# fact4.mips - Given a number n passed on the stack,
# calculate n!.  Return the result in $1.  No other
# registers are modified. Uses a recursive implementation.
#
# Register Usage:
# $1 - Used to return result  $2 - Temporary storage of n
# $31 - Return address        $3 - Temporary storage of n-1

.globl fact4

fact4:  addi $30, $30, -12
        sw $2, 0($30)
        sw $3, 4($30)
        sw $31, 8($30)
        lw $2, 12($30)    # Copy n to $2
        beq $0, $2, base  # Check for n=0
        addi $3, $2, -1   # Calculate n-1
        addi $30, $30, -4
        sw $3, 0($30)
        jal fact4         # Calculate (n-1)!
        addi $30, $30, 4
        mult $1, $2       # Calculate (n-1)!*n
        mflo $1           # Get product from Lo register
        j done
base:   addi $1, $0, 1    # Return 1
done:   lw $2, 0($30)
        lw $3, 4($30)
        lw $31, 8($30)
        addi $30, $30, 12
        jr $31            # Return to caller