A Quite Obscure C128 Video Mode Hack

30 years ago, [Dave] found himself up a C128D creek without a paddle. His main monitor was on the fritz, and he needed to use his C128D in 40-column mode to run old C64 programs for development purposes. Normally this is only possible through the low quality composite out, but no composite monitor was available. Needs must, so he got to coding a workaround that would allow the C128D to output in 40 column mode through the higher-quality RGBI output. 

It’s a proper old-school hack in the spirit of the 8-bit era. The C64 ROM is copied into RAM, where it’s then modified to instead update a 40-column image that’s sent to the RGBI display hardware. The original C64 character ROM is also copied over to ensure everything displays correctly.

It’s not bulletproof, and a few pokes to the wrong memory locations have a high likelihood of crashing the system, seeing as the ROM is now in RAM. However, it does allow the user to enable FAST mode and use all the C128 extended keys. [Dave] recommends experimenting in an emulator first, lest you scare your vintage monitor with angry signals it can’t understand.

The C128 was Commdore’s last 8-bit computer on the market, and there’s a heck of a story behind its creation.