CP/M for the Commodore 64

by Dave McMurtrie · December 21, 2021

CP/M was an operating system written by Gary Kildall, the founder of Digital Research, Inc, in 1974 which enjoyed early widespread popularity. As early as 1979, Commodore’s VP of Marketing, Dennis Barnhart, proposed CP/M compatibility for the Commodore PET. That discussion never materialized because CP/M was designed around the Z80 CPU and Commodore’s systems were all based around the MOS 6502.

Commodore 64 CP/M cartridge box

CP/M for the Commodore 64

When the Commodore 64 was being developed, Kit Spencer proposed that CP/M should be made available (in addition to the built-in BASIC) for Commodore’s newest computer to counter any possible negatives that competing computer manufacturers might put forward, and to be able to release with an extensive pre-existing software library to drive sales. Commodore began a project to create a Z80-based CP/M cartridge for the Commodore 64. Robert Russell assigned the project to an engineer dubbed “Shooting Star”, and gave him the Apple II CP/M schematic and documentation as a starting point. There were considerable problems trying to get the cartridge to work at all, and the project was significantly delayed.

US Federal Trade Commission order

When the Commodore 64 was released in August, 1982, the CP/M cartridge was not yet available, but all of Commodore’s marketing materials including the box the c64 came in stated that a CP/M option was available. This eventually caught the attention of the United States Federal Trade Commission who ordered Commodore to cease advertising the CP/M cartridge until it was available for public sale in sufficient quantities.

Commodore 64 retail packaging with the “CP/M Option” marketing text highlighted.

CP/M release

The CP/M cartridge was eventually released in the summer of 1983, just about a year after the Commodore 64 hit the market. The cartridge apparently only works with earlier C64 board revisions due to slight timing differences in later models, but I don’t know the details about which boards work. Regardless, the Commodore 64 became so wildly popular in its own right that the allure of having the library of CP/M software available for it was diminished.