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Debug is a program that comes with modern versions of DOS (I do
not know when I started shipping out with DOS). Anyway, all Windows
users should have it already.
It's a great tool for debuging programs, unassembling and cracking,
and reading "hidden" memory areas like the boot sector, and much
more.
The following was copied from an assembly tutorial who's author
we cannot credit, because we have no idea who he is.
Get into DOS and type "debug", you will get a prompt like this:
-
now type "?", you should get the following response:
| assemble |
A [address] |
| compare |
C range address |
| dump |
D [range] |
| enter |
E address [list] |
| fill |
F range list |
| go |
G [=address] [addresses] |
| hex |
H value1 value2 |
| input |
I port |
| load |
L [address] [drive] [firstsector] [number] |
| move |
M range address |
| name |
N [pathname] [arglist] |
| output |
O port byte |
| proceed |
P [=address] [number] |
| quit |
Q |
| register |
R [register] |
| search |
S range list |
| trace |
T [=address] [value] |
| unassemble |
U [range] |
| write |
W [address] [drive] [firstsector] [number] |
| allocate expanded memory |
XA [#pages] |
| deallocate expanded memory |
XD [handle] |
| map expanded memory pages |
XM [Lpage] [Ppage] [handle] |
| display expanded memory status |
XS |
Lets go through each of these commands:
Assemble:
-a
107A:0100
At this point you can start assembling some programs, just like
using a assembler. However the debug assembler is very limited as
you will probably notice. Lets try to enter a simple program:
-a
107A:0100 MOV AH,02
107A:0102 MOV DL,41
107A:0104 INT 21
107A:0106 INT 20
-g
A
Program terminated normally
That's the same program we did at the end of the previous chapter.
Notice how you run the program you just entered with "g", and also
notice how the set-up part is not there? That's because debug is
just too limited to support that.
Another thing you can do with assemble is specify the address at
which you want to start, by default this is 0100 since that's where
all .COM files start.
Compare:
Compare takes 2 block of memory and displays them side by side,
byte for byte. Lets do an example. Quite out of debug if you haven't
already using "q". Now type "debug c:\command.com"
-c 0100 l 8 0200
10A3:0100 7A 06 10A3:0200
This command compared offset 0100 with 0200 for a length of 8 bytes.
Debug responded with the location that was DIFFERENT. If 2 locations
were the same, debug would just omit them, if all are the same debug
would simply return to the prompt without any response.
Dump:
Dump will dump a specified memory segment. To test it, code that
assembly program again:
C:\>debug
-a
107A:0100 MOV AH,02
107A:0102 MOV DL,41
107A:0104 INT 21
107A:0106 INT 20
-d 0100 l 8
107A:0100 B4 02 B2 41 CD 21 CD 20
...A.!.
The "B4 02 B2 41 CD 21 CD 20" is the program you just made in machine
language.
B4 02 = MOV AH,02
B2 41 = MOV DL,41
CD 21 = INT 21
CD 20 = INT 20
The "...A.!." part is your program in ASCII. The "." represent
non-printable characters. Notice the A in there.
Enter:
This is one of the hard commands. With it you can enter/change
certain memory areas. Lets change our program so that it prints
a B instead of an A.
| -e 0103 |
<-- edit program at segment 0103 |
| 107A:0103 41.42 |
<-- change 41 to 42 |
| -g |
| B |
Program terminated normally
-
Wasn't that amazing?
Fill:
This command is fairly useless, but who knows....
It fills the specified amount of memory with the specified data.
Lets for example clear out all memory from segment 0100 to 0108,
which happens to be our program.
| -f 0100 l 8 0 |
<-- file offset 0100 for a length of 8 bytes with 0 |
| -d 0100 l 8 |
<-- verify that it worked |
| 107A:0100 00 00 00 00 00 00 00 00 |
....... |
| Yep, it worked. |
Go:
So far we used go (g) to start the program we just created. But
Go can be used for much more. For example, lets say we want to execute
a program at 107B:0100:
| -r CS |
<-- set the CS register to point to 107B |
| CS 107A |
| :107B |
| -g =100 |
You can also set breakpoints.
| -a |
<-- enter our original program so we have something |
| 107A:0100 MOV AH,02 |
to work with |
| 107A:0102 MOV DL,41 |
| 107A:0104 INT 21 |
| 107A:0106 INT 20 |
|
| -g 102 |
<-- set up a break point at 107A:0102 |
At this point the program will stop, display all registers and
the current instruction.
Hex:
This can be very useful. It subtracts and adds two hexadecimal
values:
| -h 2 1 |
| 0003 0001 |
<-- 2h + 1+ = 3h and 2h - 1h = 1h |
This is very useful for calculating a programs length, as you will
see later.
Input:
This is one of the more advanced commands, and I decided not to
talk about it too much for now. It will read a byte of data from
any of your computers I/O ports (keyboard, mouse, printer, etc).
-i 3FD
60
-
Your data may be different.
