Re: C64 MMU POC

On 7/10/2019 12:26 PM, laughton_at_cyg.net wrote:
>
> How strong a guarantee do we need?  In the 1980's I built a computer 
> that uses 65C02 (CMOS) undefined opcodes, so I'm familiar with those 
> -- and some of them are very ill suited for cycle-saving branches 
> etc.  I'm *not* familiar with NMOS undefined opcodes.  But it's 
> reasonable to suppose there may be some which could serve the MMU 
> function but otherwise are never (or almost never) used.  It's true 
> this can't be 100% guaranteed.
>
> As for the missing SYNC pin: one solution consumes PLD resources to 
> create a state machine.  That seems like (and may indeed be) a rather 
> expensive solution.  But it would be no surprise if someone who's 
> willing to engage with the challenge (this is crucial) found it 
> simpler than initially imagined.
The Faks6509 project uses a similar idea, a small state machine to 
reverse engineer the SYNC pin function.
>
> Failing that, I know of a $10 IC that's available in surface-mount 
> which exactly matches the cycle-by-cycle behavior of an NMOS 
> 6502/6510  ;o)

Hmm, you thinking of the 65C02S?  Because, I think we determined that 
the CMOS 6502 and the NMOS 6502 are not cycle exact.  I know the 
Rockwell 65C02 differs in the execution pipeline and thus it fails with 
the Fake6509 project.

Jim