My recommendation is that you take a digital logic class (maybe two of them)
and a VLSI design class. You will then understand what an "undocumented
opcode" is, why it would exist in many processors, etc. It often comes down
to minimization of digital logic.
Generally, you don't want to use undocumented opcodes. The chief reasons
a)These may be reassigned at any time by the manufacturer, rendering your
code unable to run. The reassignment could even be silent, meaning that the
manufacturer does not change part numbers.
b)The opcode may have effects (in the digital logic) that you are not able
to detect with the testing you do. For example, it is possible although
unlikely that one of these opcode might have built into it "clear the
'dirty' flag in the 213'th memory cache entry". Point is it may do
something that you can't detect with the testing you are able to do.
David T. Ashley ( XXXX@XXXXX.COM )
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