Re: 65816 abort line on the AppleIIgs

[Matthew Montchalin <mmontcha@OregonVOS.net>]

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.