RE: CBM Game Cartridge Configuration

From: Marko Mäkelä (
Date: 2001-07-25 11:15:17

On Wed, 25 Jul 2001 wrote:

> Ok, thanks Marko, but I assume the program is trying to do something like
> this, 
> sta $DFFF (this activates /IO2 and in turn pulls EXROM low)
> jmp $8xxx (run code from ROM)
> The FastLoad Cart uses a capacitor ensure that the EXROM signal stays low
> for the 3us between the store and jump. Is this not necessary due to some
> "internal" or circuit delay?

Okay, now I understand what you were trying to say.  By the way, the EXROM
signal should stay active for much longer than the 3 microseconds you
mention in the above example.  In fact, the program would work, even if
it took the EXROM signal up to 3 clock cycles to react to the I/O2 signal.

The Action Replay fastloader and utility cartridge makes use of something
like what you are describing. It has one 6-bit register that is triggered
on every I/O1 access.  (It's better not to try _reading_ from $dexx when
the cartridge is active, since the register on the cartridge will accept
the value that happened to be on the data bus also in that case.)  The
I/O2 area is mapped to $9f00-$9fff on the ROM.  You can get a very short
documentation of the Action Replay cartridge hardware by reading the
prlink source code.  Look for the "actionreuplay" option that (together
with a hardware modification) allows simultaneous use of the Action Replay
cartridge and the Commodore REU (RAM Expansion Unit).

Note that the Action Replay uses a latch (maybe an 8-port D-flipflop) in
order to implement the register.  You can get the schematic diagram of an
Action Replay clone from FUNET.


       Message was sent through the cbm-hackers mailing list

Archive generated by hypermail 2.1.1.