Rob Robson <PICLIST@mitvma.mit.edu> wrote:

> My first (unsuccessful) attempt to locate some routines on Page 1
> and to call them from Page 0 has sent me scrambling .... could I
> not just make every single CALL in my program an LCALL and call it
> good?  Would that work, or would that cause the PIC's PC to go on
> some unwanted long-call sallies?

Rob:

LCALL will set the PCLATH bits appropriately for the destination of
the CALL, but it won't reset PCLATH after the RETURN.  Therefore,
you'll have to change EVERY ONE of your CALLs and GOTOs to LCALL and
LGOTO.

Macros that adjust PCLATH on both sides of the CALL have been posted
numerous times to the list; I'm sure Olin will post a link to his
macros.

Be careful when using long-call (and long-goto) macros; if you
replace a fragment like:

    BTFSS   PORT,BIT
    CALL    BIT_IS_LO

with:

    BTFSS   PORT,BIT
    XCALL   BIT_IS_LO

your code will break, since that second fragment REALLY says
something like:

    BTFSS   PORT,BIT
    BSF     PCLATH,3
    BCF     PCLATH,4
    CALL    BIT_IS_LO
    BCF     PCLATH,3
    BCF     PCLATH,4

For cases like that, what you really want is:

    BTFSC   PORT,BIT
    GOTO    $+6
    BSF     PCLATH,3
    BCF     PCLATH,4
    CALL    BIT_IS_LO
    BCF     PCLATH,3
    BCF     PCLATH,4

... and the easiest, least-error-prone way to do THAT is to never use
BTFSS/CALL at all; instead, write "LCALLIF" and "LCALLIFNOT" macros
that do the whole thing for you, from the initial BTFSC/BTFSS to the
final PCLATH adjustment.

For a long-call macro to be correct in all cases, it must adjust all
the relevant PCLATH bits, even when only one bit actually NEEDS to be
adjusted.  Just about everyone tries to avoid this by writing a macro
that conditionally includes the second bit-adjustment only if it's
necessary.  Unfortunately, MPASM can't deal with macros that aren't
fixed-length, so the conditional macros don't work.

If you want maximal efficiency, then, you need a whole bunch of
macros, one for each combination of source and destination page.  To
ensure that you never use the wrong one, each macro should include
IF/ERROR lines that check for source and destination pages that match
the ones for which the macro was written, then display an error
message if there's an inconsistency.

Personally, I wouldn't bother maximizing the efficiency for most
projects.  If you're only using two pages, it's pretty easy to
arrange things so very few cross-page calls are necessary; if you're
using more, it usually isn't necessary to squeeze every last byte and
instruction cycle out of your code.

-Andy

=== Andrew Warren -- aiw@cypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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