Kawari and C128-like 2MHz mode

Questions related to hardware compatibility/modding etc
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Kawari and C128-like 2MHz mode

Post by Goodwell »

This might be based on some wrong assumptions or pure incompetency on my side - if so, I apologize.

The original VIC-II doesn't only act as a video chip, it also provides the system with clock speed.

On the C128, the VIC-IIe also provides the clock for 2 MHz operation. While the video capabilities are disabled in this mode, the chip still drives the system.

So: if the Kawari was able to switch to 2 MHz mode, and the CPU was capable of running at 2 MHz (rumor has it, that 8500 CPUs most frequently and quite some 6510's can actually handle 2 MHz), would it be possible that the system was just running at 2 MHz - while still driving the rest of the system at 60 Jiffies per Second?
I guess the Kawari's graphics capabilities wouldn't need to be restricted to the same 1 MHz speed limit as the original VIC-II is.

If this is utter nonsense, please feel free to delete it. I don't want to spread any false rumors.
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Re: Kawari and C128-like 2MHz mode

Post by rrossi »

Yes, you are correct! I actually tried this a while back. I added a register that changed the clock frequency to 2Mhz. I tried this on a C64-C short board with an 8500 and made sure the CIAs were also capable of 2Mhz. But even though it appeared the CPU was active, the system would always lock up. I made sure the VIC's DRAM access was turned off (equivalent to turning off DEN bit). But since the Kawari has its own video ram ,the hires modes could still function. I must have missed something since in theory it should have worked. Maybe I will revisit it some day. But even if the CPU works, there will be other issues related to I/O. The c128 used a 'clock stretching' technique which required an extra line to the video chip to accomplish it's 2mhz mode (and keep I/O access working).
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