First, after studying the 74LS783 SAM datasheet, I thought I could make the expansion card almost fully compatible with how the Dragon 64 works, enabling the extra RAM while in SAM "map type 1" (all-RAM). But after some trials and looking more closely at the Dragon 32 schematics I understood this could not work.
So instead this card adds a new "map". When poking $FFBF, it disables the BASIC ROM and the card's RAM is available at $8000-FEFF. Poking $FFBE brings back the ROM again. (This "external map" will only activate in SAM map type 0.)
This is different from a Dragon 64:
- Video RAM access must stay in the lower 32K (because the 6847 VDG can only reach the internal memory). So for instance NitrOS-9 needs a small patch.
- The 32K cannot be swapped into the lower 32K (because the cartridge cannot disable the internal lower 32K on writes). Very few 64K programs use this "page" switch on the Dragon 64.
- To go 64K RAM, poke $FFBF instead of $FFDF. NitrOS-9 needs this modification as well.
It can also be used on a Dragon 64 to have a total of 96K RAM. But there is no software (yet) to use it. If someone ports Fuzix to it, they'll have free boards
There are some unpopulated headers and spare signals for use as a development board. It is made more in that spirit than a "consumer" product.
I would like to charge 20 USD for an assembled board, plus shipping (economy is 3 USD to Europe, 5 USD world). I have 7-8 boards that I can part with. Send e-mail to: