ehs@jumbo.dec.com (Ed Satterthwaite) (11/12/89)
In article <115200047@uxa.cso.uiuc.edu>, tle33710@uxa.cso.uiuc.edu writes: > I'd be VERY interested to hear what the undocumented 6502 opcodes are!!!!! A quick pass through my 6502 file didn't uncover anything. I believe these opcodes were revealed in an article in the early 1980's, perhaps in Micro, a 6502 and 6809 oriented magazine that folded by the mid-80's. As I recall, the effects of these opcodes were not too surprising if you looked at the bit patterns and matched them with defined opcodes -- they no doubt fell out of the internal logic implementation. In the early days of cheap microprocessors, discovering such opcodes was a popular game. I think the 8085 had quite a few extensions of the 8080 instruction set that were intentionally added by Intel but, for some reason, never made part of the official spec. The undocumented 6502 instructions were not particularly useful. See the 65c02 for a reasonably well-thought-out and much more useful set of extensions. The 65c02 also fixed a more widely documented 6502 bug/feature. In a 6502 JMP (256*n+255) loads the PC with contents of locations 256*n+255, 256*n. In a 65c02, it gets the contents of (256*n+255, 256*n+256). Ed Satterthwaite ehs@src.DEC.COM {...}!decwrl!ehs