;;  call subroutine
;;  main
;;     c = sum(4,5)
;;
;;  sum(a,b)
;;     return a+b
;;
.org 0
  jmp main
retads: 0
retads2: 0
a: 0
b: 0
c: 0
.org 100H
main:
  lda #4
  psh
  lda #5
  psh
  jsr sum
  pop
  sta c
  jsr 1001
;; subroutine
sum:
  pop
  sta retads
  pop 
  sta retads2
  pop
  sta b
  pop
  sta a
;;  do a+b
  lda a
  add b
  psh   ;; push a+b
  lda retads2
  psh
  lda retads
  psh
  rts
.end
