18. DOSEMU debugger v0.6

This section written on 98/02/08. Send comments to Max Parke <mhp@light.lightlink.com> and to Hans Lermen <lermen@fgan.de>

18.1. Introduction

This is release v0.6 of the DOSEMU debugger, with the following features:

18.2. Usage

To run, start up DOSEMU. Then switch to another virtual console (or remote login or use another xterm) and do:
If there are more then one dosemu process running, you will need to pass the pid to dosdebug, e.g:

      dosdebug 2134

NOTE: You must be the owner of the running dosemu to 'debug-login'.

You should get connected and a banner message. If you type 'q', only the terminal client will terminate, if you type 'kill', both dosemu and the terminal client will be terminated.

It may be desirable to debug the DOS or its drivers itself during startup, to realize that you need to synchronize DOSEMU and your debugger terminal. This can be done using the -H1 command line option of DOSEMU:
      $ dosemu -H1
DOSEMU will then lock before jumping into the loaded bootsector waiting for dosdebug to connect. Once connected you are in `stopped' state and can set breakpoints or singlestep through the bootstrap code.

18.3. Commands

See mhpdbgc.c for code and cmd table.

(all numeric args in hex)


Print a help page


Quit the debug session


Kill the dosemu process (this may take a while, so be patient) See also Section 20

console n

Switch to console n


list regs

r reg val

change contents of 'reg' to 'val' (e.g: r AX 1234)

e ADDR valuelist

modify memory (0-1Mb) `ADDR' maybe just a `-' (minus), then last (incremented) address from a previous `e' or `ed' command is used (this allowes consecutive writes).

`valuelist' is a blank separated list of


such as 0F or C800


enclosed in single quotes such as 'A' or 'b'


any valid register symbol, in this case the current value (and size) of that registe is take (e.g AX is 2 bytes, EAX is 4 bytes)


enclosed in double quotes such "this is a string"

The default size of each value is one byte (except registers), this size can be overridden by suffixing a `W' (word, 2 bytes) or `L' (long, 4 bytes) such as C800w or F0008123L

ed ADDR valuelist

same as above `e' command, except that the numbers are expected as decimals per default. To write a hexvalue with `ed' you may prefix it with `0x' as in C or write an octal value prefixing a `0'.


dump memory (no limit)


unassemble memory (no limit)


go (if stopped)


stop (if running)

mode 0|1|+d|-d

set mode (0=SEG16, 1=LIN32) for u and d commands +d enables DPMI mode (default on startup), -d disables DPMI mode.


single step (may jump over IRET or POPF)


single step, loop forever until key pressed


single step, force over IRET and POPF NOTE: the scope of 't' 'tf' or a 'come back for break' is either 'in DPMI' or realmode, depending on wether a DPMI-client is active (in_dpmi).


dump regs in 32 bit format

bp addr

set int3 style breakpoint NOTE: the scope is defined wether a DPMI-client is active (in_dpmi). The resulting 'come back' will force the mode that was when you defined the breakpoint.

bc breakp.No.

Clear a breakpoint.

bpint xx

set breakpoint on INT xx

bcint xx

clr breakpoint on INT xx

bpintd xx [ax]

set breakpoint on DPMI INT xx optionaly matching ax.

bcintd xx [ax]

clear breakpoint on DPMI INT xx.


set one shot breakpoint at entry point of the next loaded DOS-program.


list active breakpoints

bplog regex

set a breakpoint on logoutput using regex. With this the normal DOSEMU log output (enabled via the -D commandline option or the dosdebug `log' command) is monitored via the regular expression `regex' (look at GNU regex manual) and when a match is found emulation is set into `stopped' mode. There may be 8 log breakpoint active simultaneously. Without the `regex' given `bplog' such prints the current active breakpoints.

bpclog number

clears a log break point.

log [flags]

get/set debug-log flags (e.g 'log +M-k')

log on|off

redirect dbug-log output to the dosdebug terminal

ldt sel [lines]

dump ldt starting at selector 'sel' for 'lines' 'sel' may be a symbolic register name.


(internal command to read /usr/src/dosemu/dosemu.map at startup time)

rusermap org fn

read microsoft linker format .MAP file "fn" code origin = "org". for example if your code is at 1234:0, org would be 12340.

Addresses may be specified as:

  1. a linear address. Allows 'd' and 'u' commands to look at both DOSEMU kernel and DOS box memory (0-1Mb).

  2. a seg:off address (0-1Mb) seg as well as off can be a symbolic registers name (e.g cs:eip) 'seg' under DPMI is resolved via LDT, if so a numeric 'seg' value is prefixed by # (e.g. #00af:0000. You may force a seg to treaten as LDT selector by prefixing the '#'. Accordingly to the default address mode 'off' under DPMI is 16 or 32 bit. When in DPMI mode, and you want to address/display realmode stuff, then you must switch off DPMI mode ('mode -d')

  3. a symbolic address. usermap is searched first, then dosemu map. ( not for DPMI programms )

  4. an asterisk(*): CS:IP (cs:eip)

  5. a dollar sign($): SS:SP (ss:esp)

18.4. Performance

If you have dosemu compiled with the debugger support, but the debugger is not active and/or the process is not stopped, you will not see any great performance penalty.

18.5. Wish List

Main wish is to add support for hardware debug registers (if someone would point me in the direction, what syscalls to use, etc.) Then you could breakpoint on memory reads/writes, etc!

18.6. BUGS

There must be some.

18.6.1. Known bugs

  • Though you may set breakpoints and do singlestep in Windows31, this is a 'one shot': It will bomb after you type 'g' again. ( I suspect this is a timer problem, we really should freeze the timer and all hardware/mouse IRQs while the program is in 'stop'). Debugging and singlestepping through DJGPP code doesn't have any problems.

  • INT3 type breakpoints in DPMI code are very tricky, because you never know when the client has remapped/freed the piece of code that is patched with 0xCC ( the one byte INT3 instruction ). Use that with caution !!

  • Single stepping doesn't work correctly on call's. May be the trap-flag is lost. However, when in DPMI the problems are minor.

  • popf sometime clears the trap-flag, so single stepping results in a 'go' command. 'tf' works around, but we should do it better.

  • When stopped for a long period, the BIOS-timer will be updated to fast and may result in stack overflow. We need to also stop the timer for dosemu.

  • When not stopped, setting break points doesn't work properly. So, as a work around: Setting breakpoints while not in stop is disabled.