RE: 6502 Clock issues

From: Baltissen, GJPAA (Ruud) (
Date: 2005-05-19 11:39:50

Hallo allemaal,

Gábor wrote:
> we can replace eg 6510 in C64 with 65816. There were some
> article to replace the CPU in the C64,

The 65816 has the ABORT input, which IMHO is meant for page faults etc.
Some articles:

I'm updating the last page at this moment and therefore the GIFs aren't
available yet. Another thing is that this page mentions that "Replacing the
6510 is quite forward and doesn't need an explanation IMHO" but that is not
true if you want to use the complete 16 MB range.

So her is the extra info you need:
1) an extra 74ALS573 latches the data to generate the address lines A16..23,
clocked by an ivertor
2) a 688 8-bit-comparator checks which 64 KB segment is addressed.
3) extra 541's buffer the addresslines towards the C64
4) a 245 buffers the databus
5) a 125 buffers R/W
6) a number of pull-up resistors

The idea is that when the 65816 addresses an address outside the first 64
KB, the address lines AND the R/W line towards the C64 are disabled. The
effect for the system is that it seems that the CPU wants to read address
$FFFF, completely save IMHO, whether it is the ROM or the RAM under it.
One could even go a step further by disabling RDY during that period as
there is no need to halt the 65816 as it is addressing an area outside he
original C64. Tri-stating the 65816 is not needed at all here as the buffers
can perform this function.

I've build an hack where I put the buffers described above between the 65816
and the C64 just to proof that the idea could work. No 688 and no I/O but
the C64 worked fine as long as a program didn't try to change the I/O port.

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