Thanks for taking the time to post the explanation
Andrew!:o)
It's always good to hear from the oldtimers who
learned their magic in the dawn of the PIC age! ;o)

I understand your example but still find it amusing
as in my ram_clear loops I tend to DEC the counters
and test for the end, so a DECFSZ probably makes more
sense to me, although I prefer a concrete test like
>=X as you don't always need to clear all the ram.

Either way it's extremely annoying to "read" a register
and receive crap, then have to mask the unused bits
out before you know what the register value is.
Hopefully Microchip will stick to the convention that
unused bits==0 now and when you read a register you
get its darn value! :o)
-Roman


Andrew Warren wrote:
>
> Roman Black <PICLIST@mitvma.mit.edu> wrote:
>
> > So what happened to the Microchip convention that unused bits read
> > as zeros???
>
> Roman:
>
> The 16C505's FSR-register behavior (along with the datasheet's
> description of it as "a 5-bit register") is left over from the 16C54.
>
> On that part, it made sense to hardwire the upper bits to 1, since it
> allowed loops like this one, which clears all 16C54 RAM, to be short
> and fast:
>
>       MOVLW   PORTA
>       MOVWF   FSR
>
>     LOOP:
>
>       CLRF    INDF
>       INCFSZ  FSR
>       GOTO    LOOP
>
> If the upper bits were hardwired to 0, INCFSZ would cease to work on
> the FSR register, so the above loop would have to become:
>
>     LOOP:
>
>       CLRF    INDF
>       INCF    FSR
>       TSTF    FSR
>       SKPZ
>       GOTO    LOOP
>
> It's not a BIG deal... But I guess the guys who'd been contracted to
> design the PIC figured that, since it didn't cost anything to
> hardwire those bits to 1, it was better not to break the INCFSZ
> instruction.  Also, if the larger-RAM PICs that the C54's designers
> had anticipated had been designed with contiguously-addressed RAM,
> hardwiring the upper bits to 1 would have made code portable from the
> 54 to other PICs.
>
> -Andy
>
> === Andrew Warren --- aiw@cypress.com
> === IPD Systems Engineering, CYSD
> === Cypress Semiconductor Corporation
> ===
> === Opinions expressed above do not
> === necessarily represent those of
> === Cypress Semiconductor Corporation
>
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