Debug (command)

debug is a command in operating systems such as DOS, OS/2 and Microsoft Windows (only in 32bit[1]) which runs the program debug.exe (or DEBUG.COM in DOS version 4.x and older).[2]

Original author(s)Tim Paterson
Developer(s)Microsoft, IBM, Paul Vojta
Operating systemDOS, MS-DOS, OS/2, Windows
LicenseMS-DOS, FreeDOS: MIT
86-DOS, OS/2, Windows: Proprietary commercial software

Debug can act as an assembler, disassembler, or hex dump program allowing users to interactively examine memory contents (in assembly language, hexadecimal or ASCII), make changes, and selectively execute COM, EXE and other file types. It also has several subcommands which are used to access specific disk sectors, I/O ports and memory addresses.


Traditionally, all computers and operating systems have included a maintenance function, used to determine whether a program is working correctly. Debug was written by Tim Paterson to serve this purpose in QDOS. When Paterson began working for Microsoft in the early 1980s he brought the program with him. Debug was part of DOS 1.00 and has been included in MS-DOS and certain versions of Microsoft Windows. DOS debug has several limitations:

Windows XP and later versions include debug and other 16-bit commands (nonnative) for the MS-DOS subsystem to maintain MS-DOS compatibility. The 16-bit MS-DOS subsystem commands are not available on 64-bit editions of Windows.[3]

Enhanced DEBUG package includes a 32-bit clone "DEBUGX" version supporting 32-bit DPMI programs as well.[4] Andreas "Japheth" Grech, the author of the HX DOS extender, developed enhanced DEBUG versions 0.98…1.25, and former PC DOS developer Vernon Brooks added versions 1.26…1.32.[5]

The operating systems Intel ISIS-II[6] and iRMX 86,[7] DEC TOPS-10[8] and TOPS-20[9], THEOS/OASIS,[10] Zilog Z80-RIO,[11] Stratus OpenVOS,[12] PC-MOS,[13] and AROS[14] also provide a debug command. The FreeDOS version was developed by Paul Vojta.[15]


debug [[drive:][path] filename [parameters]]

When Debug is started without any parameters the Debug prompt, a "-" appears. The user can then enter one of several one or two-letter subcommands, including "a" to enter the assembler mode, "d" to perform a hexadecimal dump, "t" to trace and "u" to unassemble (disassemble) a program in memory.[16] Debug can also be used as a "debug script" interpreter using the following syntax.

debug < filename

A script file may contain Debug subcommands and assembly language instructions.[17] This method can be used to create or edit binary files from batch files.[2]

Using for non-debugging purposes

The Debug utility was useful for editing binary files in an environment where only MS-DOS is installed without anything else. It can also be used to edit disk sectors, which was one method of removing boot-sector viruses in the MS-DOS era.

See also


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