; ***********************************************************************
; *									*
; *      Compare Two Numbers (First Version in Assembly Language)       *
; *									*
; ***********************************************************************

; Authors:  Saeid Nooshabadi and John Zaitseff
; Date:     5th May, 2003
; Version:  1.5

; This program compares two integer numbers.  The difference between this
; program and cmp-s.s is that this one uses a function, larger.  Beware:
; there are some subtle bugs in this program, as explained in ELEC2041
; Experiment 3.


; -----------------------------------------------------------------------
; Assembly-language preamble for the main module

	.text			; Executable code follows

_start: .global _start		; "_start" is required by the linker
	.global main		; "main" is our main program

	b	main		; Start running the main program


; -----------------------------------------------------------------------
; Start of the main program

; This function loads the variables "a" and "b", compares them and stores
; the larger of the two in the variable "c".  The registers R0, R1 and R3
; are used.

main:	ldr	r3, =a		; Load R3 with the address of "a"

	ldr	r0, [r3]	; R0 = 0xE0001234 (the value of "a")
	ldr	r1, [r3,#4]	; R1 = 0x00004567 (NB: Address of "b" is R3+4)

	bl	larger		; Call the function "larger"; store the return
				; address (address of next instruction) in
				; R14 (also called LR).  Result returned in R0

	ldr	r3, =c		; Load R3 with the address of "c"
	str	r0, [r3]	; Store result in "c" (0x00004567)

exit:	mov	pc, lr		; Stop the program (Caution: bugs are present!)


; -----------------------------------------------------------------------
; Function: int larger (int first, int second)

; This function calculates the larger of the two unsigned integers in
; registers R0 and R1.  It returns the larger of the two in register R0.

larger: cmp	r0, r1		; Compare "first" and "second" (cmp performs
				; first - second, but does not store the
				; result of a - b)
	movle	r0, r1		; Place R1 in R0 if and only if R0 ("first")
				; <= R1 ("second")
	mov	pc, lr		; Return to function main(): restore saved copy
				; of LR into PC.


; -----------------------------------------------------------------------
; Program variables (stored together with the code)

a:	.word	0xE0001234	; Variable "a"
b:	.word	0x00004567	; Variable "b"
c:	.word	0		; Variable "c"


; -----------------------------------------------------------------------

	.end