RE: Virtual6510 - Can a PC act as a CPU Accelerator?
Date: 2001-11-20 02:43:05

Hi Guys,

>As far as I understand the PC LPT port is simply too slow to handle the
>maximum data transfer rate. So I doubt that full synchronized access is 
>possible. Maybe implementing some microcode program that kind of burst-
>loads/stores from/to C64 may do the trick. Is that what you intend?

That's correct, the memory is not fully nor continuously synchronised. A
very tight loop runs on the Commodore updating or skipping memory bytes in
sequence. The memory that is synchronised is dependent upon the "setting".
"NO memory" means only I/O reads/writes are done, "BASIC" means only the 1k
Video RAM and 1k Colour RAM are done. In a tight loop, the screen RAM worse
case rate (WCR) would be refreshed at 33Hz. If the whole 16k bank was
refreshed, the WCR is slower => 4Hz. As the code allows skipping of
unchanged bytes the refresh rate is faster. All the memory mirroring modes
from the SCPU plus more could be handled (in PC code the optimisations can
be even smarter).

The problem is though I/O reads/writes take priority over memory writes,
they are too slow for doing things like IFLI. For this the copy of the code
on the real Commodore needs to be called. I envisage a "trainer", that lets
the emulator learn when calls need to be made to the real resident code.

>RAM can be completely mirrored in the PC, only I/O read/writes and 
>video memory writes must be taken into account, so this also reduces
>accesses a lot.


>The PC can count the number of cycles it has executed. However, how is
>this count synchroniyed with the C64 cycles? In the VICE emulator we
>always have to synchronize CPU cycles with the "more real time" cycles
>defined by the frame rate. But in VICE we know how many cycles have

Can you explain why I need the synchronisation? I'm interested in
understanding if I've missed something here.  Do you mean things like raster
coding, etc?




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