Re: Additional cartridge ROM question

Re: Additional cartridge ROM question

From: Marko Mäkelä <>
Date: Wed, 25 Mar 2009 08:56:53 +0200
Message-ID: <20090325065653.GA22825@x60s>
On Tue, Mar 24, 2009 at 09:08:32PM -0500, Jim Brain wrote:
> Bil Herd wrote:
>> If you want to send me your schematic when done I can look to see if it matches all of the basic rules.
> I'd appreciate it:
> I'm also not sure about C128 mode.  I read somewhere that EXROM + GAME   
> = 0 would always switch into C64 mode.  I wonder if the EPROM code can  
> switch back to 128 mode for those with 128 machines.

On the C128 MMU, EXROM and GAME are general purpose input/output pins.
In C128 mode, you can use them as outputs: the PLA doesn't care about
them.  If you programmatically switch to C64 mode, you can set EXROM
and GAME as you wish, but you can't control them afterwards, since the
MMU won't be addressable in the C64 mode.

At reset, both the Z80 BIOS (which boots the system) and the C128 KERNAL
check if either line is pulled low or if the C= key is pressed.  If any
of that happens, the computer will be switched to C64 mode.  You can't
switch back to C128 mode without RESET.  The C65 is different: it boots
up in C64 mode, and by writing a special sequence to the VIC-III registers,
you can switch to the C65 mode.

Bil, the PLA equations I referred to in my last message are the logic terms
of the 82S100 PLA in the Commodore 64.  They were first read in 1994.  The
equation for CASRAM was simplified later.


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Received on 2009-03-25 08:13:04

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