[Coco] 6309 microprocessor project: 11-01-2003

[tim lindner]

John Collyer <johncollyer at zoominternet.net> 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?

The three games Tandy produced that used Bank switching all did it the
same way:

You wrote to $ff40 (the hardware was tied to the SCS line), to choose
the bank number.

Mind-Roll had two banks.
Predator had four banks.
Robocop had eight banks.

I don't think any of the paks included circuitry to read back the
current bank number.

Mind-Roll is also interesting becuase it contains a CoCo 1/2 _and_ CoCo
3 version of the game in the same cart.

> Also I'd like to learn more about the Multi-Pak and how it switched
> between rom cartridges?

The entire bus is shared (unswitched) between the four ports except the
following three lines:

SCS (CoCo pin number 36)
CTS (CoCo pin number 32)
Cart (CoCo pin number 8)

On power up the front switch determines which of the four slots these
three lines are connected to. But writing to the location $ff7f can
change the behiavor:

76543210
\  /\/\/
 \/  | |
  |  | +--- SCS slot number
  |  +----- CTS slot number
  +-------- Ignored

Also the Cart signal is selected by bits 1 and 0 or bits 3 and 2, but
for the life of me I can't remember which. Any help is appreciated.

-- 
tim lindner
tlindner at ix.netcom.com                                            Bright