Re: C64 2 Mhz Hack

From: Nick Coplin (
Date: 2001-02-24 17:46:34

>> In other words, it ran the 6502 at 4 MHz for 2
>> cycles and at 0 MHz while
>> the video chip was doing its transparent DMA.  In
>> this way, the computer
>> ran at 2 MHz with the screen turned on.
>> Another possibility is to use a C128 video chip to
>> produce the exact
>> equivalent of the C128 "2 MHz mode" on a C64.
>Wow, I didn't know you could do that!

Not sure that you can.... I've been trying to do a couple of things along these lines and it seems the 8502 is right on the limit for rating at 2MHz. The two tricks I've tried are:
1) thenNTSC /PAL clock divider change to give 30% improvement.  This works when cold, but not for long.

2) using the 8MHz dot and glue logic including the IOACC line from the MMU to try the clock multiplying trick.  Doesn't work either (locks up)

In both cases the RAM has been upgraded to the faster 41256 chips I could find (100ns).

As an interesting side effect of 2) I came up with a clock circuit that can slow down the CPU to 0.5MHz... not much use?

The only way to use the 2MHz mode seems to be in the borders with a patched Kernal (there is also a Patched GEOS Kernal out on the net which which will run 2MHz in the borders)

- Nick

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