>     There's a major flaw in the Change-on-Port-B Interrupt logic.  If you
>     read from Port B at the same instant that a change on RB4-7 occurs, the
>     RBIF flag will NOT be set.  BSF/BCF instructions perform a read, of
>     course, so if you ever do BCFs, BSFs, or any other Port-B read operation,
>     you can pretty much forget about using the Change-on-Port-B interrupt.

Does MOVWF perform a read-before-write? [I realize, of course, that if it
did so it would ignore the value read, but it's probably easier to have the
hardware do the read for all of the ALU-freg ops].  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?