Modified 6502 Interrupt Returns

Question

I am trying to switch normal program flow while an interrupt returns:

START
    SEI
    LDX #<IRQ
    LDY #>IRQ
    STX $FFFE
    STY $FFFF
    CLI

LOOP1
    INC $D020
    JMP LOOP1

LOOP2
    INC $D021
    JMP LOOP2

IRQ
    STA SAVEA+1
    STX SAVEX+1
    STY SAVEY+1

    // Some Routines

    LDA #$00
    PHA
    LDA #<LOOP2
    PHA
    LDA #>LOOP2
    PHA

SAVEA   
    LDA #$00
SAVEX   
    LDX #$00
SAVEY   
    LDY #$00
    RTI

I wrote this code accourding to that source: http://6502.org/tutorials/interrupts.html#1.3

But PHA's cause crash, how to switch normal flow LOOP1 to LOOP2 in an interrupt?

Answer 1

The simplest thing is probably to have two stack areas -- one for every task. $100-$17f and $180-$1ff, for example. Then, you would have your interrupt task switching code like this:

  pha
  txa
  pha
  tya
  pha ;saving task's registers on its stack,
      ;where flags and PC are already saved
      ;by entering the interrupt

  tsx
  stx ... ;save task's stack position

  ... ;select new task to run/etc.

  ldx ...
  txs ;load other task's stack position

  pla
  tay
  pla
  tax
  pla ;restore other task's registers

  rti ;and finally continue other task