Hello Dave,

You are on the right track with the macros. On any of the Microchip devices
with multiple pages of RAM or ROM, we always use macros that state your
present address ($) and the address of the routine you wish to access, then
let the assembler figure out the proper page bits to set and / or clear.
Unfortunately with the code space, the macro has to always set and / or
clear using the the same number of instructions since it gets upset if
differing number of addresses are used. For example, if one called a routine
from the same page, no instructions would be needed, but the macro has to
generate them. Actually, in versions of the compiler long gone, the task was
easier since the assembler would make another pass through the code to see
if the addresses had been modified. Here's a crude example of something that
might work:

;**************************************
;Long Jump
;**************************************
; If on Different Pages, Do Page Set,
LJump   MACRO   AddFrom,AddTo
        NOLIST
        IF      (AddTo>>.9) == 0
     LIST
; Page 0
         BCF    STATUS,PS0
         BCF    STATUS,PS1
        NOLIST
          ENDIF
         IF     (AddTo>>.9) == 1
     LIST
; Page 1
          BSF   STATUS,PS0
          BCF   STATUS,PS1
        NOLIST
          ENDIF
          IF    (AddTo>>.9) == 2
     LIST
; Page 2
           BCF  STATUS,PS0
           BSF  STATUS,PS1
        NOLIST
          ENDIF
          IF    (AddTo>>.9) == 3
     LIST
; Page 3
           BSF  STATUS,PS0
           BSF  STATUS,PS1
        NOLIST
          ENDIF
        errorlevel      -306
          LIST
          GOTO  AddTo
          NOLIST
        errorlevel      +306
        LIST
        ENDM


The same type of "Bankswitch From, To" works great with the RAM since it
doesn't rely on your knowing or caring which bank the variables are in. This
method can, in fact set and clear variable numbers of bank bits (including
none) since it doesn't self modify the address space.

Hope this helps.

Tom

-----Original Message-----
From:   pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU] On
Behalf Of David Thompson
Sent:   Tuesday, January 09, 2001 6:44 AM
To:     PICLIST@MITVMA.MIT.EDU
Subject:        [PIC]: Anyone have a solid strategy for code that spans pages?

Hi guys,

The code for my project has been growing beyond the first 2k page of my
16F877 for some time now. I have moved as much code and subroutines as
possible into Page 1. I have all the right macros for doing cross-page calls
and gotos, but *@#% it, it is never quite that simple, is it?.

There is always something that has a problem with running in a certain page
or being accessed from another page. To save me from
death-through-debugging, I wanted to ask the group what strategies people
had for managing large code the spans pages on these chips.

It is a bit more complex than saying "Put the less-used subs in Page 1" as
nothing in my code is much less used than anything else.

A typical problem is as follows: You have a large number of calls to a
subroutine from various spots, all residing in page 0. You decide to move
the subroutine to page 1, because you're desperate for space in page 0. You
then have to change all calls to the subroutine to xcalls/longcalls/whatever
to make it all work. You recompile and discover that while moving the
subroutine gave you say, 100 bytes free space in page 0, you lost 60 bytes
of those because of the 30 times you now call it with your macro!. If the
subroutine is small, you can actually be worse off than you were.

As my code tries to be as size efficient as possible (in a linear addressing
world) there are a lot of subroutines and a great many calls to them. Moving
any to another page is perilous or pointless because of the code that the
macros generate. I of course try to minimise this code by using the single
page call macros, you know...

pcall   MACRO   LABEL           ;Page call - only for jumps to page 1 from
page 0
        bsf     PCLATH,3
        CALL    LABEL
        bcf     PCLATH,3        ;Needed for sanity
        ENDM

I am interested to hear if anyone has been down this path before and thought
out a really good way to have a large piece of code with LOTS of subroutines
run happily across multiple pages with minimal page swapping macros.

Anyone?.

Dave. (Totally paged out... >:-o)

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