# MIPS Assembly Calculator

# $s0 = first number
# $s1 = second number
# $s2 = operator
# $s3 = result

# DATA
.data
prompt:     .asciiz "> Enter a number: "
op:         .asciiz "> Enter operation (+,-,*,/): "
nl:         .asciiz "\n"

# CODE
.globl main
.text
main:
  # LOAD OPS
    li      $t1, 0x2B   # +
    li      $t2, 0x2D   # -
    li      $t3, 0x2A   # *
    li      $t4, 0x2F   # /

  # FIRST INPUT
    la      $a0, prompt # print string prompt
    jal     PRINT_STRING

    jal     READ_INT
    move    $s0, $v0    # save to $s0

  # SECOND INPUT
    la      $a0, prompt # print string prompt
    jal     PRINT_STRING

    jal     READ_INT
    move    $s1, $v0    # save to $s1

  # OPERATION CHARACTER
    la      $a0, op     # print string op
    jal     PRINT_STRING

    la      $v0, 12     # read_char = 12
    syscall
    move    $s2, $v0    # save operator to $s2
    la      $v0, 12
    syscall             # read_char again to get rid of enter key

  # OPERATION BRANCHES
    beq     $s2, $t1, opadd
    beq     $s2, $t2, opsub
    beq     $s2, $t3, opmul
    beq     $s2, $t4, opdiv
    j       exit

opadd:
    add     $s3, $s0, $s1
    j       result

opsub:
    sub     $s3, $s0, $s1
    j       result

opmul:
    mult    $s0, $s1
    mflo    $s3         # load result from LO
    j       result

opdiv:
    div     $s0, $s1
    mflo    $s3         # load result from LO, HI contains remainder
    j       result

result:                 # result is saved in $t2
    move    $a0, $s3    # load result to $a0
    la      $v0, 1      # print_int = 1
    syscall
    jal     NEWLINE

  # EXIT
exit:
    li $v0, 10
    syscall

# Routine:      Print out a new line
NEWLINE:
    la      $a0, nl     # print string nl
    la      $v0, 4
    syscall
    jr      $ra         # return from method

# Routines:     commonly used syscall routines
PRINT_STRING:
    la      $v0, 4
    syscall
    jr      $ra

READ_INT:
    la      $v0, 5
    syscall
    jr      $ra