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MS-DOS Debug Commands
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 value 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] [number] unassemble U [range] write W [address] [drive] [firstsector] [number]
H o w t o u s e t h e
C O M M A N D S
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: 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.
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 D
- e ffcb d 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 CC h (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):
- a 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, xxxx:012A mov dx, xxxx:012D int 21 xxxx:012F mov ax,4c xxxx:0132 int 21 xxxx:
- g = 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, xxxx:012A BA0201 MOV DX, 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
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 test.com. 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\test.com
- l which will load the file test.com into DEBUG's Memory starting at location CS: ( 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\doswinok.com
- 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, cs :0138 mov dx, cs :013B mov ah, cs :013D int 21 cs :013F mov ax,4c cs :0142 int 21 cs :
- rcx CX 0000 : 44
- w Writing 00044 bytes [ 68 bytes in decimal ]
- q 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\choice.com 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=
DS=0ED8 ES=0ED8 SS=0ED8 CS=0ED8 IP=0100 NV UP EI PL NZ NA PO NC 0ED8:0100 E90E01 JMP 0211
Flags Register
Flag Name Set Clear Overflow(yes/no) OV NV Direction(increment/decrement) DN UP Interrupt(enable/disable) EI DI Sign(negative/positive) NG PL Zero(yes/no) ZR NZ Auxiliary carry(yes/no) AC NA Parity(even/odd) PE PO Carry(yes/no) CY NC 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. Write: W [address] [drive] [firstsector] [number]
