file extension DEB

File extension DEB description:

In 1980, Tim Paterson began working on a 16-bit OS for the 8086 S-100 Bus card he had designed for SCP (Seattle Computer Products) the previous year. To help in getting QDOS (later called 86-DOS) to work correctly, Tim created a debugger in a ROM chip; the code for that ROM version was released into the Public Domain. Later, Tim adapted the code to run as a .COM program under QDOS, and also added the ability to disassemble 8086 machine code. In the meantime, Microsoft® had been busy purchasing the rights to sell Tim's QDOS to IBM® for their 'secret' PC project. Tim was then hired by Microsoft as the primary author of their first OS. When he completed his work on IBM's Personal Computer™ DOS 1.00 in 1981, his DEBUG.COM utility was included with it. All the functionality that Tim put into DEBUG is still there and little has been added to it (the major exception being the Assemble command; added under DOS 2.0).

With the release of DOS 2.0, DEBUG gained the ability to assemble instructions directly into machine code (the A command). This is one of the most important commands for many of its users. Though lacking much of the functionality of a stand-alone Assembler, e.g., all Jumps must be to hexadecimal addresses (no labels can be used), many useful .COM programs have been assembled with this command. Under DOS 3.0, the P (Proceed) command was added, so DEBUG could quickly execute subroutines; at the same time, it became possible to attempt stepping through Interrupts with the T (Trace) command. When DOS made EMS (Expanded Memory) functions available under DOS 4.0, the four commands xa, xd, xm and xs were also added to DEBUG. It appears they were rarely, if ever used though, even by programmers. For most of us, the only noticeable change in DEBUG was the addition of the help command (type a '?' while inside DEBUG) under DOS 5.0; when all DOS commands finally got the /? command-line switch.

DEBUG's code went through a number of changes (and 'bug fixes' too) over the years! Some of these internal changes were related to DOS system calls and screen output, then there was the change in file type from a .COM to an .EXE program under DOS 5.0. But in spite of all those changes and others which followed, DEBUG has never had an official revision since 2.40 (those digits have been embedded inside all versions of DEBUG since DOS 3.0). We can only guess about the real reasons that Microsoft® never updated DEBUG to handle instructions beyond those of the Intel® 8086/8087/8088 processors. Microsoft® did create their own Assembler (MASM), 'C' compiler and Debugger (CodeView ); which you could use too, if you were willing to pay extra, so that could have been one of their reasons. Rather than using MASM and CodeView, many opted for the less expensive Borland® assembler (TASM) and Turbo™ Debugger when they appeared, or some other commercial product. However, users and students alike can still learn a great deal about Assembly language by using DEBUG.