RE: Building my monster C64 accelerator

From: Baltissen, R (Ruud) (
Date: 2003-08-19 09:56:44

Hallo Nick,

> The SCPU actually assumes mirror-ALL as its default ....

So if I understand you correctly, the SCPU starts up in a mode where reading
is done at full speed from its own memory and writing is done at 1 MHz to
both the systems. The speedgain lays in the fact that normally there are
more reads then writes to be done. (horrible translation IMHO) More speed
can be gained by windowing the RAM-area running the risk that graphics
disapear or never show. 

The advantage of the 64'er project was that it worked without the need for
windows. Slowdown for some windows is still needed but some native C64
signals can tell a circuit when or when not to activate this slow-down. No
external programming needed! 
The disadvantage was that the CPU was only used for plm. 50%. With a 14 Mhz
65816 one could run the system on 15 MHz which would end up with a C64
running on 8 MHz ALL-THE-TIME !!! The critical timing of this design was
mentioned but that was a point in 1994 (or was it 1992) but not nowadays
IMHO. The only thing that worries me is the fact that the VIC still can
claim the bus for quite some ns. after the positive edge of CLK2. This means
that in case of 8, we only can use 7 cycles.

Keeping things more simple would mean using the systems own DOTCLOCK which
provides us with 8 MHz and ending up with a 4 MHz C64. Disadvantage: only
works with a 8 MHz 65xxx which doesn't exist thus forcing one to use a 14
Mhz 65816 which means waisting CPU-power.

Remark for Oliver: when using the 65816, what about using its 16 MB
capabilities? Can provide you with quite some nice ideas.

   / __|__
  / /  |_/     Groetjes, Ruud
  \ \__|_\


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