; ***********************************************************************
; *									*
; *                 Simple Subroutines (Function Calls)                 *
; *									*
; ***********************************************************************

; Author:   John Zaitseff <J.Zaitseff@unsw.edu.au>
; Date:     9th September, 2002
; Version:  1.3

; This program contains simple subroutine calls (also known as a function
; calls).  It illustrates the "bl" (branch and link) instruction, as well
; as the standard method of returning from such calls.


	.text
	.global	_start

_start:

	mov	r0,#15		; Set up parameters
	mov	r1,#32
	bl	f_add		; Call the function "f_add"; result is in R0

	mov	r1,#5		; Set up the second parameter
				; (other parameter is still in R0)
	bl	f_sub		; Call the function "f_sub

exit:	swi	0x11		; Terminate the program


f_add:				; Function "f_add" for addition

; A function (subroutine) and its caller need to agree on a "calling
; convention", a statement about what parameters are expected on entry
; (and in what registers or memory locations) and what is returned (and in
; what registers or memory locations).

; For the function "f_add", R0 and R1 on entry are the addends; on exit,
; R0 is the sum.

	add	r0,r0,r1	; Perform R0 := R0 + R1
	mov	pc,lr		; and return to the caller


f_sub:				; Function "f_sub" for subtraction

; On entry, R0 is the minuend and R1 is the subtrahend.  On exit, R0 is
; the difference.

	sub	r0,r0,r1	; Perform R0 := R0 - R1
	mov	pc,lr		; and return to the caller

	.end