Using DOS DEBUG
This course makes extensive use of the DOS DEBUG command.
DEBUG can be used
to examine the contents of files, both plain and executable, and to examine
the contents of disk sectors from your disk drives.
In all cases,
knowing how to supply DEBUG with a prepared sequence of instructions and
then saving the output to a file for later printing is an essential skill.
This page contains the following sections:
For more detailed understanding of the power of DEBUG, here are some links to
general descriptions of how to use the DOS DEBUG command:
Some of the course assignments require you to capture the output of DEBUG.
Sometimes, you can use the mouse to do this; sometimes, you have to save the
output in a file for later printing and hand-in.
Cut-and-Paste under Windows 9x
If you're running DEBUG in a DOS Window under Windows 9x, you can always
use the mouse to copy text from your DOS window and paste it into another application such as
Wordpad, or Write. (Do not save the entire bitmap window as a graphic
using Print Screen - the resulting file is huge and unnecessary. Save
the text only, using cut-and-paste into another application such as
Write or Wordpad and print the text from there.)
Saving output under DOS (pure DOS)
If you're running in pure-DOS mode, without Windows running, the Print
Screen button won't work unless you have an attached text printer on your
computer, and neither will the mouse function to cut-and-paste.
Study the examples
below under the heading Writing DEBUG Scripts.
You can enter a few DOS DEBUG commands into a text file (using a program such
as DOS EDIT), and have DEBUG read the
file and execute the commands while you redirect the output into an output
file. (You can also type the commands blind into DEBUG with just the
Once you have your saved DEBUG output in a file, you can restart Windows on your computer and print the output files (using a Courier
or Terminal fixed-width font).
One of the handy things you can do with DEBUG is prepare small text files
of DEBUG commands (and even include assembler instructions) and run them
through DEBUG using command line redirection to get a quick idea of how
C:\> debug <inputcommands >outputfile
Writing Scripts for Program Debugging
See the script file debug_script.txt
and its corresponding output file debug_script_out.txt. I've edited the output
file to add comments explaining what I was doing in the script file, and how to
create the script file.
Writing Scripts for Disk Sector Debugging
If you're trying to dump disk sectors (as you do in Project 4 in this
course), you may find these kinds of DEBUG scripts, used with command line
redirection, necessary for saving your DEBUG output into files for later
printing. Here is an example of a script file that dumps the BOOT sector
and first ROOT directory sector of a floppy disk (drive A:):
Input file for DEBUG: debug_dump.txt
Output from DEBUG: debug_dump_out.txt
When you finally print some screen text, dump information, traces, or programs, you must use a Courier or Terminal fixed-width font so
that the text lines up. Do not print with a variable-width font
such as Times, Tahoma, or Arial!
Wordpad or Write let you choose a smaller font size for printing your
screen dumps, so that the text doesn't overflow the right margin and wrap
around the page.
Do not use Notepad to print screen text - the output font is almost
always too large, causing the lines to wrap.
DEBUG is able to load and let you single-step through ".COM"
format executables. The quickest way to get the executable loaded into
DEBUG is to give the program you wish to debug as a command argument.
All .COM programs start at segment offset 0100h, so that's where you'll find
your first instructions, ready to be traced:
C:> debug myprog.com
1454:0100 BE8100 MOV SI,0081
1454:0103 803C0D CMP BYTE PTR [SI],0D
1454:0106 7421 JZ 0129
1454:0108 803C20 CMP BYTE PTR [SI],20
1454:010B 7511 JNZ 011E
1454:010D BA2F01 MOV DX,012F
1454:0110 B409 MOV AH,09
1454:0112 CD21 INT 21
1454:0114 83C601 ADD SI,+01
1454:0117 803C20 CMP BYTE PTR [SI],20
1454:011A 74F8 JZ 0114
1454:011C EBE5 JMP 0103
1454:011E 8A14 MOV DL,[SI]
With your program in memory, you are now ready to debug it by
single-steping through it using the "Trace" or "Proceed"
commands, watching the flow of control and the values of the registers.
If you want to supply command line arguments to a program being debugged,
simply add them to your DOS command line when you call DEBUG:
C:> debug myprog.com argument1 argument2 arg3 ...etc...
Using "Trace" vs. "Proceed" in DEBUG
Don't use "Trace" to trace an INT instruction; use
"Proceed" instead. If you trace the INT instruction,
you will end up tracing the call to the interrupt service routine in DOS that
actually performs the interrupt service, and that can be many hundreds or
thousands of instructions! The "Proceed" instruction will not
trace the interrupt service routine; it will simply let it execute (without
tracing each instruction) and it will pause when the service routine returns
to your program.