My understanding, and hence my code, is that you must allocate the same memory
location in all banks for, as you note it below, "temp_w". This is because as
 you say
there is no assurance as to which page is active at the time of the interrupt.
 So
I allocate the first RAM location in all pages to this temp location. Uck.

stevec@rain.org


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From:   Jim Main[SMTP:jmain@CQM.CO.UK]
Sent:   Wednesday, May 22, 1996 9:32 AM
To:     Multiple recipients of list PICLIST
Subject:        Context saving in interrupts

This is the start of my interrupt service routine - it saves W, STATUS, FSR
and PCLATH.  I'm thinking however that there'll be a problem if an interrupt
occurs when STATUS,RP0 is set.... temp_w is then in the wrong register bank,
and it'll overwrite whatever that location's being used for..

The only way round it is to leave the corresponding bank1 register free, so
that it doesn't matter - but then you'd have to know which bank to look in
when it comes time to restore the registers at the end of the routine..  eg
you'd have to restore the STATUS register before restoring the W register

Does this sound ok, or is there a simpler method?



        ORG     04                      ;This is the interrupt vector

        movwf   temp_w                  ;store the contents of the w register
        swapf   STATUS,w                ;and the status register, so that they
        bcf     STATUS, RP0             ;(set RAM page 0)
        movwf   temp_stat               ;can be restored before leaving the
 interrupt

        movf    FSR,w
        movwf   temp_FSR

        movf    pclath,w
        movwf   temp_lath