Reverse-Engineered Final Cartridge III Source Code

The Final Cartridge III was one of the major multi-function extension cartridges for the Commodore 64. It contained BASIC extensions, floppy and tape speeders, centronics printer support, screen editor extensions including F-key shortcuts, a monitor, a freezer – and a GEOS-like windowing system called “Desktop”. In all this, the FC3 integrated seamlessly with the look-and-feel of the stock Commodore 64: It did not change anything (same screen colors and banner!), it only extended functionality in consistent ways.

The 64 KB ROM in the FC3 consists of 4 banks. I reverse engineered and commented the first bank, which contains the BASIC and editor extensions, the floppy and tape speeder, fast format, the centronics printer driver, and the monitor. The cc65 suite can compile the set of .s files into the exact original binary.

The project can be found at

The source is useful in many ways. Since the components are quite independent of each other, they can be extracted and used in other projects. The build system is configured so it can build a stand-alone RAM version of the monitor, for example. Another idea would be to use the source as a base for an improved version of the Final Cartridge. Components that are obsolete nowadays can be swapped with more useful functionality, for example. Refer to the README in the repository for more information.

Contributions welcome!

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