>.   movf PORTB, w
>.   andlw 0x30  ; mask out 4,5
>.   xorwf last_input, f

If you only want to test EITHER bit changing 0->1, then you can just use:
.    btfss STATUS, Z
.    goto error_path

This will save one cycle, bringing this one to 5 cycles, regardless of which
bit changed, 6 cycles if neither changed.  However, you won't know WHICH one
changed, of course.

>.   btfss last_input, 5
>.   btfss last_input, 4
>.   goto error_path
>;; Either 4 or 5 changed 0->1
>.   movf PORTB, w
>.   movwf last_input
>.
>.   etc...
>error_path
>;; Otherwise, neither changed
>;   do error stuff here
>;

--Andrew
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