Emile van Bergen

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Real-mode debugger for 80386 processors


Before I started using Unix, I did a lot of development and debugging on MS-DOS/DRDOS. When applications started using more and more of the features of the 80386 that are available in real mode, such as 32-bit data registers and extra segment registers, the standard debugger 'debug' became less and less useful, as it only understands the 8086 instruction set.

So I wrote my own. Other than correctly disassembling the complete 80386 instruction set in both 16- or 32-bit code segments, it also takes advantage of the 386's hardware breakpoints, which make it possible to single-step through ROM code and allow you to trap accesses to memory locations.

Other features are remote debugging facilities through the serial port and support for an NMI key, which can be used to interrupt the program even when normal interrupts are disabled by the CLI instruction. Both features are especially useful for debugging graphics routines and in embedded environments.

It's written for Borland's TASM assembler. You need to assemble both disasm.asm and monitor4.asm, and then link the resulting .obj files into monitor4.exe using Borland tlink. Earlier versions are also available in the download directory.
 

Usage

Invocation:

monitor4 executable[,arguments]

Commands for use at the '-' prompt:

a		Set arguments, you get a prompt
b[seg:]ofs	Set breakpoint at address seg:ofs, uses CS if no segment given
c		Clear all breakpoints and watchpoints
d[[seg:]ofs]	Dump 256 bytes of memory at address seg:ofs or last address
g		Go, continue program
i		Inspect breakpoints
k		Keep resident and exit, after debugged program has done same
lexecutable	Load a new executable using DOS' exec function
m[seg:]ofs	Set a data watchpoint that triggers when read or written
n[seg:]ofs	Set a data watchpoint that triggers only when written
p		Set breakpoint at instruction below current one and proceed
q		Quit
r		Display registers
rreg,value	Set register reg
rf		Set flags, you get a prompt
s[seg:]ofs,s,f	Save 's' bytes of memory memory at seg:ofs to file 'f'
t		Trace to instruction to be executed next
u[[seg:]ofs]	Disassemble code at seg:ofs, assuming a 16-bit code segment
v[[seg:]ofs]	Disassemble code at seg:ofs, assuming a 32-bit code segment
w		Clear redirection to serial port and use int 10h I/O again
wport		Redirect to serial port using UART at base I/O address 'port'




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