Path: utzoo!attcan!uunet!mcsun!ukc!harrier.ukc.ac.uk!dac From: dac@ukc.ac.uk (David Clear) Newsgroups: comp.sys.atari.8bit Subject: Re: 6502 error conditions Message-ID: <5501@harrier.ukc.ac.uk> Date: 21 Sep 90 10:53:38 GMT References: <1990Sep19.171649.20159@ingres.Ingres.COM> Reply-To: dac@ukc.ac.uk (David Clear) Distribution: comp Organization: Computing Lab, University of Kent at Canterbury, UK. Lines: 32 In article <1990Sep19.171649.20159@ingres.Ingres.COM> dalekim@llama.Ingres.COM (Dale Kim) writes: >On the topic of illegal 6502 opcodes, I once heard that there was a >'Crash and Burn' instruction on the Apple II's 6502 chip (which supposedly >melts down the chip). Have any of you 8-bit enthusiasts heard of anything >like this for the 6502 in an Atari computer? I have an expendable Atari 800 >at my disposal. :-) I too heard about this. Whether I really believe it or not is another matter. What I heard is that it was a memory address (maybe for an i/o device) which, when written to the right way, would send some current to part of the computer which blew something. Technically, I suppose this sort of thing is possible, but what a design bug!!! As for other 6502 opcodes, Personal Computer World (or maybe Practical Computing - which ever one had the "Subset" section) had a pretty comprehensive list of "illegal" opcodes back in 1983 (I think). The instructions were pretty weird - too weird for me to remember. What I do remember was that I saw no real use for any of them, although there were two and three byte NOPs with which you can write very sneaky code as the first byte was the NOP2 or NOP3 code and the next one or two bytes were just ignored and so could be any other instruction/data you wanted. Of course, the use of such illegal codes is "bad practice", but then so is self modifying code and I know no-one who programs 6502 and doesn't use that :-) Dave. -- % cc life.c | David Clear dac@ukc.ac.uk +44 227 764000x7592 % a.out | Local Area Networks, Computing Laboratory, Segmentation fault (core dumped) | University of Kent, Canterbury, England. >>> Kernel R0M. His Mission: To rid the world of wobbly ZX-81 16K RAM packs. <<<