Implement factorial using MIPS

This was a computer organization class assignment. I implemented a factorial using the following algorithm:

$ F(n) = \begin{cases} 1, & n = 0 \\ F(n - 1) * n, & n \in \mathbb{N} \end{cases} $

In MIPS, I use $a0 as the parameter n of F(n). For example, letting $a0 equal to 5 and jumping to the fact label means F(5), and at the end of the recursion, the result will be stored in $v0.

main:
    addi $a0, $zero, 5        # let the parameter n be 5
    jal fact                  # jump to fact label, i.e. calling F(5)
    j exit

fact:
    addi $sp, $sp, -8         # allocate 8 bytes to this stack
    sw $ra, 0($sp)            # save the address of the instruction that calls fact label (instruction address)
    sw $a0, 4($sp)            # save the value of n

    slti $t0, $a0, 1          # $t0 is used for conditions. If n < 1 then $t0 = 1, else $t0 = 0
    beq $t0, $zero, L1        # if $t0 == 0 then jump to branch L1
    addi $v0, $zero, 1        # let $v0 be 1
    addi $sp, $sp, 8          # let $sp point to upper stack
    jr $ra                    # jump to the next instruction of the instruction calling fact

L1:
    addi $a0, $a0, -1         # n = n - 1
    jal fact                  # jump to fact label again, like as calling F(n - 1)
    lw $a0, 4($sp)            # recover the value of n
    mul $v0, $a0, $v0         # $v0 *= $a0, like as F(n) = n * F(n - 1)
    lw $ra, 0($sp)            # recover instruction address
    addi, $sp, $sp, 8         # let $sp point to the upper stack
    jr $ra                    # jump to the next instruction of the instruction calling L1

exit:

The execution result of the MARS simulator looks like this:

You can see that the value of $v0 is 0x78, 120 in decimal, which is the result of F(5).