Section 5
CODE EXAMPLE: SIMPLE IO PROGRAM

INTRODUCTION

The following example illustrates a simple program that gets keyboard input from a user, adds two numbers together, and displays output to the standard video output. This routine uses three basic DOS calls: the DOS function 1 (StdConInput) for input, the DOS function 2 (StdConOutput) for output, and the DOS function 4CH (Exit) to terminate the routine. This routine gets two single-digit numbers from a user, then adds the numbers together and displays the results. The routine continues to operate until the user enters a keyboard character that is not a number.

;Code routine to add numbers
.MODEL small
;######################################
.STACK 500
;######################################
.DATA
data_1  dw      0
;######################################
.CODE
start  proc    near
       mov     dx,@data     ;get data segment index
       mov     ds,dx        ;set data segment
add_loop:
       call    get_number
       jc      number_error
       mov     data_1,ax
       call    get_number
       jc      number_error
       add     ax,data_1
       call    display_number
       jmp     add_loop
;!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
number_error:
      mov     ah,4CH    ;set for DOS terminate function
      mov     al,0      ;set terminating variable code
      int     21H       ;call DOS to terminate
start   endp
;************************************
get_number proc near
;on exit if carry clear then number in AX is OK else ERROR
     call    display_new_line
     mov       ah,1   ;set for read keyboard function
     int     21H    ;call DOS to get keyboard data
     cmp       al,’0’  ;test for valid number
     jb      get_number_bad  ;jump if bad number
     cmp       al,’9’
     ja      get_number_bad  ;jump if bad number
     an        ax,0FH
     clc          ;set OK exit code
     ret          ;exit
get_number_bad:
     stc          ;set ERROR exit code
     ret          ;exit subroutine
get_number endp
;************************************
display_number proc near
;on exit binary number in AX is displayed (between 0 to 19)
     call   display_new_line
display_number_1:
     cmp   al,9
     ja   display_number_2
     or   al,30H
     mov   dl,al
     mov   ah,2   ;set for DOS display character
     int  21H    ;call DOS function
     ret       ;exit subroutine
display_number_2:
     sub  al,10 ;adjust number
     push  ax   ;save adjusted number
     mov   dl,’1’
     mov   ah,2 ;set for DOS display character
     int  21H   ;call DOS function
     pop   ax   ;restore adjusted number
     jmp   display_number_1
display_number endp
;************************************
display_new_line proc near
;on exit cursor is at start of next line down
     push    ax      ;save AX register
     push    dx      ;save DX register
     mov     dl,0DH  ;start of line character
     mov     ah,2    ;set for DOS display character
     int    21H      ;call DOS function
     mov     dl,0AH  ;new line character
     mov     ah,2    ;set for DOS display character
     int    21H      ;call DOS function
     pop     dx      ;restore DX register
     pop     ax      ;restore AX register
     ret             ;exit subroutine
display_new_line endp
;************************************
     end    start