How the Final Cartridge III Freezer Works
19 hours ago
- #Retro Computing
- #Commodore 64
- #Hardware Hacking
- Freezer cartridges enhanced the Commodore 64 by allowing users to pause programs, apply cheats, and save the computer's state.
- Ultimax mode, a feature from the failed Commodore Ultimax console, is crucial for freezer cartridges to function on the C64.
- The Final Cartridge III (FC3) uses a combination of NMI interrupts and Ultimax mode to take control of the C64 when the freeze button is pressed.
- FC3's freeze process involves a delay before activating Ultimax mode to avoid memory access issues during instruction completion.
- The FC3 firmware initializes the freezer by finding and using compressible memory sections for temporary storage and register backups.
- Displaying the freezer menu leverages the VIC-II's invalid bitmap mode and Ultimax mode to show graphics without using C64 RAM.
- Backups are created by compressing memory with RLE and saving to disk, with a loader that restores the system state upon loading.
- The FC3's game trainer modifies memory reads of CIA and VIC-II registers to inject cheat codes, disabling disk access in the process.
- Screenshot functionality in FC3 supports various printers and graphics modes, including sprites, with a preview feature.
- The FC3's design reflects deep understanding of the C64's hardware, achieved without modern tools or extensive documentation.