Re: External Kernal PDF

From: Jim Brain <>
Date: Sun, 21 Nov 2010 21:17:43 -0600
Message-ID: <>
On 11/21/2010 8:03 AM, Rainer Buchty wrote:
> On Sat, 20 Nov 2010, Jim Brain wrote:
>> An English translation of the article would help clarify...
> The explanation is on page 7, but rather wordy, so I restrict it to 
> the technical content:
> The module needs to know whether ROM or RAM configuration is used for 
> Kernal accesses in order to properly bank-in the external ROM. 
> However, that information is not present on the Expansion port. Hence, 
> the cartridge simulates a module in the BASIC space.

Thanks for the translation.

Skoe (Thomas Winkler) provided another explanation that might help 
some.  If the address bus is not being driven, and you drive GAME and 
EXROM low and drive A14 low to simulate an access to $a000-$bfff, the 
state of ROMH will indicate whether HIRAM is active or not.  It's pretty 
ingenious, though very timing sensitive.

Essentially, you wait for PHI2 low, ensure BA is not low, and then you 
drag EXROM/GAME/A14 low and read ROMH.  You clock it in very quickly, 
and store it.

Given you now have HIRAM, you bank the KERNAL ROM in on A13 * A14 * A15 
* PHI2 * BA * !HIRAM and set GAME and EXROM to LOW at the same time.


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Received on 2010-11-22 04:00:02

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