2 Megabyte Nascom

Ian J Clemmett

The possibility of having 2 megabytes of RAM hung onto a Nascom 2 is remarkable but with the new 256K RAM boards from M.A.P.-80 Systems this becomes a reality. You will require a large motherboard of some desciption with at least 9 slots (8 for the RAM cards and 1 for the Nascom main board) but suprisingly little money.

When I was first contacted about reviewing this new board I imagined that it would be more a case of the RAM controlling the Nascom but I was pleasantly suprised to find that the 256K RAM cards were a convenient 8 x 8 with nothing more than 4 banks of 64K RAM chips and some decode and paging circuitry. I had been expecting at least an additional processor but, unless it was disguised as a 74LS series chip, there definitely wasn’t one.

All products have a distinguishing feature of some kind or other and this board is no exception. Its blue. None of your common or garden green or grey but blue. (The writing paper that they sent me was blue as well.)

I was sent, or rather had delivered, the prototype of the board included in a Gemini disc system but I was also shown the card working in a Nascom (my own Nascom in fact). The Gemini system consisted of a processor card, video card, FDC and the 256K RAM card. I was also loaned 2 disc drives so that I could see the card under the control of CP/M.

As I said earlier, the RAM is arranged in four 64K blocks made up of 4864 chips. The minimum configuration is the board populated to only 64K but it ts a simple matter of adding the additional RAM to make the board up to 256k but as the chips are quite pricey upgrading should be done carefully.

As the Z80 is only capable of directly addressing 64K paging must be used to allow access to the RAM. Its not much use having 2 Meg. if you can not use it. The page allocation is done using 2 link blocks which select which 64k block is on which page. The page control is performed via port FE which leaves port FF free for use by other cards eg. RAM B card. The normal Nascom paging system only allows 4 pages but the 256k card gives a five bit decode which gives the user 32 ‘pages’ to play with which works out at 2 Megabytes. The RAM card works with most Nascom/​Gemini ROM/​RAM cards but, unfortunately, your faithful RAM A cards are incompatible though I would imagine that if a bit of paging could be added to then they could be reprieved. If you want to use your G802 board in the system in the 32k paged mode you need to do a minor hardware mod but nothing difficult or complicated.

The bits on port FE are assigned as follows: