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Debug (command)

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Original author(s)
  
Tim Paterson

Type
  
Debugger

Developer(s)
  
Microsoft

Website
  
Debug

Debug (command)

Operating system
  
DOS, MS-DOS, OS/2, Microsoft Windows

License
  
Proprietary commercial software

debug is a command in DOS, OS/2 and Microsoft Windows (only in 32bit) which runs the program debug.exe (or DEBUG.COM in older versions of DOS). 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.

Contents

Background

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 Microsoft Windows. DOS debug has several limitations:

  • It can only access 16-bit registers and not 32-bit extended registers.
  • When the "n" subcommand for naming files is used, the filename is stored from offset DS:5D to DS:67 (the Program Segment Prefix File Control Block area), meaning that the program can only save files in FAT 8.3 filename format.

    • Enhanced DEBUG package includes a 32-bit clone "DEBUGX" version supporting 32-bit DPMI programs as well. 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.

    Syntax

    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. 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. This method can be used to create or edit binary files from batch files.

    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.

    References

    Debug (command) Wikipedia
    (Text) CC BY-SA