MS-DOS Debug Commands






range address






address [list]



range list



[=address] [addresses]



value1 value2






[address] [drive] [firstsector] [number]



range address



[pathname] [arglist]



port byte



[=address] [number]








range list



[=address] [number]






[address] [drive] [firstsector] [number]

H o w t o u s e t h e

P a r a m e t e r s

NOTE: Parameters listed in brackets ( [ ] ) are optional. Optional parameters usually indicate there are a number of different ways that a command can be used. I've listed the meanings of all the parameters here for you:

address - Memory location specified in hexadecimal. You can use either a simple Offset all by itself (in which case, the present CS 'Code Segment' will be assumed), or you can enter the full Segment:Offset location using either all hex numbers or substituting the name of a segment register for a number. Leading zeros are not required; thus 1F all by itself would be the location 'CS:001F' ( CS meaning whatever the CS happened to be at the time you entered this ). Examples:

100 DS:12 SS:0 198A:1234

range - Two hexadecimal addresses separated by a single space. They may be listed as either full Segment:Offset pairs or just an Offset alone ( in which case, the Segment is assumed to be that of the present CS or "Code Segment" ). NOTE: Some commands, such as Compare (C), may require that the second address be given only as an offset.

list - A string of Hexadecimal bytes separated by a space, or ASCII data enclosed within single or double quote marks. You can list any number of bytes from a single one up whatever number fits on the line before having to press the Enter key. A single byte, such as 00 is most often used with the FILL (f) command whereas an ENTER (e) command will most likely have a string of many hex bytes or ASCII characters per line; for example:

e 100 31 C0 B4 09 BA 50 02 CD 21 B8 4C 00 CD 21

e 250 'This is an ASCII data string.$'

number - Remember that all numbers and values used in any DEBUG commands are understood as being Hexadecimal only! That includes the number of sectors in the LOAD or WRITE commands and even the number of instructions you want DEBUG to step through in the TRACE or PROCEED commands. It's all HEX all the time in here!

A Simple DEBUG Tutorial

Details of each Command

NOTE: In the Examples below, commands which are entered by a user are shown in bold type; data displayed in response by DEBUG is in normal type. DEBUG (from MS-DOS 5.0 or later) will display the following usage message, if you enter debug /? at a DOS prompt:

C:\WINDOWS>debug /?

Runs Debug, a program testing and editing tool.

DEBUG [[drive:][path]filename [testfile-parameters]]

[drive:][path]filename Specifies the file you want to test.

testfile-parameters Specifies command-line information required by

the file you want to test.

Quit: Q

Immediately quits (exits) the Debug program! No questions ever asked... should be the first command you remember along with the "?" command.

Hex: H value1 value2

A very simple (add and subtract only) Hex calculator. Never forget that all numbers inside of DEBUG are always Hexadecimal. Enter two Hex values (no more than four digits each) and DEBUG shows first the SUM, then the DIFFERENCE of those values. Examples:

-h aaa 531 -h fff 3 -h dbf ace

0FDB 0579 1002 0FFC 188D 02F1

Dump: D [range]

D [address] [length]

Displays the contents of a block of memory. The Memory locations near the beginning of Segment C000 (even under Windows 2000/XP) should display information about the kind of video card installed on your PC. This first example shows we have a Matrox card in this system.


-d c000:0010

C000:0010 24 12 FF FF 00 00 00 00-60 00 00 00 00 20 49 42 $.......`.... IB

C000:0020 4D 20 43 4F 4D 50 41 54-49 42 4C 45 20 4D 41 54 M COMPATIBLE MAT

C000:0030 52 4F 58 2F 4D 47 41 2D-47 31 30 30 20 56 47 41 ROX/MGA-G100 VGA

C000:0040 2F 56 42 45 20 42 49 4F-53 20 28 56 31 2E 32 20 /VBE BIOS (V1.2

C000:0050 29 00 87 DB 87 DB 87 DB-87 DB 87 DB 87 DB 87 DB )...............

C000:0060 50 43 49 52 2B 10 01 10-00 00 18 00 00 00 00 03 PCIR+...........

C000:0070 40 00 12 10 00 80 00 00-38 37 34 2D 32 00 FF FF @.......874-2...

