.include "devices.xyw"

; Main program: calculate and print factorial of 5
$5 fact JSRw 
print8 JSRw
$0A terminal.output STB
end JMPw

; Factorial subroutine
; a8 -- | [xy]
.label fact
  ; Save argument to both registers
  POPxy
  .label fact.loop 
    PSHy $0 EQU fact.end0 JCNw
    PSHy $1 EQU fact.end1 JCNw
    ; Decrement y and multiply
    DECy PSHx PSHy MULx POPx
    fact.loop JMPw
  .label fact.end0
    ; Edge case: factorial of 0 is 1
    $1
    CLRxy
    RTS
  .label fact.end1
    ; Return result
    PSHx
    CLRxy
    RTS

.include "print8.xyw"

.label end