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Forums > C64 Coding > How to use the stack area?

2016-03-07 21:31

Trap

Registered: Jul 2010
Posts: 222

How to use the stack area?

Hi,

I'm having some problems here that I need your help with.

I need to put some graphics in the area of the stack, $0100-$01ff. I can easily store data up to $01f0, but when I go beyond that, my code crashes.
So, I'm guessing here ... any branch or jump will store the PC on the stack or what? So, in order to make this work I would have to eliminate all loops in my initialization code? Would a simple TSX and TXS in my interrupt fix the remaining problem?

Am I on the right path here or have I strayed off it? ;)

Cheers
Trap

2016-03-07 21:33

Trap

Registered: Jul 2010
Posts: 222

Ah ok. Found that only JSR, PLA, PLP, RTS, PHA, PHP and RTI use the stack.

So, branches are ok I guess.

2016-03-07 21:37

Hoogo

Registered: Jun 2002
Posts: 102

Quoting Trap

Would a simple TSX and TXS in my interrupt fix the remaining problem?

I guess your interrupt is also your problem. You can't prevent that 3 bytes are pushed to the stack, and more if you're using the usual interrupt handler.
If you do not want to mess the stack, you have to avoid pha, pla, php, plp, jsr, rts, rti, brk. Turn off IRQs, and lock NMIs too if you want protection from the restore key.

EDIT: At least make sure that no IRQ appears in areas where your graphic is shown. An own IRQ handler can pick flags and return address from the stack, repair the graphic bug and return to the main loop without RTI. But handling the flags could be tricky enough that you will easily find a way without interrupts.

2016-03-07 22:09

Trap

Registered: Jul 2010
Posts: 222

Thanks Hoogo.

I managed to fix it. I just moved the SP to a safe spot and replaced the RTS I had at the end of my init code with a jmp back to main code. Viola!

Happy coder :)

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