Changing Letters Lower To Uppercase

@Program written by Hamed Adefuwa
@Part of computer architecture university module
@ ---------------------------------------------------------------------------------------------
@
@ 3. Assume the that you have an array of chars with the following sentence stored in it:
@ 3.1. “Computer Architecture Module preparation questions\0”. This arrays ends with a null char.
@ Implement an assembly program to convert each char from lower to upper case. 
@ 
@ My theory: load each character into a register and compare it to see if its below or over 91
@ if over 91 then subtract 32
@ if under 91, then skip and go to next position in array 
@ use ldrb / strb.
@ ---------------------------------------------------------------------------------------------
.data
.balign 4
string: .asciz "Computer Architecture Module preparation questions\0" @ String is an array of chars. It is stored in memory.
return: .word 0

.text
.global main
main:
	ldr r0,=string
loop:
	ldrb r9, [r0]
	cmp  r9, #0
	beq exit	
	cmp r9, #91
	bgt subtract

skip:
	add r0, r0, #1
	b loop		
subtract:
	sub r9, #32
	strb r9, [r0]
	b skip
exit:
	swi 0