Re: C64 2 Mhz Hack

From: MagerValp (
Date: 2001-02-25 21:49:37

>>>>> "gb" == g baltissen <> writes:

gb> Hallo allemaal, I have two question that possibly fits in this
gb> discussion: The SCPU uses its own memory to maintain maximum
gb> speed. But the the content has to be copied in some or another way
gb> to the original memory - how is this done? Probably DMA?

Seems reasonable.

gb> - what range is copied and how does the mechanism know?

There are "optimization" registers in the SuperCPU that determines
what ranges are copied. The default in SuperCPU V1 is to mirror every-
thing, and the default in V2 is to mirror everything but page 0 and 1.
So unless a program wants to display $0000-$01ff, it'll run without
problems. Of course, this slows down all memory writes to 1 MHz, so a
SuperCPU optimized/aware program uses the optimization registers to
only mirror the parts of memory that will actually be used for memory,
such as $4000-$7fff.

gb> We already discussed replacing a non 6502 CPU by 6502 + Hardware.
gb> I prooved it can be done. Now to find a way to be able to use fast
gb> memory.

If you have a way of intercepting write cycles to memory it shouldn't
be too hard to implement mirroring. This way the CPU will read at X
MHz and write a 1 MHz, running at full speed most of the time.

gb> This triggered another question: the 65816 can run on 20 MHz. The
gb> SIMMs SCPU uses ar mostly of the 70 ns type. 20 Mhz means 25 ns
gb> cycles. How does that fit?

Wait states. I don't have the numbers with me, but access to SuperRAM
can take several cycles.

    ___          .     .  .         .       . +  .         .      o   
  _|___|_   +   .  +     .     +         .   .  Per Olofsson, konstnär
    o-o    .      .     .   o         +
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