[Coco] Coco virus/worms are possible!

[Robert Gault]

A topic that comes up occasionally is "can a Coco be infected in a 
manner similar to current PCs". Generally the answer is no but I've just 
come across a method that could make infection a possibility.

I was asked about a Coco game that was stated to start running when the 
DIR command was issued. Now how can that be possible as DIR just prints 
text to the screen. Well, DIR does much more in order to get that text 
for printing.

One of the things done by DIR is to load the disk FAT table into a 
buffer at $600. As this table is no larger than a 256 byte sector, this 
should be safe. However, the game formats track 17 with six 1024 byte 
sectors. That means when DIR loads T17S2 (the FAT) it gets more than 
bargained for. The buffer is flooded way past the NMI vector at $983.

Since all disk I/O makes use of the NMI, this game probably puts an 
address into $982 that vectors to some code loaded from T17S2. Nothing 
really bad can happen on a floppy system but on a hard drive system a 
malicious program could erase the hard drive.