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The Commodore 128 disabled the VIC-II chip when running at 2 MHz.
But then there'd be no video output. Simply disabling the chip is a good way to get a lot better performance.
I've toyed with a couple of ideas. The a6, a7 address lines on the VIC-II can be made to be inputs on the emulator. then with a few jumper wires to the C64 motherboard the emulator would be able to tell when the cpu is writing memory and maintain it's own copy of the memory. It could then scan the memory internally to generate the display while not using the external bus. That would speed up the system a bit because it would never need to steal bus cycles.
I note also the VIC-II has access to the R/W line so in theory it could be writing to memory. That allows for possibly an accelerator of some sort. For instance memory to memory copy.
I think I found a way to implement a 2MHz turbo mode but only for the test system. By doubling (or even tripling) the frequency of an internal clock the whole bus can be made to run twice as fast but it wouldn't work in a C64. Since there's a scan converter it doesn't matter that the VIC emulator is running too fast. Unfortunately I don't think chips on the C64 can keep up with a speed doubled bus access. I wonder just how fast the chipset in the C64 could be made to run. With heat sinks and a good power supply perhaps the system could run at a slightly faster clock rate.