Posted By
Csabo
on 2004-05-15
20:01:10
|  Re: XAA!
Many thanks for the feedback everyone. Especially to Ulysses777, thanks for doing all this work. It was reassuring to see that at least two of the Plus/4's (4 and 5) produce exactly the "expected" behaviour. (YAPE produces the same results as well.)
This still doesn't mean that the opcode is useless though. A demo can easily test whether the opcode works as expected, and then (through code generation) either use it or not. And of course, if it is used, you can produce those extra 2 soft-sprites on screen, or add those extra 100 dots to your plotscroller. BTW, Questionmark has some code in the initialization routine, that may display message like "invalid processor type". Not sure if it's testing for illegal opcodes.
What about >FF7F on C16's? In some documentations it is mentioned that this byte represents the computer type. $5E = Plus/4, $2A = C16. No value mentioned for the C116. But seeing that it's the same as the Plus/4, maybe this info is completely incorrect?
About the reverse engineered 6502 page: I've seen it before, it was mentioned on the Hungarian forum. It doesn't say much about the actual instructions though. It would be WAY too much work to translate, and to be honest I think the real value there are the pictures. BTW, Utasításdekódoló means Instruction Decoder. In that section they actually mention this opcode: ANE: A = (A | #$EE) & X & #byte , and say that the constant ($EE) is not guaranteed. There's our answer again then, let's spell it out completely: XAA is unreliable.
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