RE: BASIC and System ROM replacements

From: Baltissen, GJPAA (Ruud) <>
Date: Mon, 27 May 2013 06:45:18 +0000
Message-ID: <>
Hallo Jim,

> ... and only reads are required to select the bank.

My first idea was that that should be doable. IMHO one condition is that it should be compatible to the original C64. There are enough unused places in the ROM that could serve for this purpose. The nicest one is at $FFF6 but had only four bytes, meaning only four banks. 
But then I thought of this: I test some changes to the System ROM by trying it out in RAM first. The first thing I do is copying the _whole_ ROM. Thus reading the bank switching mechanism as well. OK, I as person could avoid that. But what about programs out of the box that use the same trick?
Just came to my mind: what about JiffyDOS and other altered ROMs? There is no guarantee at all that the unused places are still free.

There is a possible solution, it looks like the one a FlashRAM uses. I recently learned, thanks to this list, that first some 'magical' addresses have to be written to. What about first reading some particular addresses before reading the address that sets the bank?
- the reading of the addresses will cost more time
- you'll need more hardware than a few simple address decoders.

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Received on 2013-05-27 07:01:33

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