zero page allocation context example for c64jasm

context.js

module.exports = {
  create: ({}, initial) => {
    const stack = [initial];
    return {
      push: (elt) => {
        stack.push(elt)
      },
      pop: () => stack.pop(),
      top: () => {
        return stack[stack.length-1];
      }
    }
  }
}

main.asm

!use "context" as zp

!let defaultZp = {
    tmp0: $20,
    sprite_idx: $22
}

; Zeropage allocation context for macro expansion
!let zpctx = zp.create(defaultZp)

; Macro expansion will look at the 'current' zp allocation
; by getting the top of the zp allocation stack.
!macro test() {
    !let zp = zpctx.top()
    lda #13
    sta zp.tmp0
    ldx zp.sprite_idx
}

func: {
    +test()
    rts
}

;; IRQ routine -- overload default ZP slots
;; used by above macro.  Overloading uses
;; a stack to save/restore whatever is currently
;; assigned as the ZP allocation.
irq_func: {
    !! zpctx.push({ 
        tmp0: $40, 		;; $40 instead of $20 here
        sprite_idx: $42 ;; $42 instead of $22 here
    })

    +test()

    !!zpctx.pop()
    rti
}

;; This should use defaults again
func2: {
    +test()
    rts
}