"Blot" Virus (5-June-1990)
Entry............... "Blot" Virus
Alias............... ---
Strain.............. ---
Detected when....... May 1988
         where...... Amherst (USA)
Classification...... Boot sector virus
Length of virus..... 681 Bytes
------------------------Preconditions----------------------------------
Operating System(s). Atari TOS
Version\Release..... ROM TOS from 02.06.1986; in other versions,
                         no action is performed.
Computer models..... All Atari ST
-------------------------Attributes------------------------------------
Easy identification. In memory at Phystop +34 and in the boot sector at 
                     the same offset, the following bytes can be found:
                         $0206198600FC0018 
Type of infection... Boot sector of drive A.
Infection trigger... Usage of drive A.
Media affected...... Drive A. 
Interrupts hooked... 200 Hz interrupt , Bios Parameter Vector.
                     Critical Error Handler at infection time.
                     Phystop is decremented by 1 KByte.
Damage.............. The screen is blacked out from bottom to middle 
                     and from top to middle at same time. 
Damage trigger.....  Virus is about 3 hours in Ram. (Depends on value 
                     of 200 Hz timer).  
Particularities..... Only boot sectors that aren't executable are in-
                     fected.
Similarities........ The general concept of the virus is similar to the 
                     'Screen'-Virus
-----------------------------------------------------------------------
Countermeasures..... Make sure that the virus is not in memory.
                     Search bootsector for the string mentioned above.
                     Modify last byte in boot sector to another value.
Standard means...... Clear all bytes in bootsector (sector 0) beginning  
                     at offset 30 (dec) and clear all bytes in sector 5.
-------------------- Acknowledgements----------------------------------
Location............ Virus Test Center, University of Hamburg FRG
Classifcation by.... Ralf Stegen
Documentation by.... Ralf Stegen
Information Source.. George R. Woodside
Date................ 5-June-1990

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