C000:0080 E8 26 56 8B D8 E8 C6 56-74 22 8C C8 3D 00 C0 74 .&V....Vt"..=..t


Search: S range list

Searches within a range of addresses for a pattern of one or more byte values given in a list. The list can be comprised of numbers or character strings enclosed by matching single or double quote marks. Examples:

-s fe00:0 ffff "BIOS"



-d fe00:0

FE00:0000 41 77 61 72 64 20 53 6F-66 74 77 61 72 65 49 42 Award SoftwareIB

FE00:0010 4D 20 43 4F 4D 50 41 54-49 42 4C 45 20 34 38 36 M COMPATIBLE 486

FE00:0020 20 42 49 4F 53 20 43 4F-50 59 52 49 47 48 54 20 BIOS COPYRIGHT

FE00:0030 41 77 61 72 64 20 53 6F-66 74 77 61 72 65 20 49 Award Software I

FE00:0040 6E 63 2E 6F 66 74 77 61-72 65 20 49 6E 63 2E 20 nc.oftware Inc.

FE00:0050 41 77 03 0C 04 01 01 6F-66 74 77 E9 12 14 20 43 Aw.....oftw... C

FE00:0060 1B 41 77 61 72 64 20 4D-6F 64 75 6C 61 72 20 42 .Award Modular B

FE00:0070 49 4F 53 20 76 34 2E 35-31 50 47 00 DB 32 EC 33 IOS v4.51PG..2.3

Compare: C range address

Compares two blocks of memory. If there are no differences, then DEBUG simply displays another prompt (-). Here's an example of what happens when there are differences:

-c 140 148 340

127D:0143 30 6D 127D:0343

127D:0146 10 63 127D:0346

127D:0148 49 30 127D:0348

The bytes at locations 140 through 148 are being compared to those at 340 (through 348, implied); the bytes are displayed side by side for those which are different (with their exact locations, including the segment, on either side of them).

Fill: F range list

This command can also be used to clear large areas of Memory as well as filling smaller areas with a continuously repeating phrase or single byte. Examples:

-f 100 12f 'BUFFER'

-d 100 12f

xxxx:0100 42 55 46 46 45 52 42 55-46 46 45 52 42 55 46 46 BUFFERBUFFERBUFF

xxxx:0110 45 52 42 55 46 46 45 52-42 55 46 46 45 52 42 55 ERBUFFERBUFFERBU

xxxx:0120 46 46 45 52 42 55 46 46-45 52 42 55 46 46 45 52 FFERBUFFERBUFFER

Enter: E address [list]

Used to enter data or instructions (as machine code) directly into Memory locations. Example: First we'll change a single byte at location CS:FFCB from whatever it was before to D2

-e ffcb d2

The next two examples show that either single(') or double(") quote marks are acceptable for entering ASCII data. By allowing both forms, you can include the other type of quote mark within your entry string:

-e 200 'An "ASCII-Z string" is always followed by '

-e 22a "a zero-byte ('00h')." 00

Go: G [=address] [addresses]

Go is used to run a program and set breakpoints in the program's code. As we saw in an Example for the ENTER command, the '=address' option is used to tell DEBUG a starting location. If you use 'g' all by itself, execution will begin at whatever location is pointed to by the CS:IP registers. Optional breakpoints ( meaning the program will HALT before executing the code at any of these locations) of up to any ten addresses may be set by simply listing them on the command line.

Requirements: Breakpoints can only be set at an address containing the first byte of a valid 8088/8086 Opcode. So don't be surprised if picking some arbitrary address never halts the program; especially if you're trying to DEBUG a program containing opcodes DEBUG can't understand (that's anything 'requiring' a CPU above an 8088/8086)!

CAUTION: DEBUG replaces the original instructions of any listed breakpoint addresses with CCh (an INT 3). The instructions at these locations are restored to their originals ONLY if one of the breakpoints is encountered... If DEBUG does not HALT on any breakpoint, then all your breakpoints are still enabled! So, don't ever save the code as is, unless you're sure that DEBUG has hit one of your breakpoints! ( Saving to a backup copy before using breakpoints is often a better way. )

Assemble: A [address]

Creates machine executable code in memory beginning at CS:0100 (or the specified address) from the 8086/8088 (and 8087) Assembly Language instructions which are entered. Although no Macro instructions nor labels are recognized, you can use the pseudo-instructions 'DB' and 'DW' (so you can use the DB opcode to enter ASCII data like this: DB 'This is a string',0D,0A ).
The 'A' command remembers the last location where any data was assembled, so successive 'A' commands (when no address is specified) will always begin at the next address in the chain of assembled instructions. This aspect of the command is similar to the Dump command which remembers the location of its last dump (if no new address is specified).

