Re: Applesoft and RESET

[gids.rs@sasktel.net]

> The machine is barefoot IIc (255) without any disk booted, not even
> BASIC program entered.

I guess this makes it a hardware problem.  Since you say it is a
barefoot (never heard that slang before - maybe bare bones instead :)
but I take it you opened it up and no surprises such as a No Slot
Clock or any expansion RAM.  A hardware failure would be more random
and would even mean that the Stack Pointer would even count up
occasionally.


> > Also to see the value of the stack pointer, change to this
>
> > 310:BA 8A 20 DA FD 4C 3C D4
>
> > and hit reset.  If you run a few different programs, then hit reset
> > after each one, the number that shows up on the screen should always
> > be $FB.  If the number is creeping lower then something is interfering
> > with the reset routine and it is not resetting the SP properly.
>
> That really caught it. Every RESET gives random SP value, on average
> decreasing by about 16 bytes, but could be anything.

Since the Stack Pointer is always counting down, which it does
normally, and never randomly counts up (which would mean more hardware
failure related), I would also lean more to an interrupts pushing
addresses on the stack and never taking them off.  There are a few
interrupt related sources built into a IIc -  the mouse port, a
special register that enables keyboard buffering, don't know if the
serial ports use or enable interrupts.

And also I am not quite sure if interrupts would be enabled on any
hardware until something is at least plugged in although keyboard
buffering has the keyboard always plugged in but needs to be turned on
with software.


> So there must be either IRQ handler or the software flashing cursor
> diving into stack heavily and then RESET interrupting it. And the
> culprit is leaving SP unitialised.

I would try things in this order
If you can, plug another IIc keyboard in to eliminate sticking keys
and other keyboard problems
look for any black or cracked capacitors
and lastly, the big one, remove and replace all chips that can be
removed.

Oh and there is also, replace motherboard, buy another IIc, pull out
your hair and then lather, rinse and repeat
:)

Rob