Re: mmu for 65c02

From: A. Fachat (
Date: 2005-04-21 22:09:58

Gábor Lénárt wrote:

>It can be solved anyway. In case of fault, a FlipFlop (FF for short) should
>be set. Any further memory operation (either read/write) should be ignore
>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.

>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 :-)


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