RE: mmu for 65c02

From: Gideon Zweijtzer (
Date: 2005-04-22 06:58:26

> 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.
> >  
> >
> It gets worse.  The opcode in question could have updated an internal 
> register:
> $67ff and #$10
> 67ff (and) gets pulled.
> $6800 is page fault, so force 0.  Uh oh, as now .A is 
> corrupt, and there 
> is no way to reset it.

That same FF should also disable writes to the flags. Actually, the FF
should disable any change to the internal state of the CPU.


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