On Sun, 29 Feb 2004, Rob wrote:
> On Sunday 29 February 2004 17:59, Oliver Achten wrote:
> > But why do you bother? No one will ever uses these opcodes
> > anyway, because it is too dangerous.
>
> I suppose, if you consider the authors of about half the homebrew
> games out there "no one"...
No, there are differences between undocumented and unstable opcodes.
I forget the mnemonics for examples offhand...Thomas's favorite
instruction (is it LAX?) is undocumented, but stable. The thing that was
recently tested (need a little help remembering) was listed as unstable,
and that was verified. The unstable instructions rely on the state of
the bus, ie what happened outside the 6502. The undocumented commands
are things that weren't explicitly designed as part of the instruction
set, but they only depend on internal register values and will always
give the same operation. These usually result from the microcode
partially decoding the opcode for 2 or more "real" instructions.
-Chris
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