Re: ROMs replacement.. solved!

From: Gerrit Heitsch <>
Date: Fri, 19 Oct 2012 08:02:46 +0200
Message-ID: <>
On 10/19/2012 02:30 AM, wrote:
> On 2012-10-18, at 19:00, Gerrit Heitsch wrote:
>>> * - how about using the extra space for selecting-in some cart images? I mean I have a need for three KERNALs. Can imagine a need for four. Beyond that I find it difficult to imagine a daily use setup that would need more KERNALs always present. But I can imagine using  e. g. three or four KERNALs and one or two or three ROM cart images that would autoboot with CBM80 when selected at $8000. To make it best as I can think of now I would use the room for four KERNALs and two ROM carts.
>> That will need some extra wiring since you need to grab the _EXROM or _GAME line from the PLA and also their corresponding chip select lines.
>> The logic would then only decode the cart image if it also detects the matching _EXROM/_GAME line being low.
> Why? If I want to blend-in the content - say - as if it was loaded in RAM?

Yes, but you will need to tell the PLA about putting ROM where RAM would 
usually be. Otherwise the PLA will generate the _CAS signal for that 
address space and activate the RAM. That would mean RAM and ROM output 
drivers fighting it out and the system crashing.

This 'telling the PLA about it' is done with the _EXROM and _GAME signals.


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Received on 2012-10-19 07:00:16

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