[Coco] Banking ROMS (was-6309 microprocessor project:11-01-2003)

[Robert Gault]

There was another approach taken by Greg Zumwalt for the RoboCop and Predator
cartridges. These paks used a 64K ROM, a SN74LS10N, and a SN74LS175N to switch
the ROM as four 16K banks. This meant that if the programs did not use Coco3
specific programming (which they did), the 64K pak could be used with a Coco1 or
2.

The specifics of the technique were detailed in Rainbow, June 1990, p.58-64,
where Zumwalt mentioned in passing the Super Program Pak II using 1024K by 8 bit
ROMS or higher!

Steve Bjork wrote:
> 
> Good Day John,
> 
> Trying to remember from a "A Long Time ago on a computer far far way...."
> (Back when I worked with Tandy on creating Games for the CoCos)
> 
>   The Color Computer 1&2 had a simple ROM limit of 16k.  With the
> introduction of the CoCo 3, the ROM limit was double to 32k by using the
> option of disabling internal 16k BASIC ROM.  But that is the limit to the
> standard ROM Pak.
> 
> There are two reasons for the limit.  First is the size to the CPU's
> addressing window to the ROM Pak. In the case of the COCO 1&2 the limit was
> always 16K.  In the case of the COCO 3, the internal ROM could be disabled
> to expand the CPU's addressing window to 32k.
> 
> The second reason for the limit is ROM addressing did NOT use the same
> addressing system as the RAM.  While RAM uses a bank control system (via
> the GIME) to page in the memory, the ROM get's its addresses straight from
> the CPU without any page bank.
> 
> So how do you access more than 32k of ROM on a COCO 3? You must put the ROM
> banking circuit on the cartridge. While this is not unheard of (common on
> the Atari 2600), not so for the COCO because this would double if not
> triple the cost of the COCO 3 Game Pak.  I don't recall any Game Paks
> released with more than 32k because of the overriding cost issue.  (But I
> could be wrong.)
> 
> My Rampage game did use 32k ROM, but it was hard to fit in that small of a
> package.
> 
> I remember hearing a story about how useful Rampage.  The COCO guys back at
> Tandy had display a CoCo 3 and Tandy 1000 side by side with both running
> the game.  In walk the execs for the COCO and PC (Tandy 1000) lines and
> start looking over the game running the two computers.  After a few
> minutes, the PC guy said to all of the other execs  . . .
> 
> "This is why we need to kill off the Color Computer, just look how much
> better the PC version is over the Color Computer version!"
> 
> One the COCO guys steps up and said "Sorry, But that's the Color Computer
> you are pointing to."
> 
> Not only did the PC exec have egg on his face, but the Color Computer line
> was saved that day!
> 
> Just a moment in Color Computer History.
> 
> Steve (6809er) Bjork
> 
> At 08:20 AM 11/1/2003 -0500, you wrote:
> >Could someone explain how the newer coco3 rom cartridges worked.  The ones
> >that accessed
> >more then 32K ROM?  Where the ones that did use more then 32K using the
> >same memory map
> >and switching methods?  How did these rom cartridges switch memory
> >in&out?  What was
> >the most ROM, a cartridge ever used?
> >
> >Any help appreciated; thanks.
> >
> >John Collyer
> 
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