The Commodore 64 proved to be a next-level challenge for the C74-6502. C64 programmers are notorious for exploiting every nuance of the machine’s hardware in pursuit of new and exciting effects. As a result, many games and demos on this machine depend on subtle details in the CPU’s function and timing. To run the C64 software catalogue, the C74-6502 would need to replicate the operation of the MOS 6510 microprocessor in meticulous detail.
Doing so required extensive testing, and several careful modifications to the prototype boards. Below is one such modification, worked out on a scratch-pad, and used to correct the action of an NMI when interrupting an IRQ already in progress:
Despite enduring several such patches and fixes, in the end the C74-6502 was able to run a wide variety of C64 games; and the prototype boards performed like a champ!
Over seventy-five games were selected from various “best of” lists (along with several demos, utilities and cartridges) to assemble a representative test suite of both complex and popular software. The C74-6502 ran them all without a problem. (See the complete list of currently verified titles here).
For all practical purposes, the C74-6502 appears to be fully “plug-compatible” with the original NMOS 6502 and 6510 CPUs. Please feel free to suggest any games or demos which might exercise the CPU in unique ways. It will be interesting to see whether any compatibility issues remain undiscovered.