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A DOS 2.0 filter for word processing document files
;
; This program reads text from the standard input device and writes
; filtered and transformed text to the standard output device.
;
; 1.  High bit of all characters is stripped off.
; 2.  Tabs are expanded.
;       3.  Removes all control codes except for line
;           feeds, carriage returns, and form feeds.
;       4.  Appends an end-of-file mark to the text, if
;           none was present in the input stream.
;
; Can be used to make a WordStar file acceptable for
; other screen or line editors, and vice versa.
;
;
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII line feed
ff equ 0ch ; ASCII form feed
eof equ 01ah ; End-of-file marker
tab equ 09h ; ASCII tab code

command equ 80h ; buffer for command tail

; DOS 2.0 Pre-Defined Handles

stdin equ 0000 ; standard input file
stdout equ 0001 ; standard output file
stderr equ 0002 ; standard error file
stdaux equ 0003 ; standard auxilliary file
stdprn equ 0004 ; standard printer file

cseg segment para public 'CODE'

assume cs:cseg,ds:cseg

org 100H ; start .COM at 100H

clean proc far ; entry point from PC-DOS.
push ds ; push a long return back
xor ax,ax ; to DOS onto the stack.
push ax

clean3: call get_char ; get a character from input.
and al,7fh ; turn off the high bit.
cmp al,20h ; is it a control char?
jae clean4 ; no.  write it to output.
cmp al,eof ; is it end of file?
je clean6 ; yes, go write EOF mark and exit.
cmp al,tab ; is it a tab?
je clean5 ; yes, go expand it to spaces.
cmp al,cr ; is it a carriage return?
je clean35 ; yes, go process it.
cmp al,lf ; is it a line feed?
je clean35 ; yes, go process it.
cmp al,ff ; is it a form feed?
jne clean3 ; no. discard it.
clean35:
mov column,0 ; if it's a legit ctrl char,
jmp clean45 ;  we should be back at column 0.

clean4: inc column ; if it's a non-ctrl char,
clean45: ;  col = col + 1.
call put_char ; write the char to output.
jnc clean3 ; if OK, go back for another char.

mov bx,stderr ; not OK.  Set up to show error.
mov dx,offset err_msg
mov cx,err_msg_len ; error = Disk full.
mov ah,40h ; write the error message
int 21h ; to the standard error device. (CON:)
ret ; back to DOS.

clean5: mov ax,column ; tab code detected, must expand
cwd ; expand tabs to spaces.
mov cx,8 ; divide the current column counter
idiv cx ; by eight...
sub cx,dx ; eight minus the remainder is the
add column,cx ; number of spaces to send out to
clean55: ; move to the next tab position.
push cx
mov al,20h
call put_char ; send an ASCII blank
pop cx
loop clean55
jmp clean3

clean6: call put_char ; write out the EOF mark,
ret ; and return to DOS.

clean endp


get_char proc near
mov bx,stdin ; get chars from std. input
mov cx,1 ; # of chars to get = 1
mov dx,offset input_buffer ; location = input_buffer
mov ah,3fh
int 21h ; do the function call
or ax,ax ; test # of chars returned
jz get_char1 ; if none, return EOF
mov al,input_buffer ; else, return the char in AL
ret
get_char1:
mov al,eof ; no chars read, return
ret ; an End-of-File (EOF) mark.
get_char endp

put_char proc near
mov output_buffer,al ; put char to write in buffer.
mov bx,stdout ; write to std. output
mov cx,1 ; # of chars = 1
mov dx,offset output_buffer ; location = output_buffer
mov ah,40h
int 21h ; do the function call
cmp ax,1 ; check to see it was really done.
jne put_char1
clc ; really done. return carry = 0
ret ; as success signal.
put_char1:
stc ; not really done. return carry = 1
ret ; as error signal (device is full).
put_char endp

input_buffer db 0
output_buffer db 0

column dw 0

err_msg db cr,lf
db 'clean: Disk is full.'
db cr,lf
err_msg_len equ (this byte)-(offset err_msg)

cseg ends

end clean

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