Expression Evaluator

expr.s

.global _main

.text

parse_num:
    mov x1, #0
1:
    ldrb w2, [x0]
    cmp w2, #'0'
    blt 2f
    cmp w2, #'9'
    bgt 2f
    sub w2, w2, #'0'
    mov x3, #10
    mul x1, x1, x3
    add x1, x1, x2
    add x0, x0, #1
    b 1b
2:
    ret

_main:
    mov x4, #0              // Arthematic Accumulator

    adrp x1, buf@PAGE
    add  x1, x1, buf@PAGEOFF
    mov x0, #0
    mov x2, #256
    mov x16, #3
    svc #0x80               // x0 = bytes read

    adrp x0, buf@PAGE
    add  x0, x0, buf@PAGEOFF // x0 now points to the buffer
   
    bl parse_num
    mov x4, x1

1:
    add x0, x0, #1
    ldrb w5, [x0]
    add x0, x0, #2

    bl parse_num

    cmp w5, #'+'
    beq do_add
    cmp w5, #'-'
    beq do_sub
    cmp w5, #'*'
    beq do_mul
    cmp w5, #'/'
    beq do_div

2:
    ldrb w5, [x0]
    cmp w5, #' '
    beq 1b
    b done

do_add:
    add x4, x4, x1
    b 2b
do_sub:
    sub x4, x4, x1
    b 2b
do_mul:
    mul x4, x4, x1
    b 2b
do_div:
    udiv x4, x4, x1
    b 2b
done:
    mov x0, x4
    mov x16, #1
    svc #0x80

.data
buf:
    .space 256