[Coladam] CV Ultimate SD Cart on ADAM
dalemwick at hotmail.com
Thu Jul 15 14:51:18 CEST 2010
> Hi Rich,
> Thanks for the reply! I'm guessing that memory expansion cards have access
> to the signals for port in/out I/O, so I would not be able to emulate a
> traditional memory expansion as the cartridge port only has read strobes for
> the rom area.
> I'm encouraged that the whole cartridge can be mapped directly into the ADAM
> memory space. It should be possible for any game or program to access the
> onboard CPU or RAM the same way the Colecovision does then.
> In fact it should be much easier for code running on the ADAM, as the
> cartridge is unavailable while the CPU is busy and the Colecovision has wait
> for the I/O to complete before it can continue to execute code from the ROM
> area. With the ADAM I/O operations could be pended and code could continue
> to run without interruption from other areas of RAM.
So there are two tricks to consider if you don't have access to the I/O pins.
It turns out that the RAM expansion slot also doesn't have access to the I/O signals, so for those, they run a jumper wire over to the RAM card. It is much more elegant because the cards are beside each other, and in side the case.
The other trick is that some memory address of the ROM (maybe the last two for example) are reserved for selecting which bank is available. By using writing a value to that address, it would page in the appropriate bank.
Rich Drushel is quite right: making the SD cart look like a harddisk image seamlessly for example can't really be done in any obvious way, since there is no reserved operating system memory in the bottom half of RAM, to use as a buffer to do the minimum needed: bank switch, transfer data, bank switch, and record in the disk I/O buffer. But it could probably be supported in select programs. It just would be super tedious.
More information about the Coladam