Re: BASIC and System ROM replacements

On 5/25/2013 4:30 PM, Michał Pleban wrote:
> Hello!
>
> Jim Brain wrote:
>
>> Not to take this too far off topic, but I would be interested in an idea
>> that would allow the ROM areas to be "banked" in such a way that you
>> could fit 16kB of KERNAL or BASIC into a 8kB space.  I know the trivial
>> is to use a "secret" location, but that's too simplistic.  I was
>> thinking of something where a sequence of address accesses would trip
>> the bank, and then if there was an IRQ when in the wrong bank, any
>> access to the normal locations would "trip" it back by writing to a
>> single location.  That way, accesses to ROM routines would incur no more
>> than a few cycles to flip back to the correct bank.
> How would you like to return to the previous ROM bank upon exit from the
> interrupt handler? You can't detect a RTI instruction because the 6510
> doesn't have the SYNC pin.
>
> Regards,
> Michau.
>
>
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To be honest, I was struggling with that.  I could see watching the 
vector locations for a key to snap back into regular ROM mode, but I 
have no idea about returning to the previous bank.



-- 
Jim Brain
brain@jbrain.com
www.jbrain.com


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