.reg IAR main
.reg SP -1 ; the last addressable address

; In this example, the stack pointer points to the next free address
; below the stack. The stack grows downwards from large to small
; addresses. The stack has no stack frames to simplify the program.

counter: .lit 0

.org 100
main:
  ; set counter bit 0
  LDV counter
  ADC 0b00001
  STV counter
  
  ; Since we're top-level, we don't need to (re-)store our RA when calling
  CALL sub-1

  ; set counter bit 1
  LDV counter
  ADC 0b00010
  STV counter

  HALT

.org 200
sub-1:
  ; Set counter bit 2
  LDV counter
  ADC 0b00100
  STV counter
  
  ;; Store the current RA on the stack
  ; Store the RA at current position of stack pointer
  LDRA
  STRS 0
  ; Move stack pointer by 1 since the stack grew
  LDSP
  ADC -1
  STSP
  
  ; Call the subfunction
  CALL sub-2

  ;; Pop and restore the RA from the stack
  ; Read RA from the top of the stack
  LDRS 1
  STRA
  ; Remove top element from stack
  LDSP
  ADC 1
  STSP

  ; Set counter bit 3
  LDV counter
  ADC 0b01000
  STV counter

  RET

.org 300
sub-2:
  ; Set counter bit 4
  LDV counter
  ADC 0b10000
  STV counter

  RET