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In quite many instruction descriptions it says:
if d == 15 then UNPREDICTABLE;
What does this mean?
Can the instruction really work in some unexpected way in each such case or what?
I guess if I use a bit-reversing instruction on PC I should expect that the address
the processor jumps is hard to follow, but as such it's not UNPREDICTABLE.
It would be UNPREDICTABLE, however, if the next instruction can be fetched from an address
where the bits are only partially reversed.
I'm wondering, because every second instruction (or maybe more) seems to have that
'warning' in the description.
Yes that is what it is saying in effect.