mips lab 5 - recursivity - factorial

.data

# factorial(n):
# if n == 0:
# return 1
# return n * factorial(n-1)

startText: .asciiz "Enter a number\n n = "
endText: .asciiz "n! = "
invalidText: .asciiz "Invalid number."

.text

start:
    puts    startText                               # print the text
    geti    $a0                                     # get the number

    bltz    $a0,            textIsInvalid           # if the number is negative, print the error message and exit

# -----------------
# before the call
# $ra, $fp, $a0   -- the order is important
    subu    $sp,            $sp,            12      # allocate space for the return address. 12 = 4 * 3 (3 registers) [unsigned subtraction]
    sw      $ra,            8($sp)                  # save the return address. Decrementing offset since we're going *DOWN* the stack
    sw      $fp,            4($sp)                  # save the frame pointer
    #       sw,             $a0,            0($sp)  # we do NOT need to save $a0 because it's done automatically by the jal instruction. Doing so doesn't break anything, but it's redundant.

    addu    $fp,            $sp,            12      # move FP to the base again.

# -----------------


    jal     Factorial                               # call the factorial function

# after the call
    addu    $sp,            $sp,            12      # pop all the registers we pushed before the call

display:
    puts    endText
    puti    $v1

    j       finish

textIsInvalid:
    puts    invalidText
    j       finish

finish:
    done


Factorial:


    subu    $sp,            $sp,            12      # Same steps as above.
    sw      $ra,            8($sp)
    sw      $fp,            4($sp)
    sw      $a0,            0($sp)                  # we DO need to save it now

    addu    $fp,            $sp,            12

    lw      $v1,            ($sp)                   # v1 = n

    bgtz    $v1,            continue                # if n > 0, continue


    li      $v1,            1                       # else, v1 = 1 (return 1)
    j       func_finish




continue:
    lw      $a0,            0($sp)                  # a0 = n
    sub     $a0,            $a0,            1       # a0 = n - 1

    jal     Factorial                               # call factorial(n-1)

    lw      $a0,            ($sp)                   # a0 = n
    mul     $v1,            $v1,            $a0     # v1 = v1 * a0 (v1 = n * factorial(n-1))


func_finish:
    lw      $ra,            8($sp)                  # restore the return address
    lw      $fp,            4($sp)                  # restore the frame pointer
    addu    $sp,            $sp,            12      # pop all the registers we pushed before the call

    jr      $ra