prg-lib.s

# PURPOSE:  Find factorial of number
# INPUT:
#           number - first argument
#
# OUTPUT:
#           !number
#
# VARIABLES:
#           eax - holds current number
#           ebx - holds current number minus 1
#
.type f_factorial, @function
.globl f_factorial
    f_factorial:
        f_factorial_start:
            pushl %ebp
            movl %esp, %ebp

        f_factorial_body:
            movl 8(%ebp), %eax      # eax = number

            cmpl $1, %eax           # if number == 1
            je f_factorial_end      # jump end of function

            decl %eax               # otherwise, decrease the value
            pushl %eax              # push it for our call to factorial
            call f_factorial        # call factorial

            movl 8(%ebp), %ebx      # %eax has the return value, so we
                                    # reload our parameter into %ebx

            imull %ebx, %eax        # multiply that by the result of the
                                    # last call to factorial (in %eax)
                                    # the answer is stored in %eax, which
                                    # is good since that’s where return
                                    # values go.

        f_factorial_end:
            movl %ebp, %esp
            popl %ebp
            ret