Re: C64 2 Mhz Hack

From: Larry Anderson (
Date: 2001-02-25 21:33:15 wrote:
> Hallo allemaal,
> I have two question that possibly fits in this discussion:
> The SCPU uses its own memory to maintain maximum speed. But the the content
> has to be copied in some or another way to the original memory
> - how is this done? Probably DMA?

I recall the CMD page has a Theroy of Operation file on-line for the
SCPU for reading...  It might contain that info, my instruction book
does not mention how that works.

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

There are settings called 'optimation modes' which you set to determine
which memory areas are mirred back to the 64 (beyond the chip
registers), these are usually done for the sake of the video hardware -
at best optimization only the minimum neccesary is copied back:
0400-0700 (screen memory) the rest is  ran solely off the SCPU.

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

Isn't there an issue of bus speed?  (I'm not a hardware person but in
previous articles I recall that everything - chips, memory, I/O - is
geared for 1mghz.)

> This triggered another question: the 65816 can run on 20 MHz. The SIMMs
> SCPU uses ar mostly of the 70 ns type. 20 Mhz means 25 ns cycles. How does
> that fit? Either someone is cheating or I missed something. I hope the last
> and that someone can tell the trick.

I think it all depends on the SCPU bus speed  I have Macs that have way
higher then 20mgz but still use 70ns RAMs. :/

All I know isy the SCPU is an almost complete computer, it is doing much
of the gruntwork and using the Commodore 64/128 as it's I/O System.  It
makes BASIC look like ML and ML lookm insanely fast.  (=))

> Groetjes, Ruud
> -
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