Non-Recursive version of the Factorial procedure that uses a loop.

TITLE Factorial Procedure
INCLUDE Irvine32.inc
;----------------------------------------------------
.data
     MSG_iNumber     BYTE  "Enter a number N:", 0
     MSG_oNumber     BYTE  "N! = ", 0
;----------------------------------------------------
.code
main PROC
     ;// Ask the user to input a number N
     MOV edx, OFFSET MSG_iNumber
     CALL WRITESTRING
     CALL CRLF
     CALL READINT ;// N is stored in eax

     ;// Calculate the factorial of N
     CALL Factorial

     ;// Output the result on the screen
     MOV edx, OFFSET MSG_oNumber
     CALL WRITESTRING
     CALL WRITEINT

     ;// Exit the program
     EXIT  
main ENDP
;----------------------------------------------------
Factorial PROC
     ;// The value of N is already stored in eax
     PUSH eax
     DEC eax
     MOV ecx, eax ;// Loop (N - 1) times
     POP eax
     MOV ebx, 1 ;// Set the initial value of ebx to 1.

     L1:
          MUL ebx ;// eax = eax * ebx
          INC ebx

          ;// This loop will eventually create the following function:
          ;// eax = eax * (eax - 1) * (eax - 2) * ... * (eax – eax + 1)
     LOOP L1
     ret
Factorial ENDP

END main

Testing assembly program was pretty hard for me and my colleague in the Assembly Courses. It might be a bit complicated to execute an assembly program, but I’m sure that you will get used to it. Assembly is a very important language, it’s the mother of all other languages. Ok now, enough about talking, here is how:

1) Install software MASM 6.15 on your PC.

2) Use notepad as text editor.

3) Type your assembly code on the text editor and save it in the \BIN directory of the working environment with file extension *.asm. For example, type copy the following code and save it as C:\MASM615\BIN\test.asm.