.MODEL SMALL
 .STACK 100H

 .DATA
   PROMPT_1  DB  'Enter a Positive Binary number (max. 1000) : $'
   PROMPT_2  DB  0DH,0AH,'The Factorial of the given number is : $'
   ILLEGAL   DB  0DH,0AH,'Illegal character. Try again : $'

 .CODE
   MAIN PROC
     MOV AX, @DATA                ; initialize DS
     MOV DS, AX

     LEA DX, PROMPT_1             ; load and display the string PROMPT_1
     MOV AH, 9
     INT 21H

     CALL BINARY_INPUT            ; call the procedure BINARY_INPUT

     CALL FACTORIAL               ; call the procedure FACTORIAL

     LEA DX, PROMPT_2             ; load and display the string PROMPT_2
     MOV AH, 9 
     INT 21H

     CALL BINARY_OUTPUT           ; call the procedure BINARY_OUTPUT

     MOV AH, 4CH                  ; return control to DOS
     INT 21H
   MAIN ENDP

 ;**************************************************************************;
 ;**************************************************************************;
 ;-------------------------  Procedure Definitions  ------------------------;
 ;**************************************************************************;
 ;**************************************************************************;

 ;**************************************************************************;
 ;----------------------------  BINARY_INPUT  ------------------------------;
 ;**************************************************************************;

 BINARY_INPUT PROC
   ; this procedure will read a number in binary form    
   ; input : none
   ; output : store binary number in BL
   ; uses : MAIN

   JMP @START                     ; jump to label @START

   @ERROR:                        ; jump label

   LEA DX, ILLEGAL                ; load and display the string ILLEGAL
   MOV AH, 9
   INT 21H

   @START:                        ; jump label

   MOV CX, 4                      ; initialize loop counter
   XOR BX, BX                     ; clear BX
   MOV AH, 1                      ; set input function

   @INPUT:                        ; loop label
     INT 21H                      ; read a digit

     CMP AL, 0DH                  ; compare input and CR
     JE @END                      ; jump to label @END if input is CR

     CMP AL, 30H                  ; compare AL with 0
     JL @ERROR                    ; jump to label @ERROR if AL<0

     CMP AL, 31H                  ; compare AL with 1
     JG @ERROR                    ; jump to label @ERROR if AL>1

     AND AL, 0FH                  ; convert ascii to decimal code
     SHL BL, 1                    ; shift BL by 1 position towards left
     OR  BL, AL                   ; place the input decimal digit in BL
   LOOP @INPUT                    ; jump to label @INPUT if CX!=0

   @END:                          ; jump label

   RET                            ; return control to the calling procedure
 BINARY_INPUT ENDP

 ;**************************************************************************;
 ;----------------------------  BINARY_OUTPUT  -----------------------------;
 ;**************************************************************************;

 BINARY_OUTPUT PROC
   ; this procedure will display a number in binary form    
   ; input : BX  
   ; output : none                       
   ; uses : MAIN

   MOV CX, 16                     ; initialize loop counter
   MOV AH, 2                      ; set output function

   @OUTPUT:                       ; loop label
     SHL BX, 1                    ; shift BX by 1 position towards left
     JC @ONE                      ; jump to label @ONE if CF=1
     MOV DL, 30H                  ; move 0 to DL
     JMP @DISPLAY                 ; jump tp label @DISPLAY

     @ONE:                        ; jump label
       MOV DL, 31H                ; move 1 to DL

     @DISPLAY:                    ; jump label
       INT 21H                    ; display a digit 
   LOOP @OUTPUT                   ; jump to label @OUTPUT if CX!=0

   RET                            ; return control to the calling procedure
 BINARY_OUTPUT ENDP 

 ;**************************************************************************;
 ;-----------------------------  FACTORIAL  --------------------------------;
 ;**************************************************************************;

 FACTORIAL PROC
   ; this procedure will computes the factorial of a given number
   ; input : BL
   ; output : store the factorial of the number in BX
   ; uses : MAIN

   MOV AX, 1                      ; set AX=1

   XOR CX, CX                     ; clear CX
   MOV CX, BX                     ; set CX=BX

   @LOOP:                         ; loop label
     MUL CX                       ; multiply CX with AL i.e. AX=AL*CX
   LOOP @LOOP                     ; jump to label @LOOP if CX!=0

   MOV BX, AX                     ; set BX=AX

   RET                            ; return control to the calling procedure
 FACTORIAL ENDP

 ;**************************************************************************;
 ;--------------------------------------------------------------------------;
 ;**************************************************************************;

 END MAIN

 ;**************************************************************************;
 ;**************************************************************************;
 ;------------------------------  THE END  ---------------------------------;
 ;**************************************************************************;
 ;**************************************************************************;