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

[Steve Bjork]

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