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about quarter of a page of typing. The first sector on a diskette is known as the 'boot sector' and its function is something like that of the starter motor on a car: it kicks the machine into operation. When a computer is on, the machine bursts into life and carries out some simple self-diagnostic checks. If no fault is found the machine looks to see if there is a diskette in the diskette drive. The diskette drive is like a record player, and the diskette its record. If a diskette is in place, the drive rotates and the boot sector is read by the computer. The boot sector then directs the computer to the three actual start-up programs, and the computer is operational.

The Brain virus hides itself in the boot sector, waiting for the computer to start up from the diskette. It then loads itself into the computer's memory, as though it were a legitimate start-up program.

Brain is around 2,750 bytes long, much too big to fit entirely within the boot sector. So the virus does two things: it places its first 512 bytes in the boot sector and then stores the rest of its code, together with the original boot-sector data, in six other sectors on the diskette. When the computer starts up, the head of the virus jumps into memory, then calls up its tail and the original boot sector.

Brain is one of the most innocent viruses imaginable - though that wasn't known at the time at the University of Delaware, which spent a week and expended considerable manpower cleaning out its computer system and destroying infected diskettes. Brain's payload is simply the tagging of infected diskettes with the label 'Brain'. This label is the name a user can give to any diskette he may be using, and is of no real importance. Most users don't even bother to label their diskettes, and if a virus suddenly names it for them, they are unlikely to notice. Or care, for that matter.

However, like all viruses, Brain can cause unintended damage. If a diskette is very nearly full, then it is possible for some sectors to be accidentally overwritten when Brain is attaching its tail, thereby wiping out the data contained on those sectors - which is almost certainly how the student at Delaware lost his thesis. Also, copying can render the virus unstable and it could unintentionally overwrite systems areas (the controlling instructions on a diskette), thus rendering them useless.

Paramount to the viability of a computer virus is an effective infec-

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