The assembly process will stop after you ENTER an empty line.

Example (you enter the characters in bold type):


xxxx:0100 jmp 126

xxxx:0102 db 0d,0a,'This is my first DEBUG program!'

xxxx:0123 db 0d,0a,'$'

xxxx:0126 xor ax,ax

xxxx:0128 mov ah,9

xxxx:012A mov dx,102

xxxx:012D int 21

xxxx:012F mov ax,4c

xxxx:0132 int 21


-g =100

This is my first DEBUG program!

Program terminated normally


Unassemble: U [range]

Disassembles machine instructions into 8086 Assembly code. Without the optional [range], it uses Offset 100 as its starting point, disassembles about 32 bytes and then remembers the next byte it should start with if the command is used again. ( The word 'about' was used above, because it may be necessary to finish with an odd-number of bytes greater than 32, depending upon the last type of instruction DEBUG has to disassemble.

NOTE: The user must decide whether the bytes that DEBUG disassembles are all 8086 instructions, just data or any of the newer x86 instructions (such as those for the 80286, 80386 on up to the latest CPU from Intel; which are all beyond the ability of DEBUG to understand)! Example:

-u 126 133

xxxx:0126 31C0 XOR AX,AX

xxxx:0128 B409 MOV AH,09

xxxx:012A BA0201 MOV DX,0102

xxxx:012D CD21 INT 21

xxxx:012F B84C00 MOV AX,004C

xxxx:0132 CD21 INT 21


Input: I port

The use of I/O commands while running Windows™9x/Me is just plain unreliable! This is especially true when trying to directly access hard disks! Under Win NT/2000/XP, the I/O commands are only an emulation; so don't trust them. Though the example below still works under Win2000/XP, it's most likely using some WinAPI code to show what's in the Windows clock area; not directly from an RTC chip.

Long ago (when DOS was the only OS for PCs), there were dozens of BASIC programs that used I/O commands for handling tasks through parallel and serial ports (e.g., to change the font used by a printer or values in a modem's control registers). Under real DOS, they can still be used for direct communications with keyboards or a floppy drive's control chips along with many other hardware devices.
Here's an example of how to read the hours and minutes from a computer's "real time clock" (RTC):

-o 70 04 <-- Check the hours.

-i 71

18 <----- 18 hours (or 6 p.m.)

-o 70 02 <-- Check the minutes.

-i 71

52 <----- 52 minutes

Output: O port byte

See comments under the Input command.


L [address] [drive] [firstsector] [number] or program! (See the N command for more on this)

This command will LOAD the selected number of sectors from any disk's Logical Drive under the control of MS-DOS or Windows into Memory. The address is the location in Memory the data will be copied to (use only 4 hex digits to keep it within the memory allocated to DEBUG), the drive number is mapped as: 0=A:, 1=B:, 2=C:, etc., firstsector counts from ZERO to the largest sector in the volume and finally number specifies in hexadecimal the total number of sectors that will be copied into Memory (so a floppy disk with 0 through 2,879 sectors would be: 0 through B3F in Hex).

Move: M range address

This command should really be called: COPY (not Move) as it actually copies all the bytes from within the specified range to a new address.


1) -m 7c00 7cff 600

Copies all the bytes between Offset 7C00 and 7CFF (inclusive) to Offset 0600 and following...

2) -m 100 2ff 70

This second example shows that it's very easy to overwrite most of the source you're copying from using the Move command. Apparently, DEBUG stores the source bytes elsewhere before writing them; otherwise, this example would cause a problem when it started overwriting what it hadn't copied yet! This copies the 512 bytes between Offsets 100h and 2FFh (inclusive) to Offset 0070 overwriting the first 368 bytes in the process.

Name: N [pathname] [arglist]

This command can be used to load files into DEBUG's Memory after you have started the program, but it's main function is to create a new file under control of the Operating System which DEBUG can WRITE data to.

