----- Original Message -----
From: "Scott Dattalo" <scott@DATTALO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, July 19, 2002 9:00 AM
Subject: Re: [PIC]: Code optimization


> On Fri, 19 Jul 2002, Carlos Ojea wrote:
>
> > Hello :
> >
> > I want to optimize this :
> >
> > If number = 2 then var = xxABxxxx
> > If number = 1 then var = xxxxABxx
> > If number = 0 then var = xxxxxxAB
> >
>

Scott Dattalo's answer:

> If you know that 'number' can only be 0,1,2 then:
>
>      movlw  _AB_          ; assume number == 0
>
>      btfsc  number,0      ; LSB set?
>       movlw (_AB_ << 2)
>      btfsc  number,1      ; Bit 1 set?
>       movlw (_AB_ << 4)
>
>    ; write the _AB_ bit pattern
>
>      xorwf  var,w         ;
>      xorwf  var,f
>

Er, Scott. Not such a good idea. The last two instructions are equivalent
to:

    var = var xor w xor var

which of course is the same as

    var = w

which wipes out all the other ('xx') bits in varr

Bob Ammerman
RAm Systems

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