(Picture donated by a reader)
This was an alternative to the Nascom RAM A and B cards. It was the first to allow more than 64k to be installed on a single NAS-BUS/80-BUS computer. Gemini had already produced a 64k RAM card and the GM813 64k processor card. The latter mapped ROM into the RAM area by outputting selection values to port FE.
Capacity was 256kB. It was available as a PCB (42.5UKP in 1983), to which you had to add all the components yourself! Total cost including RAM came to just over 200UKP so it was quite an expensive device. It was also available ready built with sockets for the RAM in 64k and 256k versions. The 64k version was upgradable to 128k, 192k or 256k.
The address mapping allowed this card to work with the Nascom-1, Nascom-2 and Gemini GM811 using 32k memory blocks. It would also work with the GM813 (using a small hardware change) by adding extra 4k blocks! With a Nascom 2 and GM811 the page size is 32k. With a GM813 any 4k page can be mapped into any 4k slot. The maximum supported paged RAM is 1MB. MAP were able to supply special versions of SIMON and RP/M to work with their products.
One of the things that made this card famous was the standard of the documentation. It was dire. Written in a complex style with no circuit diagram and no examples of how to program using it.
MAP80_RAM_Construction MAP80_RAM_Instructions
However, we can now add a circuit diagram, a much clearer re-type of the construction info and quite a bit more information:
MAP80_256_RAM_Construction_Issue_4_retyped MAP80_RAM_256K MAP80_RAM_Circuit
MAP80_RAM_Manual MAP80_RAM_page_Select
Modifications to upgrade the board to 1MB:
MAP80_RAM_1MBdoc MAP80_RAM_1MBmod
Note that the NASBUS does not support direct addressing above 512kB. See the NASBUS page.
nascom.info 