.data
prompt:
    .asciiz "Enter a nonnegative integer: "
msg:
    .asciiz "! = "

    .text
    .globl main

main:
    li $v0, 4
    la $a0, prompt
    syscall
    li $v0, 5
    syscall
    move $s0, $v0   # $s0 contains input number n

    move $a0, $v0
    li $v0, 1
    syscall
    li $v0, 4
    la $a0, msg
    syscall

    move $a0, $s0
    jal fact

    move $a0, $v0
    li $v0, 1
    syscall

    li $v0, 10
    syscall

# recursive factorial
fact:
    beqz $a0, baseCase
    
# begin pushing $ra & $a0 on the stack
    addi $sp, $sp, 8
    sw $ra, 4($sp)
    sw $a0, 0($sp)
# end pushing $ra & $a0 on the stack

    subi $a0, $a0, 1
    jal fact

# begin popping $ra & $a0 off the stack
    lw $ra, 4($sp)
    lw $a0, 0($sp)
    addi $sp, $sp, -8
# end popping $ra & $a0 off the stack

    mul $v0, $v0, $a0
    jr $ra

baseCase:
    li $v0, 1
    jr $ra