I have ran into things like this before, I think on some occasions the programmer may have used this trick to fool a disassembler and keep their code a little more secure but without thinking about possible problems that you mention.

Cool, thanks for the explanation. That's used a few times in the MS Basic code. Unfortunately I didn't get any time to play with it in the weekend. My PC melted down on Saturday so I ended up building a new one. Still trying to get the on board parallel port working so I can use my ROM programmer.

Simon

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My 6502 related blog: http://www.asciimation.co.nz/bb/category/6502-computer