Normally, when you want to 'debug' a file, you'd start DEBUG with a command like this: C:\WINDOWS>debug . But it's also possible to load a file into DEBUG's Memory from within DEBUG itself by using the 'N' command and then the 'L' command (with no parameters) like this:
-n c:\temp\
which will load the file into DEBUG's Memory starting at location CS:0100 (you cannot specify any other location when using the L command like this!).

The 'N' command makes it quite easy to save data or an Assembly program created in DEBUG to a file on your hard drive!
For example, these commands (
in bold; along with DEBUG's reponses):

-n c:\temp\
a 100
cs:0100 jmp 138
cs:0102 db 0d,0a,"It's OK to run this "
cs:0118 db "program under DOS or Windows!"
cs:0135 db 0d,0a,24
cs:0138 mov dx,102
cs:013B mov ah,9
cs:013D int 21
cs:013F mov ax,4c01
cs:0142 int 21
CX 0000
Writing 00044 bytes [ 68 bytes in decimal ]

will create a 68-byte file called DOSWINOK.COM in the C:\TEMP folder; even when running DEBUG in a DOS-window. The file names, however, are still limited to DOS's eight characters plus three for the extension (an 8.3 filename as it's often called)!

Homework: Follow the steps above to Assemble and save this program under DEBUG, then use DEBUG to debug it! Use the P(roceed) command to step through most of the instructions, since this will keep you from accidentally stepping into an INT(errupt) instruction! If you ever do use the T(race) command on an INT, you'll end up inside nests of BIOS routines which often crashes DEBUG!

Register: R [register]

Entering ' r ' all by itself will display all of the 8086 register's contents and the next instruction which the IP register points to in both machine code and an unassembled (Assembly Language) form. For example, if you start DEBUG in a Windows 95B DOS-box with the command line:
>debug c:\windows\command\
and then enter an ' r ' at the first DEBUG prompt, DEBUG will display someting similar to this:

AX=0000 BX=0000 CX=1437 DX=0000 SP=FFFE BP=0000 SI=0000 DI=0000


0ED8:0100 E90E01 JMP 0211

Flags Register

Flag Name


















Auxiliary carry(yes/no)









Trace: T [=address] [number]

The T command is used to trace (step through) CPU instructions one at a time. If you enter the T command all by itself, it will step through only ONE instruction beginning at the location specified by your CS:IP registers, halt program execution and then display all the CPU registers plus an unassembled version of the next instruction to be executed; this is the 'default' mode of the TRACE command. Say, however, you wanted DEBUG to trace and execute seven instructions beginning at address CS:0205; to do so, you would enter:

-t =205 7

Remember that the value for the number of instructions to execute must be given in hexadecimal just as all other values used in DEUBG. (Since the T command uses the "hardware trace mode" of the CPU, it's possible to step through instructions in a ROM - Read Only Memory - chip.)

Proceed: P [=address] [number]

Proceed acts exactly the same as Debug's T (Trace) command for most types of instructions... EXCEPT: Proceed will immediately execute ALL the instructions (rather than stepping through each one) inside any Subroutine CALL, a LOOP, a REPeated string instruction or any software INTerrupts. This means that you do not have to single-step through any of the code contained in a Subroutine or INT call if you use the Proceed (P) command. This means Proceed will be the command you use most often to debug programs, and Trace will only be used to step into a Subroutine or possibly check the logic of the first few iterations of a LOOP or REP instruction.


W [address] [drive] [firstsector] [number]


Do NOT experiment with the W - write command in DEBUG. It can be used effectively to create new files on your hard drive, but only if you use it properly. Trying to write directly to a sector on a hard disk would very RARELY be considered proper use of this command!

Trying to write directly to a hard disk using sector numbers will most likely result in loss of data or even a corrupted hard drive!

The WRITE (W) command is often used to save a program to your hard disk from within DEBUG. But the only safe way to do so, especially under Windows, is by allowing the OS to decide where to physically create that file on the disk. This is done by first using the Name (N) command to set up an optional path and filename for the new file (or to overwrite one that already exists). DEBUG will automatically begin saving program or data bytes from Offset 0100 of the 64 KiB Segment that the OS allocated for it. The only other requirement is to set the size of the file you wish to write by placing the total number of bytes in the combined BX and CX registers* before executing the WRITE command. The Register command is used to change the value in the CX register.