/*
 *  Source name     : ex2_asmprog.s
 *  Executable name : ex2_asmprog
 *  Author          : Jason Fritts
 *  Description     : Converts ASCII number to its numeric value
 * 
 *  Build using these commands:
 *    as --gstabs ex1_asmprog.s -o ex1_asmprog.o
 *    gcc -g ex1_asmprog.o -o ex1_asmprog
 */

.equiv MIN_ASCII, 0x30
.equiv MAX_ASCII, 0x39

.text
.global main			// program entry point

main:
    push	{lr}		// Create stack frame
    push	{r5}		// Save register r5

    // start program code
    // first zero-out registers
    mov		r0, #0

    // put an ASCII character value in %al
    ldr		r5, =num
    ldrb	r5, [r5]

    // check if character is less than ASCII value for number 0
    cmp		r5, #MIN_ASCII
    blo		not_a_number

    // check if character is greater than ASCII value for number 9
    cmp		r5, #MAX_ASCII
    bhi		not_a_number

    // if a number, compute the integer number equivalent in r0
    sub		r0, r5, #MIN_ASCII
    b		end

    // if not a number, put -1 in r0
  not_a_number:
    mov		r0, #-1		// run 'echo $?' after execution to see value

  end:
    // end program code
    pop		{r5}		// Restore register r5
    pop		{pc}		// Exit & return control to OS


.section .rodata		// read-only (e.g. string) data

.section .data			// initialized data

num:
	.byte	0x36

.section .bss			// uninitialized data