Re: [stella] Megacart Specs

Subject: Re: [stella] Megacart Specs
From: Chris Wilkson <cwilkson@xxxxxxxxxxxxx>
Date: Sun, 12 Apr 1998 00:40:35 -0700 (PDT)
> >>>
> The console's address space is divided into 4 slots of 1K each.  Any
> physical memory block can be switched into any memory slot by writing its
> block number to the "hot address" for the desired slot.  Memory locations
> $3C through $3F serve as "hot addresses" for memory slots 0 through 3,
> respectively. 
> <<<
> Is limiting ROM access to 1K chunks necessary?

It's not really necessary.  2k blocks are doable, though maybe not as
flexible.  4K blocks are also doable (replacing ROM with more ROM) but
you get a lot of software overhead and you lose the ability to access
RAM (see below).  You could use 3K blocks for ROM and permently map RAM
into the remaining 1K, but that either wastes ROM space or the decoding
gets nasty from a hardware point of view.

> Isn't it possible to swap out 4K ROM at a time and still access RAM
> somehow?

Might be possible.  But there's an inherent problem.  If you swap out 4K of
ROM and swap in 4K of RAM, 2k actually, then you've lost your code forever.
So we can't do it that way.  Another way to do it is to map RAM into the
$EXXX addresses.  I know that there'll be conflicts with the TIA, but
I can't remember which address lines are used.  I'll dig out my 2600
schematics when I get home and see if there are any unused blocks of
significant size.  Still, I'm not sure what would happen if we did this.
Even if there are holes in the TIA's memory map, I don't know what TIA
does to the data bus when one of these addresses is accessed.  Anybody
else know? The only other way I can think of to do it is to turn RAM
access on and off with hot addresses.  But this would become processor
intensive.  And I'm not even sure it could be done without a lot of hardware


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