1. About Debug/X
Debug/X is a package of debuggers. The most important ones are:
- Debug: is a clone of the MS-DOS DEBUG command - although, "clone"
isn't the correct name, because it has many features not available in
the original MS DEBUG. For details see below.
- DebugX: is an extended version of Debug that allows to debug both
real-mode and protected-mode (DPMI) DOS programs.
- additional variants like DebugXv, DebugXg, DebugB or DebugR that are
useful for special cases. For details see below.
2. Added/Missing Features
a. Debug extends MS-Debug in the following ways:
- The assembler and disassembler support all publicly documented
instructions for Intel chips through the Pentium Pro (P6), except for
the MMX instructions (as for DebugX, MMX is partially supported since
v1.18). The assembler and disassembler inform you if any instruction is
inappropriate for the current processor.
- FPU Opcodes are supported.
- The current processor (for the purposes of assembler and disassembler
warnings, above) can be declared to be something else via the following
commands:
m [x] set current processor.
x=0 current processor is 8088.
x=1..6 current processor is 80x86. 80586 is a Pentium and
80686 is a Pentium Pro.
no argument = print current CPU/FPU types.
mc [2|N] set math coprocessor.
2 = math coprocessor is a 287 (only valid if current
processor is a 386).
N = math coprocessor is absent
no argument = math coprocessor is present
- 'r register [value]' accepts 32-bit register names (for cpu 80386+).
- You can do `r cx 1234' instead of having to put the `1234' on a
separate line.
- 'rn' displays FPU register status.
Just the raw hex values of the registers are displayed, though.
- 'rx' switches among 8086 and 80386 register display (for cpu 80386+).
- When doing `debug < file', debug will not hang if it reaches an end of
file before encountering a `q' command.
- This debugger saves and restores the program's Control-C and critical
error interrupts, providing for better isolation between the debugger
and the program being debugged.
- 'tm 0|1' sets trace command mode, 0 = process INTs (default),
1 = single-step INTs (the MS-DOS Debug compatible mode).
- 'xr' allows to reallocate an EMS handle and 'xa' allows to allocate
an "empty" EMS handle (with zero pages) - but only if an EMM v4.0
is present.
- Besides 'i' and 'o' exist the variations 'iw', 'id', 'ow' and 'od'
to read/write a WORD or DWORD value from/to a port [DWORD values
require a 80386+ cpu].
- 'h' can handle dword values.
- register names can be used anywhere where a number is expected as
input. That is, things like "u cs:ip" do work.
- 'dm' displays the DOS memory control block (MCB) chain and the
current PSP.
- if DOS has set its InDOS flag, Debug will avoid to use int 21h
internally, which allows to single-step through DOS code.
- 'd', 't' and 'u' are automatically repeated if a blank line is
entered.
b. DebugX has the following additional features:
- besides real-mode applications it can debug both 16-bit and 32-bit
DPMI clients.
[To debug DPMI clients a DPMI host has to be installed. Tested with
cwsdpmi, hdpmi, WinXP NTVDM, 32rtm. In DosEmu and Windows 95/98/ME
DOS boxes it is required to single-step through the initial switch
to protected-mode, else DebugX will loose control of the debuggee.]
- 'dl': display LDT descriptor(s) (in protected-mode only)
- 'di': display interrupt vector(s)
- 'dx': display extended memory (read via Int 15h, ah=87h). Requires a
80386 cpu. 'dx' is automatically repeated if a blank line is entered.
Be aware that Int 15h, ah=87h most likely will disable "unreal"-mode
if it was enabled. This may be an unwanted side effect during debugging.
- 'rn' displays FPU registers in "readable" format.
- when running in a Windows XP DOS bos, DebugX will try to load and use
DEBXXVDD.DLL. This DLL will allow DebugX to successfully execute its
low-level disk access commands L and W in this OS - if the user has
administrator rights. DEBXXVDD.DLL is public domain, the source can
be found at http://github.com/Baron-von-Riedesel/debxxf. Please
be aware that you can do severe damage to your disk data if you use
those low-level functions without knowing what you are doing.
- 'rm' displays MMX registers.
- commandline option /2 may direct DebugX's output to a monochrome video
adapter if one is available. Useful for emulators that support this
feature ( DosBox-x or 86Box, for example ). Needs at least a VGA
compatible card, and there's no guarantee that it works as expected.
c. Optionally, one may create special-purpose versions of Debug(X).
Run file makex.bat to create them; you'll need the JWasm assembler
for a successful run. The additional versions are:
- DebugXd.COM: a version which doesn't hook int 01 and int 03. This
allows to debug this version with a "normal" version of Debug(X).
- DebugXe.COM: a version which hooks interrupts 06, 0Ch and 0Dh in
real-mode. This is for - rare - cases when such exceptions may
occur in real-mode applications.
- DebugXf.COM: a version which hooks interrupt 31h in protected-mode
and then rejects any attempt to modify exception vectors 01, 03,
0Dh and 0Eh. This allows to debug DOS-extended applications that
modify these vectors.
- DebugXg.EXE: a version that is in DOS device driver format. This
variant can be used to debug other device drivers. There are a
few peculiarities: commands N and Q are rejected, commands
R and W can't be used to read/write files and interrupt 06 (invalid
opcode exception) is hooked.
- DebugXu.COM: a version that uses "unreal" mode to access extended
memory for the DX command. This is useful to debug applications
that set/use "unreal" mode themselves, because DebugXU will not
change the current "unreal" state by its DX command.
- DebugXv.COM: a version that takes care that screen output of
debuggee and debugger are strictly separated - the new 'v'
command is provided to watch the debuggee screen while the
debugger is active. Additionally, the SysReq key is detected and
will activate the debugger. This debug version is useful to debug
fullscreen text mode programs.
- DebugB.BIN: a version that is supposed to be loaded during the
boot process, before DOS is loaded. Has to be installed in "free"
disk space ( for example, in the "reserved" sectors of a FAT partition
or behind the disk's MBR ) and must be initialized before it becomes
active. Definitely not for beginners. See "Installing and Initializing
DebugB" in DEBUG.TXT for details.
- DebugR(L).BIN: versions that runs in ring0 protected-mode. These variants
need additional software a) to be loaded and b) to handle the i/o
requests. Like DebugB.BIN no DOS is used - actually, not even the BIOS
is used - and hence these are not really "DOS debuggers" anymore. See
"DebugR Basics" in DEBUG.TXT for more details.
d. The following features of MS Debug are not implemented:
- Loading of .HEX files.
3. How to (re)build the Debug.COM/DebugX.COM Binaries
MAKE.BAT is intended for rebuilding the debugger binaries. The
recommended tool for this step is to use the JWasm assembler.
Alternatively, one can use MS Masm, but then a 16-bit OMF linker
will be needed as well (Open Watcom's WLink might do the job). Also,
the DebugRL variant cannot be created with Masm, since it contains
a 64-bit segment.
4. Files Overview
The files in this distribution are:
debug.com the executable binary.
debugx.com a DPMI aware version of Debug.
debxxvdd.dll a Win32 dll (VDD) loaded if debugx runs in a Windows XP
NTVDM "DOS box". Makes debugx's L and W command work with
sectors on this platform.
readme.txt this file.
history.txt list of changes.
debug.txt a MS-Debug tutorial originally written by Michael Webster,
extended to describe all additional features of Debug(X).
SAMPLES contains samples of 16 and 32-bit DPMI clients in Masm
syntax.
The following files, which may be distributed separately, are needed if
debug.com or debugx.com is to be rebuilt:
debug.asm Assembler source code for both debug.com and debugx.com.
Assemble with JWasm or Masm (see Make.bat).
debugtbl.inc include file for debug.asm, contains tables for Debug's
internal assembler and disassembler command.
instr.* input data for mktables.
mktables.c C program to compile the internal instruction-set
tables into source file debugtbl.inc. It's supposed to
be translated with Open Watcom's 16bit compiler WCC.
make.bat batch file to (re)create debug.com and debugx.com.
makecm.bat batch file to (re)create mktables.exe with MS C.
makecw.bat batch file to (re)create mktables.exe with OW C.
makex.bat batch file to create special versions of Debugx.
Note: files mktables.c and instr.* are to be removed in one of the next
versions of Debug. Then file debugtbl.inc is supposed to be modified
directly if new instructions are to be supported..
5. Copyright
This program is copyrighted, but feel free to distribute and use it
as you wish. For full copyright conditions, see the file debug.asm.
Paul Vojta
vojta@math.berkeley.edu