Re: Applesoft and RESET

[vladitx <vladitx@nucleusys.com>]

On Jun 4, 4:26 am, gids...@sasktel.net wrote:

> Try this little program to intercept the reset key
>
> 300:A9 10 8D F2 03 A9 03 8D
> 308:F3 03 49 A5 8D F4 03 60
> 310:68 A8 68 A2 F8 9A 48 98
> 318:48 20 EA 03 20 00 03 4C
> 320:3C D4
>
> This is a revised routine from D683 which resets the stack pointer,
> reconnects DOS, resets the RESET pointers to point back to the
> subroutine and then jumps into applesoft.
>
> If this works then something is corrupting the stack pointer on a
> reset. which could be anything from a card in a slot,  (Remove all
> cards and test again), or a corrupted DOS or program that might be
> running illegal instructions that can cause unpredictable results.

Yup, it works as it solidly puts $F8 in SP each time.

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

> 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.

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.

Rob, thanks for great insight!