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Index > DOS > Copy writing/patching program

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Joined: 06 Jan 2011
Posts: 200
This is a tool I made with FASM that allows users to place a copy write on any COM file specified. The default copy write is set to FreeDOS, but you can edit the source to put what ever you like! I suppose this could also be done for patching programs, just along as your code is adjusted to the new offset.
org     100h
mov     bx, 82h ; all command line parameters are saved to offset 82h in the PSP
mov     di, filen 
mov     ah, 1ah
mov     dx, hide_dta
int     21h
start: ; routine for receiving CMD line parameters
        cmp     byte[bx], 0dh
        jz      done
        cmp     byte[bx], 0
        jz      serror
        mov     al, [bx]
        inc     bx
        jmp     start
        mov     ah, 09h
        mov     dx, emsg
        int     21h
        int     20h
emsg    db      "Syntax: stub <filename>$"   
        mov     ah, 09h
        mov     dx, emsg2
        int     21h
        int     20h
emsg2   db      "File not found$"
done: ; file is found!
        mov     ah, 4eh ;find first matching file (direct 3d02h messes up)
        mov     dx, filen ; file found earlier
        xor     cx, cx ; write/read only!
        int     21h
        jc      error
        mov     ax, 3d02h
        lea     dx, [hide_dta+1eh] ; file name in the new DTA placed after the code
        int     21h
        jc      error
        xchg    ax, bx
        mov     ah, 3fh ; lets read the initial 3 bytes 
        mov     dx, bytes
        mov     cx, 3
        int     21h
        mov     ax, word [hide_dta+1ah]
        mov     cx, word [bytes+1]
        add     cx, vend-vstart+3 
        cmp     ax, cx ; is the jump/size equal to our program plus the other
        jz      alinf ; if so, then the file is already changed
        sub     ax, 3 ; you must subtract 3 from the size (see note after code)
        mov     word [newjump+1], ax ; put new offset after first byte
        mov     ax, 4200h
        xor     cx, cx
        cwd ; convert word(CX) to double word (CX) (DX) therefore xoring both 
        int     21h
        mov     ah, 40h ; write our jump
        mov     dx, newjump
        mov     cx, 3 
        int     21h
        mov     ax, 4202h
        xor     cx, cx
        int     21h
        mov     ah, 40h ;write our program
        mov     dx, vstart
        mov     cx, vsize 
        int     21h
        mov     ah, 3eh
        int     21h
        mov     ah, 09h ; were we successful? if so then print it!
        mov     dx, smsg
        int     21h
        int     20h
        mov     ah, 09h ; all files have been patched!
        mov     dx, fmsg
        int     21h
        int     20h
smsg    db      "File successfully copyrighted$"
fmsg    db      "File already copyrighted$"
newjump db      0e9h, 0, 0 ; 0e9h = jmp short
filen   db      20 dup (0) ; 20 byte buffer for file name
vsize    =       vend-vstart ; size of our patch
        call    begin ; obtains BP
        pop     bp ; pops BP
        sub     bp, begin ; adjusts BP to current offset
        mov     di, 100h ; beginning of the COM file
        lea     si, [bp+bytes] ; restore the bytes saved
        push    di ; save it for the jump later on
        movsw ; move 3 bytes total (jmp = 3 bytes)
display_msg: ; let's do what we need to
        mov     ah, 09h
        lea     dx, [bp+msg]
        int     21h
bytes   db      0cdh, 20h, 0
msg     db      "DOSemu FreeDOS(C) Copyright 1995-2006", 0ah, '$'
hide_dta        db 42 dup (?)
Post 07 Feb 2011, 07:08
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