Christopher Head wrote:
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> Jinx wrote:
>>> 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
> 
> With respect to read-modify-write, BSF is not safe. BSF/BCF operate by
> reading the port, altering the relevant bit, and writing the port.

But what Jinx talked about was to execute the BSF/BCF against
*another* register, not the PORTx register. And in that case it's
"safe", of course. And "IORWF PORTx,F" isn't better than BSF/BCF
in this regard.

And, remember also that *no* operation that reads the PORTx register 
is safe. Remember that it is in the *read* phase the RMW is 
introduced. So this code (from another post) :

 > MOVF PORTB,W
 > MOVWF SHADOWB
 > MOVLW B'100'
 > IORWF SHADOWB,W
 > MOVWF PORTB

isn't a single bit (!) better then a simple BCF/BSF, since it's
in the "MOVF PORTB, W" that the error is introduced. The rest
of the code can't do much about it after that, the SHADOWB
register already has the error...


> An
> operation that definitively sets the value of every single bit (e.g.
> movwf) is the *only* completely safe way to deal with RMW.

Or that you has concideded the potential RMW issues and designed
against it. That's isn't *that* hard either.

Finaly, since the O.P said he might be using an I/O pin
directly against the base of a standard transistor with
no current limiting resitor, he might have problems, some
which might look like the RMW problem.

Jan-Erik.

> 
> Chris
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