Section 6
CODE EXAMPLE: PRINT FILE PROGRAM

INTRODUCTION

The following example is a simple program to read a file and print the contents to a standard printer. It gets the filename of the file to print from the DOS command prompt input line. The prompt input information is passed to the program in a buffer area of the Program Segment Prefix (PSP). The address of the Program Segment Prefix is passed to the program in the ES and DS registers when the program execution starts. This program checks the keyboard between every character printed for an escape key code to terminate execution of the program.

; This program prints a file defined on the command line
.MODEL small
;************* Stack Section *********************
.STACK 500
;************* Data Section **********************
.DATA
psp_seg       dw   0
no_cl_mess    db  "This routine requires that a "
   db    "filename be on the command line for printing."
   db 0dh,0ah,"Please try with a filename.",0dh,0ah,"$"
file_bad_open  db "Bad file open",0dh,0ah,"$"
file_bad_read db "Bad file read",0dh,0ah,"$"
printer_bad_mess  db   "!! Printer Error !!!!",0dh,0ah,"$"
printing_mess db  "A file is being printed,",0dh,0ah
   db   "To stop printing, Press ESC key",0dh,0ah,"$"
filename       db   128 dup(0)
file_handle    dw   0
file_count     dw   0
file_pointer   dw   offset file_buffer
file_buffer    db   1024 dup(0)
; ************* ----------- *********************
;*************  Code Section  *******************
.CODE
start     proc near
  ;DS and ES are indexing PSP area
   mov  al,[DS:80H]     ;load AL with size of data line
   mov  dx,@data        ;get segment address of data area
   mov  ds,dx           ;point DS to data area
   mov  psp_seg,ES      ;save PSP address
   cmp  al,1         ;?? data in DOS command line ??
   ja   get_PSP_filename  ;branch if data found
  ;if here, there is no data on command line
   ;display error message to user and terminate
   lea  dx,no_cl_mess
;-------------------------
terminate_display:
   ;display message indexed by DX then terminate
   mov  ah,09
   int 21H     ;DOS Call
;-------------------------
terminate_program:
;terminating the program
    mov  ah,4CH    ;set AH for terminating function
    mov  al,00     ;set terminating code variable
    int 21H        ;call DOS to terminate
;------------------------------------------
; %%%%%%%%%%%%% ----------- %%%%%%%%%%%%%%%
get_PSP_filename:
  ;move PSP filename to filename buffer in our data area
   mov  ax,ds
   mov  es,ax    ;point ES to data segment
   mov  ds,psp_seg
   mov  si,82H   ;SI source is PSP data area
   lea  di,filename
   cld           ;make strings go forward
   get_PSP_data_1:
      lodsb      ;load string data byte
         ;check for end of filename
      cmp  al,21H
         ;branch if end of string
      jb  got_PSP_filename
         stosb     ;store string data byte
         jmp  get_PSP_data_1
got_PSP_filename:
   mov  al,0
   stosb     ;make ASCIIZ string with zero end
   push es
   pop  ds   ;reset data segment pointer
;try to open file
   mov  ah,3dH
   lea  dx,filename
   mov  al,0     ;read access code
   int 21H      ;DOS Call
   jnc  file_open_ok
   lea  dx,file_bad_open
   jmp  terminate_display
;+++++++++++++++++++++++++++++++++++++++++++
;############### +++++++++++ ###############
file_open_ok:
   ;save file handle
    mov  file_handle,ax
    lea dx,printing_mess  ;display start message
    mov  ah,09
    int 21H       ;DOS Call

file_read:
   ;read in block of file data
    mov  ah,3fH
    lea dx,file_buffer
    mov  cx,1024
    mov  bx,file_handle
    int 21H        ;DOS Call
    jnc  file_read_ok   ;branch if good read
       ;else read file error occurred
         ;close file
        mov  ah,3eh
        mov  bx,file_handle
        int 21H
        ;index exit error message
        lea  dx,file_bad_read
        jmp  terminate_display
file_read_ok:
   ;check to see if no more file data
    cmp  ax,0
    je   close_file   ;branch if no data left
    ;else reset data block size and pointer
    mov  file_count,ax
    lea  bx,file_buffer
    mov  file_pointer,bx
;!!!!!!!!!!!!!!!!!  ^^^^^^^^  !!!!!!!!!!!!!!!!!!!!
print_data_block:
   ;main loop to print block of file data
    ;scan keyboard to check for any keys
      mov  ah,1
      int 16H
      jz  print_data_block_1  ;branch if no key
     ;get key code out of buffer
      mov   ah,0
      int  16H          ;call BIOS keyboard
      cmp   al,01BH         ;check key code
      je   close_file    ;branch if ESC
print_data_block_1:
    mov  si,file_pointer
    mov  al,[si]
    mov  ah,0
    mov  dx,0      ;select LPT1
    int 17H       ;BIOS Call
    test ah,25H
    jnz  printer_error
    inc  si
    mov  file_pointer,si
    dec  file_count
    jnz  print_data_block    ;loop if more data
   ;else go read in next block of file data
    jmp  file_read
;!!!!!!!!!!!!!!!!  ^^^^^^^^  !!!!!!!!!!!!!!!!!!!!
close_file:
    mov  ah,3eh
    mov  bx,file_handle
    int  21H    ;DOS Call
    jmp  terminate_program
;--------------  ??????????  -------------------
printer_error:
      ;index exit error message
      lea  dx,printer_bad_mess
      jmp  terminate_display
;_______________________________________________
start  endp      ;end of start procedure
   end  start   ;define start as beginning of program