Re: mmu for 65c02

From: Gábor Lénárt (
Date: 2005-04-22 08:09:50

On Thu, Apr 21, 2005 at 10:09:58PM +0200, A. Fachat wrote:
> >by the MMU if that FF is set! In this way, we isolates CPU from the memory
> >after a fault, to be sure that it will not corrupt a bogus memory locaton.
> >The only task left is to clear that FF after the opcode when the NMI taken,
> >as fault handler.
> > 
> >
> You must clear the flag in hardware. Because after the faulting opcode 


> the CPU writes data to the
> stack and those accesses must succeed. Then it reads the interrupt 
> vectors to start the fault
> routine. So when the NMI fault handler is taken it would be too late to 
> clear the flag.

Hmmm, I've guessed - but I don't know - there is some pin of 6502 to
sign NMI acknowledgment. If there is no such an ack signal it would be
very problematic, I agree.

> >Errrrrrrrrrrrr, what a mess at the end ;-) I think I've started to really
> >overcomplicate the whole design :)
> > 
> >
> I guess that's why noone has so far - as to my knowledge - implemented 
> something like this :-)

:) Well, it's time to syntetize a brand new 6502 core in fpga :)
Welcome to the world of C=1.

- Gábor

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