# Program that will read a positive decimal number and print its factorial in decimal form using MUL and DIV instructions. ( suppose that there.........

``` .MODEL SMALL
.STACK 100H

.DATA
PROMPT_1  DB  \'Enter a Positive Decimal number : \$\'
PROMPT_2  DB  0DH,0AH,\'The Factorial of the given number in Decimal form is : \$\'

.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 INDEC                   ; call the procedure INDEC

CALL FACTORIAL               ; call the procedure FACTORIAL

PUSH AX                      ; push AX onto the STACK

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

POP AX                       ; pop a value from STACK into AX

CALL OUTDEC                  ; call the procedure OUTDEC

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

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

;-------------------------------  INDEC  ----------------------------------;

INDEC PROC
; this procedure will read a number in decimal form
; input : none
; output : store binary number in AX
; uses : MAIN

PUSH BX                        ; push BX onto the STACK
PUSH CX                        ; push CX onto the STACK
PUSH DX                        ; push DX onto the STACK

@SKIP_BACKSPACE:               ; jump label
MOV AH, 2                      ; set output function
MOV DL, 20H                    ; set DL=\' \'
INT 21H                        ; print a character

@READ:                         ; jump label
XOR BX, BX                     ; clear BX
XOR CX, CX                     ; clear CX
XOR DX, DX                     ; clear DX

MOV AH, 1                      ; set input function
INT 21H                        ; read a character

CMP AL, \"-\"                    ; compare AL with \"-\"
JE @MINUS                      ; jump to label @MINUS if AL=\"-\"

CMP AL, \"+\"                    ; compare AL with \"+\"
JE @PLUS                       ; jump to label @PLUS if AL=\"+\"

JMP @SKIP_INPUT                ; jump to label @SKIP_INPUT

@MINUS:                        ; jump label
MOV CH, 1                      ; set CH=1
INC CL                         ; set CL=CL+1
JMP @INPUT                     ; jump to label @INPUT

@PLUS:                         ; jump label
MOV CH, 2                      ; set CH=2
INC CL                         ; set CL=CL+1

@INPUT:                        ; jump label
MOV AH, 1                    ; set input function
INT 21H                      ; read a character

@SKIP_INPUT:                 ; jump label

CMP AL, 0DH                  ; compare AL with CR
JE @END_INPUT                ; jump to label @END_INPUT

CMP AL, 8H                   ; compare AL with 8H
JNE @NOT_BACKSPACE           ; jump to label @NOT_BACKSPACE if AL!=8

CMP CH, 0                    ; compare CH with 0
JNE @CHECK_REMOVE_MINUS      ; jump to label @CHECK_REMOVE_MINUS if CH!=0

CMP CL, 0                    ; compare CL with 0
JE @SKIP_BACKSPACE           ; jump to label @SKIP_BACKSPACE if CL=0
JMP @MOVE_BACK               ; jump to label @MOVE_BACK

@CHECK_REMOVE_MINUS:         ; jump label

CMP CH, 1                    ; compare CH with 1
JNE @CHECK_REMOVE_PLUS       ; jump to label @CHECK_REMOVE_PLUS if CH!=1

CMP CL, 1                    ; compare CL with 1
JE @REMOVE_PLUS_MINUS        ; jump to label @REMOVE_PLUS_MINUS if CL=1

@CHECK_REMOVE_PLUS:          ; jump label

CMP CL, 1                    ; compare CL with 1
JE @REMOVE_PLUS_MINUS        ; jump to label @REMOVE_PLUS_MINUS if CL=1
JMP @MOVE_BACK               ; jump to label @MOVE_BACK

@REMOVE_PLUS_MINUS:          ; jump label
MOV AH, 2                  ; set output function
MOV DL, 20H                ; set DL=\' \'
INT 21H                    ; print a character

MOV DL, 8H                 ; set DL=8H
INT 21H                    ; print a character

@MOVE_BACK:                  ; jump label

MOV AX, BX                   ; set AX=BX
MOV BX, 10                   ; set BX=10
DIV BX                       ; set AX=AX/BX

MOV BX, AX                   ; set BX=AX

MOV AH, 2                    ; set output function
MOV DL, 20H                  ; set DL=\' \'
INT 21H                      ; print a character

MOV DL, 8H                   ; set DL=8H
INT 21H                      ; print a character

XOR DX, DX                   ; clear DX
DEC CL                       ; set CL=CL-1

JMP @INPUT                   ; jump to label @INPUT

@NOT_BACKSPACE:              ; jump label

INC CL                       ; set CL=CL+1

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

CMP AL, 39H                  ; compare AL with 9
JG @ERROR                    ; jump to label @ERROR if AL>9

AND AX, 000FH                ; convert ascii to decimal code

PUSH AX                      ; push AX onto the STACK

MOV AX, 10                   ; set AX=10
MUL BX                       ; set AX=AX*BX
MOV BX, AX                   ; set BX=AX

POP AX                       ; pop a value from STACK into AX

ADD BX, AX                   ; set BX=AX+BX
JMP @INPUT                     ; jump to label @INPUT

@ERROR:                        ; jump label

MOV AH, 2                      ; set output function
MOV DL, 7H                     ; set DL=7H
INT 21H                        ; print a character

XOR CH, CH                     ; clear CH

@CLEAR:                        ; jump label
MOV DL, 8H                   ; set DL=8H
INT 21H                      ; print a character

MOV DL, 20H                  ; set DL=\' \'
INT 21H                      ; print a character

MOV DL, 8H                   ; set DL=8H
INT 21H                      ; print a character
LOOP @CLEAR                    ; jump to label @CLEAR if CX!=0

@END_INPUT:                    ; jump label

CMP CH, 1                      ; compare CH with 1
JNE @EXIT                      ; jump to label @EXIT if CH!=1
NEG BX                         ; negate BX

@EXIT:                         ; jump label

MOV AX, BX                     ; set AX=BX

POP DX                         ; pop a value from STACK into DX
POP CX                         ; pop a value from STACK into CX
POP BX                         ; pop a value from STACK into BX

RET                            ; return control to the calling procedure
INDEC ENDP

;--------------------------------  OUTDEC  --------------------------------;

OUTDEC PROC
; this procedure will display a decimal number
; input : AX
; output : none
; uses : MAIN

PUSH BX                        ; push BX onto the STACK
PUSH CX                        ; push CX onto the STACK
PUSH DX                        ; push DX onto the STACK

XOR CX, CX                     ; clear CX
MOV BX, 10                     ; set BX=10

@OUTPUT:                       ; loop label
XOR DX, DX                   ; clear DX
DIV BX                       ; divide AX by BX
PUSH DX                      ; push DX onto the STACK
INC CX                       ; increment CX
OR AX, AX                    ; take OR of Ax with AX
JNE @OUTPUT                    ; jump to label @OUTPUT if ZF=0

MOV AH, 2                      ; set output function

@DISPLAY:                      ; loop label
POP DX                       ; pop a value from STACK to DX
OR DL, 30H                   ; convert decimal to ascii code
INT 21H                      ; print a character
LOOP @DISPLAY                  ; jump to label @DISPLAY if CX!=0

POP DX                         ; pop a value from STACK into DX
POP CX                         ; pop a value from STACK into CX
POP BX                         ; pop a value from STACK into BX

RET                            ; return control to the calling procedure
OUTDEC ENDP

;-----------------------------  FACTORIAL  --------------------------------;

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

MOV CX, AX
MOV AX, 1                      ; set AX=1

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

RET                            ; return control to the calling procedure
FACTORIAL ENDP

END MAIN
```