This code is fine.

You're putting 0x00 in the w register, then copying it to TRISA, TRISB, and
TRISC.

As long as you don't execute any instruction that modifies w between these
instructions, you're fine. (The value of w won't change on its own.) Be
especially careful if you put a call statement in a sequence like this, as
the subroutine is likely to modify w.

The code snippet you posted is sound and correct and quite usable. An
alternate way to accomplish the same thing would be:

clrf TRISA
clrf TRISB
clrf TRISC

This not only saves you an instruction (which sometimes is important and
sometimes isn't), it also means you don't have to rely on the contents of
the w register. Preserving the value of w in an instruction sequence like
this is often useful, depending on what you have before and after the
sequence.  (In this particular case, clearing the TRIS registers, you're not
likely to have code before and after that needs to preserve w. But you might
have simiar code in other functional blocks where using clrf rather than
movlw 0x00 followed by movwf would come in handy.) 

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
solarwind
Sent: Wednesday, December 17, 2008 4:48 PM
To: Microcontroller discussion list - Public.
Subject: [PIC] movlw Moves or Copies?

If I do something like:

movlw 0x00

movwf TRISA
movwf TRISB
movwf TRISC

is it correct? Or do I need to movlw before every movwf?

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