Re: 65816 abort line on the AppleIIgs

[mjmahon@aol.com (Michael J. Mahon)]

Matthew Montchalin wrote:

>On Thu, 31 Oct 2002, David Empson wrote:
>|> Pin 35 on the 65816 is for inputting aborts.
>|>
>|> How was this line originally handled when the AppleIIgs was first
>|> marketed?
>|
>|I don't recall it ever being used for anything significant and
>|widespread.  It might have been used in some obscure hardwrae for
>|assisting with software debugging, in special hardware for testing the
>|computer at manufacturing time, etc.
>
>Ah, okay.
>
>|It can potentially be used to implement a virtual memory system, but the
>|IIgs architecture might not be able to support this.  I'd have to dig
>|out the firmware and hardware references to see if they have any
>|relevant comments.
>|
>|I don't think it is used internally by either of the IIgs accelerators
>|(ZipGS and TransWarp GS) - they just tweak the clock to the CPU as
>|necessary.
>
>I've heard conflicting information about just what happens when the
>Abort pin is pulled low.  Is it true that it wipes out the instruction
>currently being executed?  Someone told me that it tries to 'resume'
>the instruction when you finally RTI and get out of the Abort handler.
>Obviously a ROR <memory address> instruction, if interrupted in the
>middle of the rotation, is going to result in something odd after
>the processor resumes from the Abort.

The intent is that it cancels the effect of the in-process instruction,
and it should work if asserted during the first memory-access
cycle of an instruction.  This is consistent with it's intended use
for a virtual memory implementation, where it would be used to
interrupt & cancel the current instruction if it referred to un-mapped
memory--a page fault.

After the page has been made present by an interrupt service
routine, the program would be resumed at the aborted instruction
so that it could re-execute to completion.

-michael

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