The story
This cartridge put a Zilog Z80 processor into a Commodore 64 so it could run CP/M 2.2, the business operating system of the day. The idea was to give C64 owners the large library of CP/M software. In practice it was let down by the slow 1541 drive, the 40-column screen and compatibility problems with some C64 board revisions, and it sold poorly. Commodore learned the lesson: the C128 put a Z80 and CP/M 3.0 inside the computer itself.