> So the following is safe and will never introduce a transient? > > MOVLW B'100' > IORWF PORTB,F I have a feeling that operations like IOR/AND/ADD/XOR etc would read the register first, leading to a possible r-m-w You should be safe with BSF and MOVWF though. Particularly if the port contents are read into a buffer or shadow register, modified, and then written back with MOVWF eg MOVF PORTB,W MOVWF SHADOWB MOVLW B'100' IORWF SHADOWB,W MOVWF PORTB The problem with PICs < 18 is that i/p and o/p share some pin logic. PICs > 16 have separate outputs (LAT) and inputs (PORT) and r-m-w is not an issue -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist