
        octal:          01234567

  vbreak seg:off              Set a virtual address instruction breakpoint
  vb     seg:off

  lbreak addr                 Set a linear address instruction breakpoint
  lb     addr

  pbreak [*] addr             Set a physical address instruction breakpoint
  pb     [*] addr             (the '*' is optional for GDB compatibility)
  break  [*] addr
  b      [*] addr

  info break                  Display state of all current breakpoints
  bpe    n                    Enable a breakpoint
  bpd    n                    Disable a breakpoint
  delete n                    Delete a breakpoint
  del    n
  d      n

8.11.3. Memory WatchPoints

  watch read  addr            Insert a read watch point at physical address addr
  watch r     addr            Insert a read watch point at physical address addr

  watch write addr            Insert a write watch point at physical address addr
  watch w     addr            Insert a write watch point at physical address addr

  watch                       Display state of current memory watchpoints

  watch stop                  Stop simulation when a watchpoint is encountered (default)
  watch continue              Do not stop simulation when a watchpoint is encountered

  unwatch addr                Remove watchpoint to specific physical address
  unwatch                     Remove all watch points

  trace-mem on/off            Enable/Disable memory access tracing

8.11.4. Manipulating Memory

  x  /nuf addr      Examine memory at linear address addr
  xp /nuf addr      Examine memory at physical address addr
     n              Count of how many units to display
     u              Unit size; one of
                      b Individual bytes
                      h Halfwords (2 bytes)
                      w Words (4 bytes)
                      g Giant words (8 bytes)
                      NOTE: these are *not* typical Intel nomenclature sizes,
                            but they are consistent with GDB convention.
     f              Printing format.  one of
                      x Print in hexadecimal
                      d Print in decimal
                      u Print in unsigned decimal
                      o Print in octal
                      t Print in binary

    n, f, and u are optional parameters.  u and f default to the last values
    you used, or to w(words) and x(hex) if none have been supplied.
    n currently defaults to 1.  If none of these optional parameters are
    used, no slash should be typed.  addr is also optional.  If you don't
    specify it, it will be the value the next address (as if you had
    specified n+1 in the last x command).

  setpmem addr datasize val    Set physical memory location of size
                               datasize to value val.

  crc  addr1  addr2            Show CRC32 for physical memory range addr1..addr2

8.11.5. Info commands

  r|reg|regs|registers         List of CPU integer registers and their contents
  fp|fpu                       List of all FPU registers and their contents
  mmx                          List of all MMX registers and their contents
  sse                          List of all SSE registers and their contents
  sreg                         Show segment registers and their contents
  dreg                         Show debug registers and their contents
  creg                         Show control registers and their contents

  info cpu                     List of all CPU registers and their contents
  info eflags                  Show decoded EFLAGS register
  info break                   Information about current breakpoint status
  info tab                     Show paging address translation

  set $cpu = N                Cause debugger to use cpu number "N".
                              For example, "regs" will now print the registers
                              of cpuN.

8.11.6. Manipulating CPU Registers

  set reg = expr    Change a CPU register to value of expression.
                    Currently only general purpose registers are supported,
                    you may not change:
                      eflags, eip, cs, ss, ds, es, fs, gs.

    Examples: set eax = 2+2/2
              set esi = 2*eax+ebx

  registers         List of CPU registers and their contents

8.11.7. Disassembly commands

  disassemble start end       Disassemble instructions in given linear address
                              range, inclusive of start, exclusive of end.
                              Use "set $disassemble_size =" to tell
                              debugger desired segment size.  Use a value for
                              end of less than start (or zero) if you only
                              want the first instruction disassembled.

  disassemble switch-mode     Switch between Intel and AT&T disassebly styles
                              for debugger disassembler.

  disassemble size = n        Tell debugger what segment size to use when
                              the "disassemble" command is used.  Use values
                              of 0, 16 or 32 for n.  Value of 0 means
			      "use segment size specified by current CS
			      segment". Default is 0.

  set $auto_disassemble = n   Cause debugger to disassemble current instruction
                              every time execution stops if n=1.  Default is 0.
                              Segment size of current CPU context is used for
                              disassembly, so the "disassemble size" variable is

  set disassemble on          The same as 'set $auto_disassemble = 1'
  set disassemble off         The same as 'set $auto_disassemble = 0'

8.11.8. Instruction tracing

  trace on                    Disassemble every executed instruction. Note
                              that instructions which caused exceptions are
                              not really executed, and therefore not traced.

  trace off                   Disable instruction tracing.

8.11.9. Instrumentation

To use instrumentation features in bochs, you must compile in support for it. You should build a custom instrumentation library in a separate directory in the "instrument/" directory. To tell configure which instrumentation library you want to use, use the "--enable-instrumentation" option. The default library consists of a set of stubs, and the following are equivalent:

  ./configure [...] --enable-instrumentation
  ./configure [...] --enable-instrumentation="instrument/stubs"
You could make a separate directory with your custom library, for example "instrument/myinstrument", copy the contents of the "instrument/stubs" directory to it, then customize it. Use:
  ./configure [...] --enable-instrumentation="instrument/myinstrument"

8.11.10. Instrumentation commands

  instrument [command]        calls BX_INSTR_DEBUG_CMD instrumentation callback with [command]

8.11.11. Other Commands

Print the current time (number of ticks since start of simulation).
sb delta
Insert a time break point "delta" instructions into the future ("delta" is a 64-bit integer followed by "L", for example 1000L).
sba time
Insert a time break point at "time" ("time" is a 64-bit integer followed by "L", for example 1000L).
record filename
Record console input to file filename. The file consists of zero or more lines of the form "%s %d %x", where the first word is the event type, the second is a time stamp and the third is event specific data.
playback filename
Playback console input from file filename. Additional input can be given directly in the console window. Events in the file will be played back at times relative to the time when the playback command was executed.
print-stack [num words]
Print the num words top 16-bit words on the stack. Num words defaults to 16. Only works reliably in protected mode when the base address of the stack segment is zero.
Toggles CPU mode switch breakpoint.
ldsym [global] filename [offset]
Load symbols from file filename. If the global keyword is added, then the the symbols will be visible in all contexts for which symbols have not been loaded. Offset (default is 0) is added to every symbol entry. The symbols are loaded in the current (executing) context.

The symbol file consists of zero or more lines of the format

"%x %s"
show [string]
  Toggles show symbolic info (calls to begin with).
  show - shows current show mode
  show mode     - show, when processor switch mode
  show int      - show, when interrupt is happens
  show call     - show, when call is happens
  show ret      - show, when iret is happens
  show off      - toggles off symbolic info
  show dbg-all  - turn on all show flags
  show dbg-none - turn off all show flags