Re: 65816 abort line on the AppleIIgs

[david@uow.edu.au (David Wilson)]

Matthew Montchalin <mmontcha@OregonVOS.net> writes:
>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.

>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.

Page 210 of IIgs Hardware Reference:

A negative transition will inhibit modification of any internal register
during the current instruction. Upon completion of the current instruction
an interrupt sequence is generated.

I read this as meaning after the handler returns the instruction is re-tried
(same PC etc). 

>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.

Any sensible use of abort would have it activate on the read data cycle of the
ROR instruction. I don't think you could use it with an MMU and get read only
areas of memory.
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia