Path: utzoo!utdoe!generic!pnet91!ericmcg
From: ericmcg@pnet91.cts.com (Eric Mcgillicuddy)
Newsgroups: comp.sys.apple2
Subject: Re: Adding an MMU- the whole hullabaloo
Message-ID: <316@generic.UUCP>
Date: 25 Dec 90 03:50:03 GMT
Sender: root@generic.UUCP
Organization: People-Net [pnet91], Etobicoke, ON
Lines: 24

>>Perhaps, given that the program counter is saved on /ABORT, it is possible
to
>>manipulate its value so that it begins the instruction from the start,
rather
>>than continuing half way through.
>
>Abort is supposed to save the PC of the beginning of the instruction.
>one problem solved.

I'm afraid that I don't understand, it seems different from what I know about
the 65xxx series. Usually the opcode is fetched and the PC is incremented and
then 0-3 subsequent fetches and increments are performed. If any of these are
in a swapped segment then there is trouble. Are you saying that the PC is
saved in an internal temporay register, just in case, and that it is restored
once the /ABORT interupt is finished? (with the first RTI I assume)

Is this function also responsible for the VPA and VDA (Valid Program/Data
Adress) signals coming from the chip?

Rotating a ROM location would be a program bug and should fail on that count.
What does it do that makes it even worse?

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