Assembly-Language-Lab/1b_Factorial.asm at master · ShabbirMahmood/Assembly-Language-Lab · GitHub

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;Shabbir Mahmood ID: 14015139 Assembly Language Lab Problem-1b(Number Processsing(Factorial))  Date: 08-03-2017
.MODEL SMALL
.STACK 100H
.DATA
      LF EQU 0AH
      CR EQU 0DH
      MSG1 DB 'Enter the Value of N: $'
      MSG2 DB CR,LF,'Factorial of N is(N!): $'
      MSG3 DB CR,LF,'WRONG ENTRY(0<=N<=3)$'


.CODE
    MAIN PROC
        MOV AX,@DATA
        MOV DS,AX            ;ADJUSTING PROGRAM SEGMENT PREFIX(PSP)

        LEA DX,MSG1
        MOV AH,09H
        INT 21H              ; SHOWING MESSAGE FOR INPUT

        MOV AH,01H
        INT 21H
        SUB AL,30H           ; CONVERTING TO DECIMAL FROM CHARACTER

        CMP AL,3           ; ******* CHECKING N>=3 ?
        JLE OK               ; IN N IS <= 3 THEN OK

    WRONG_ENTRY:

        LEA DX,MSG3
        MOV AH,09H
        INT 21H
        JMP EXIT


    OK:
        CALL FACTORIAL


        LEA DX,MSG2
        MOV AH,09H
        INT 21H

        ADD BX,30H            ; CONVERTING TO CHARACTER
        MOV DX,BX

        MOV AH,02H            ; MULTIPLICATION(FACTORIAL)
        INT 21H

        EXIT:
            MOV AH,4CH
            INT 21H
            MAIN ENDP

    ; FACTORIAL PROCEDURE
    FACTORIAL PROC

        XOR CX,CX             ; CLEARING COUNT REGISTER ************* VERY IMPORTANT
        MOV CL,AL

        MOV AX,1              ; LOAD AX WITH 1 FOR MULTIPLICATION *****
        CMP CL,0              ; IF WE ENTER 0 AS N VALUE THEN SKIP ************
        JE SKIP               ; 0! = 1 (BX=AX=1)

    TOP:

        MUL CX                ; DX:AX = AX * CX
        LOOP TOP
    SKIP:
        MOV BX,AX             ; STORE THE REJULT IN BX
        RET

        FACTORIAL ENDP
    END MAIN