fac:
    # save registers
    addi $sp, $sp, -8
    sw $s0, 0($sp)
    sw $ra, 4($sp)
    # save n
    add $s0, $0, $a0
    # if n == 1, return 1
    bne $a0, 1, notOne
        addi $v0, $v0, 1
        j end
    # not 1, return n*fac(n-1)
    notOne:
        # get fac(n-1)
        addi $a0, $a0, -1
        jal fac
        # multiply
        mult $s0, $v0
        # get result (ignore high bits)
        mflo $v0
    end:
        # load registers again
        lw $s0, 0($sp)
        lw $ra, 4($sp)
        addi $sp, $sp, 8
        jr $ra