switch.asm

 TITLE switch : An AL Program that reads a digit, computes and prints 'P' if
              ; it is positive, 'N' if it is negative and 'Z' if it is zero
              ; ( Simulation of switch-case statement ).

 .MODEL SMALL
 .STACK 100H

 .DATA
    PROMPT  DB  'Enter the digit : $'
    MSG     DB  'The entered digit is : $'

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

     LEA DX, PROMPT               ; load and print PROMPT
     MOV AH, 9
     INT 21H

     MOV AH, 1                    ; read a character
     INT 21H

     MOV BL, AL                   ; save the input character into BL

     MOV AH, 2                    ; carriage return
     MOV DL, 0DH
     INT 21H

     MOV DL, 0AH                  ; line feed
     INT 21H

     LEA DX, MSG                  ; load and print MSG
     MOV AH, 9
     INT 21H

     CMP BL, 30H                  ; compare input digit and 0

     JL @NEGATIVE                 ; jump to label @NEGATIVE if digit<0
     JZ @ZERO                     ; jump to label @ZERO if digit=0
     JG @POSITIVE                 ; jump to label @POSITIVE if digit>0

     @NEGATIVE:                   ; jump label
       MOV DL, 'N'
       JMP @DISPLAY               ; jump to label @DISPLAY

     @ZERO:                       ; jump label
       MOV DL, 'Z'
       JMP @DISPLAY               ; jump to label @DISPLAY

     @POSITIVE:                   ; jump label
       MOV DL, 'P'
       JMP @DISPLAY               ; jump to label @DISPLAY

     @DISPLAY:                    ; jump label
       MOV AH, 2                  ; print the character
       INT 21H

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