RE: Additional cartridge ROM question

RE: Additional cartridge ROM question

From: Bil Herd <>
Date: Wed, 25 Mar 2009 08:33:23 -0500
Message-ID: <>
> 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.

Yup, I had a "throw away the key" mentality to jumping into 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.

We did this one night between 10:00PM and 3:00AM.  I had someone read me the Z80 assembler over the phone and I punched it in by hand to test it, then we added the Commodore key by making a quick request of Fred Bowen, it was our insurance policy that we could force C64 mode.  This all happened because the Commodore Speech cartridge was toggling the /Game Line on the fly so it could substitute it's own reset vectors, before we had a chance to set up the MMU.

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.

I remember the three seconds I decided not to do this.

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.

Lol... I remember casram.  Is there a link to the equations somewhere online?



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Received on 2009-03-25 13:42:59

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