The Gemini computers became the “big brothers” of the Nascom series. After the demise of Nascom, an attempt was made to capitalise on the Nasbus. There was quite sound reasoning behind this; the Nascom 2 already had a suitable edge connector so the Gemini series was initially aimed at producing an upgrade path.

Up until this point, the Nascom system (apart from the CP/M modification to the Nascom 2) had used a “fixed” memory map i.e. different areas with dedicated RAM or EPROM functions. The Gemini system, because RAM was now much cheaper, filled all the addressable area with RAM. EPROMs then “punched holes” in the memory map at any convenient address as required. Because it was designed like this from the beginning, it was far easier to implement CP/M (which requires RAM from address 0000h) than on the Nascom, with its fixed monitor and BASIC areas.

Those of you who know about such things will remember that there were already other computer busses in use. The problem with most of them was the cost. Being “professional” systems they tended to use techniques like active termination, special connectors etc. The Nasbus (later slightly modified and renamed the 80-Bus) allowed simple single-sided 0.1 inch edge connectors to be used, mounted on a passive backplane which could easily be made from Veroboard.

The original specification for the Nasbus included operation on 16 bit systems. However, it was discovered later that this wasn’t actually possible so some of the spare lines of the bus were reallocated when the 80-Bus was designed. The 80-Bus remained fully Nascom-1 and Nascom-2 compatible. The MAP80 RAM card was naughty though. It was a 256kB RAM card but it supported system memory up to 1MB by unofficially reallocating bus line 49 to the missing A19 (it is a logical position if you check). Unfortunately 49 was already allocated as a ground line to separate the address and data buses – the bus has no official support for A19 and has nowhere to put it. The official way to expand memory above 500kB is via bank switching.

The amount of hardware which became available was quite large when you consider the relatively short lifetime of the system. This was encouraged by the release of a “patch board” which allowed home users to design quite complex cards for their own systems.

Advertisements

2 Responses to “Gemini”


  1. 1 Svend Saustrup 12/11/2016 at 1:50 pm

    Being a former distributor for Gemini Microcomputers in Denmark, I find this page very interesting. GOOD JOB !!!!
    And now for a question: Do you have schematics for the Gemini cards?
    I once had them all, but at the moment they are invisible.
    Contact: svend(at)saustrup.dk

    • 2 mbnaturists 13/11/2016 at 6:39 pm

      I’m sorry, but all the details I have are on here. I’ve only been able to include information from boards that I have or that has been donated.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s