In case you want to know, 3FD is Com port 1, also known as First
Asynchronous Adapter.
Load:
This command has 2 formats. It can be used to load the filename
specified with the name command (n), or it can load a specific sector.
-n c:\command.com
-l
This will load command.com into debug. When a valid program is
loaded all registers will be set up and ready to execute the program.
The other method is a bit more complicated, but potential also more
usefull. The syntax is
L <address> <drive letter/> <sector> <amount
to load>
-l 100 2 10 20
This will load starting at offset 0100 from drive C (0 = A, 1 =
B, 2 = C, etc), sector 10h for 20h sectors. This can be useful for
recovering files you deleted.
Move:
Move takes a byte from the starting address and moves it to the
destination address. This is very good to temporary move data into
a free area, than manipulate it without having to worry about affecting
the original program. It is especially useful if used in conjunction
with the r command to which I will get later. Lets try an example:
| -a |
<-- enter our original program so we have something |
| 107A:0100 MOV AH,02 |
to work with |
| 107A:0102 MOV DL,41 |
| 107A:0104 INT 21 |
| 107A:0106 INT 20 |
| -m 107A:0100 L 8 107B:0100 |
<-- more 8 bytes starting from 107A:0100 into 107B:0100 |
| -e 107B:0103 |
<-- edit 107B:0103 |
| 107B:0103 41.42 |
<-- and change it 42 (B) |
| -d 107A:0100 L 8 |
<-- make sure it worked |
| 107A:0100 B4 02 B2 41 CD 21 CD 20 |
...A.!. |
| -d 107B:0100 L 8 |
| 107A:0100 B4 02 B2 42 CD 21 CD 20 |
...B.!. |
| -m 107B:0100 L 8 107A:0100 |
<-- restore the original program since we like the changes. |
Name:
This will set debug up with a filename to use for I/O commands.
You have to include the file extension, and you may use addition
commands:
-n c:\command.com
Output:
Exactly what you think it is. Output sends stuff to an I/O port.
If you have an external modem with those cool lights on it, you
can test this out. Find out what port your modem is on and use the
corresponding hex number below:
Com 1 = 3F8 - 3FF (3DF for mine)
Com 2 = 2F8 - 2FF
Com 3 = ??? - ??? (if someone knows, please let me know)
Now turn on the DTA (Data Terminal Ready) bit by sending 01h to
it:
As soon as you hit enter, take a look at your modem, you should
see a light light up. You can have even more fun with the output
command. Say someone put one of those BIOS passwords on "your" computer.
Usually you'd have to take out the battery to get rid of it, but
not anymore:
MI/AWARD BIOS
-o 70 17
-o 71 17
QPHOENIX BIOS
-o 70 FF
-o 71 17
QGENERIC
-o 70 2E
-o 71 FF
These commands will clear the BIOS memory, thus disabling the password.
Proceed:
Proceeds in the execution of a program, usually used together withy
Trace, which I will cover later. Like the go command, you can specify
an address from which to start
using =address
-p 2
Debug will respond with the registers and the current command to
be executed.
Quite:
This has got to be the most advanced feature of debug, it exits
debug!
-q
Register:
This command can be used to display the current value of all registers,
or to manually set them. This is very useful for writing files as
you will see later on.
-r AX
AX: 011B
:5
-
Search:
Another very useful command. It is used to find the occurrence
of a specific byte, or series of bytes in a segment. The data to
search for can by either characters, or a hex value. Hex values
are entered with a space or comma in between them, and characters
are enclosed with quotes (single or double). You can also search
for hex and characters with the same string:
| -n c:\command.com |
<-- load command.com so we have some data to search in |
| -l |
| -s 0 l 0 "MS-DOS" |
<-- search entire memory block for "MS-DOS" |
| 10A3:39E9 |
<-- found the string in 10A3:39E9 |
NOTE: the search is case sensitive!
Trace:
This is a truly great feature of debug. It will trace through a
program one instruction at a time, displaying the instruction and
registers after each. Like the go command you can specify where
to start executing from, and for how long.
| -a |
<-- yes, this thing again |
| 107A:0100 MOV AH,02 |
| 107A:0102 MOV DL,41 |
| 107A:0104 INT 21 |
| 107A:0106 INT 20 |
| -t =0100 8 |
If you leave out the amount of instructions that you want to trace,
you can use the proceed (p) to continue the execution as long as
you want.
Unassemble:
Unassembles a block of code. Great for debugging (and cracking)
| -u 100 L 8 |
<-- unassembles 8 bytes starting at offset 100 |
| 107A:0100 MOV AH,02 |
<-- debut's response |
| 107A:0102 MOV DL,41 |
| 107A:0104 INT 21 |
| 107A:0106 INT 20 |
Write:
This command works very similar to Load. It also has 2 ways it
can operate: using name, and by specifying an exact location. Refer
to back to Load for more information.
NOTE: The register CX must be set the file size in order to write!
NOTE: Write will not write .EXE or .HEX files.
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