[Coco] Fwd: Re: 32K carts?

[flexser at fiu.edu]

Joe, I'm not sure about the cartridges you mention, but some large cartridge games used a bank-switching scheme that relied on circuitry inside the cartridge to switch between 8K banks of ROM within the cartridge by poking a bank number into $FF40(?) to access different parts of the code.  You might take a look at the game's code to see if you see anything that looks like that.  They may have avoided using the 32K external mode in order to be able to run on the CoCo 1/2.

Art

---- Original message ----
>Date: Mon, 22 Oct 2007 13:21:36 -0400
>From: "Joe" <joef6809 at gmail.com>
>Subject: Re: [Coco] 32K carts?
>To: "CoCoList for Color Computer Enthusiasts"
<coco at maltedmedia.com>
>
>It seems to work like this.
>When the gime is set to 16K Internal /16K External
the internal rom runs
>from $8000 to $C000 and the External rom runs from
$C001 to Whatever. We all
>know that. When you switch to 32K External it gets
wierd.
>$C001 to Whatever is mapped to the first 16K of the
ROM and $8000 to $C000
>maps to the last 16K of the ROM. This makes sense
as any code running from
>rom say at $C??? would need to turn on 32K External
mode, but when that
>happens if it was a liner map the code that was
executing would "jump up"
>16K.
>Hope I've explained this so others can understand.
>When I get home I'll hack up a test version of VCC
and see exactly what
>Arkanoid is doing. IE what GIME registers its
trying to hit.
>
>----- Original Message -----
>From: "Roger Taylor" <operator at coco3.com>
>To: "CoCoList for Color Computer Enthusiasts"
<coco at maltedmedia.com>
>Sent: Monday, October 22, 2007 11:35 AM
>Subject: Re: [Coco] 32K carts?
>
>
>> At 09:52 AM 10/22/2007, you wrote:
>>>Hi guys,
>>>
>>>Is there anything special about a 32K cart?
>>>
>>>I'm trying to work out why I can't get Arkanoid
or Rampage to run out of
>>>EPROM on my cart board. I assumed the top 16KB
would get mapped into $C000
>>>and the ROM bootstrap code then enables the full
32KB of external ROM
>>>space.
>>>But it would appear to me that Arkanoid at least
doesn't do this.
>>>
>>>What I think is happening atm is that when I
enable the 32KB jumper on my
>>>cart, the internal and external ROMs both drive
when the CPU is accessing
>>>memory from $8000-$BFFF and it's not a pretty
sight... :(
>>>
>>>So I'm wondering how 32KB carts actually
function?!?
>>>
>>>Regards,
>>
>>
>>
>> That's something I never learned, but it seems
that the internal CoCo 3
>> ROM area from 32768 to 49151 is there no matter
what, and a 32k pak might
>> have to switch in 16K at a time starting at
49152/$C000, no?
>>
>> Hopefully you can add a jumper or two on your
boards to fix any problems
>> like this.
>>
>>
>>
>>
>>
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>
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