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cpm22 - source code for CP/M 2.2 CCP and BDOS

Hosted at the cpm22 Github repository.


Introduced in 1974, CP/M by Digital Research was one of the first microcomputer operating systems that was not tied to a single computer vendor. It could be adapted to run on almost any 8080 or Z80 microcomputer that had at least 16KB of RAM starting at address 0000h.

Originally much of CP/M was written in the PL/M programming language. With the introduction of CP/M 2.0, the command processor (CCP) and kernel (BDOS) were rewritten in 8080 assembly language.

The Digital Research ASM80 assembler allowed code to be written with mulitple assembly language instructions per line, separated by an exclamation mark ("!"). Although a semicolon (";") was used to introduce a comment, an exclamation mark in a comment would start a new instruction. There are two problems with this syntax:

  • This syntax makes the source code rather difficult to read, when one is accustomed to normal assembler syntax.

  • Few if any assemblers other than DRI's ASM80 support this syntax.

This repository contains the CP/M 2.2 CCP adn BDOS source code, reformatted to cross-assemble with Macro Assembler AS:

The source code has been verified to assemble to the exact binary present on several actual CP/M 2.2 disks, with the exception of the six-byte serial numbers present in the CCP and BDOS.

It is likely that with only minor changes, the source code could be assembled using other assemblers (native or cross).


CP/M 2.2 source code



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