Mark Peterson <map@tidepool.com> wrote:

> >> 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..
>
> After entering an ISR, the GIE bit is cleared. Save_context then
> occurs in a safe environment. If the GIE bit was set=1 before
> Save_context was done the above could happen.

    Mark:

    No offense, but I think you're confused here.  The situation Jim
    described (STATUS,RP0 set on entry to Save_Context) happens
    WHENEVER an interrupt occurs while the PIC is directly
    manipulating page-1 registers; it has nothing to do with the state
    of the GIE bit.

> An aside: One tutorial program in ECH uses the sequence below:
>
> [snip snip snip]
>
>         call    SetupForLoop            ;occurs exactly before the
>                                         ;wait loop.
> loop    goto    loop                    ;wait for an interrupt
>
>
> SetupForLoop
>         (etc.)
>         RETFIE                          ;Sets GIE=1. Don't have to
>                                         ;re-examine GIE
>                                         ;because it's set by
>                                         hardware and it overrides
>                                         ;everything else including
>                                         ;any instructions in
>                                         ;the pipeline.
>                                         ;The alternative to that
>                                         technique is... in a
>                                         ;paranoid manner be sure it
>                                         ;is set.
>                                         ;ex:
>                                  ;AreYouSure  bsf   INTCON,GIE
>                                  ;            btfss INTCON,GIE
>                                  ;           goto  AreYouSure?
>
> **The novelty here is in the unorthodox use of a RETFIE instruction
> (or is that what Microchip intended it for, dual-use?) I notice the
> statistics are in favor of the AreYouSure approach, in fact that's
> in the 16c74 manual as well.

    Again, you may be confused.  The "AreYouSure" technique is
    useful and necessary ONLY when you CLEAR the GIE bit; there's no
    need to double-check GIE when you set it (unless your ISR does
    something bizarre like ending with a "RETURN" instead of a
    "RETFIE").

    -Andy

Andrew Warren - fastfwd@ix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geocities.com/SiliconValley/2499