On Thu, Jul 13, 2000 at 09:33:23AM +0100, Geoff Wootton wrote:
> Thanks for the pointer to your web page. Regarding the section
> on CALL/RETLW usage I was wondering if anyone has an example
> of a vector table.

Yeah, goes sort of like this ...

        org     0x0
        goto    main

; vector table
alpha   goto    alpha_real
beta    goto    beta_real
delta   goto    delta_real

main
        call    alpha
        [code]
        call    beta
        [etc]
        goto    main

alpha_real
        [lots of code]
        retlw

beta_real
        [lots of code]
        retlw

delta_real
        [lots of code]
        retlw

The point being that this allows the *_real entry points to exist
anywhere within the bank memory.  You CALL the vector table entry
point, and it does a GOTO to the real entry point.  It works because
GOTO has the extra bit for the address that CALL does not.

If your code extended beyond 0x200, you could place a different
vector table at the low end of the second page ...

        org     0x200

; vector table
alpha   bank0
        goto    alpha_real

beta    bank0
        goto    beta_real

However with this trick your code would have to take care to restore
PA0 before it tries the next CALL or GOTO within 0x200 to 0x3fe ...

high    call    alpha
        bank1
        goto    high

--
James Cameron    mailto:quozl@us.netrek.org     http://quozl.netrek.org/

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]: PIC only [EE]: engineering [OT]: off topic [AD]: advertisements