>What about CLRW?  Does
>that read [IND] if the 7 lsb's of opcode are zero?  Would changing the 7
>LSB's to something else cause that something else to be read [hopefully
>harmlessly] instead?

An interesting question, if it exists this side effect could be useful for
something (not sure what yet).  If the possible extra read could be a
problem, you could replace CLRW with ANDLW 0 (exactly the same effect as
CLRW; W=0 and Z flag =1) or of course MOVLW 0 (W=0, but Z unchanged) instead
and forget about the CLRW "instruction" (as you've noticed, it's actually
coded as CLRF x,w) altogether.

-Mike