80-Bus News


January–February 1983, Volume 2, Issue 1

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supplied built in either 64K or 256K versions. The RAM block is socketed to allow easy upgrading from 64K through 128K, 192K to 256K. The quality of the pcb is of the standard expected of NASBUS/​80BUS cards, being double sided through hole plated and coated in a solder resist lacquer. A slight departure from the norm here, as the solder resist is blue in instead of the usual green. Dare I say it, adding a touch of colour to the system. The pcb edge connector is gold flashed as is to be expected. Overall quality of construction is good (the early sample I had was hand soldered, I do not know if production boards will be flow soldered).

My only complaint from the hardware point of view is the manual supplied. This excels in the trend started by the early Nascom manuals and continued by the current DRI CP/M manuals in its total incomprehensibilty. (I hasten to add that there is not a lot wrong with the current Nascom manuals.) It is the moat difficult document to understand. I wasted two evenings trying to access more than 64K, not because it didn’t work, but because I couldn’t understand the words. No examples of use were given, and the description of the mapping system was inadequate and referred to IC numbers, which in the absence of a circuit diagram, was less than helpful.

DH’s conclusion:

The MAP RAM is well built and well engineered, and arguably worth the money if you can find something to do with it (read on), let down badly by totally inadequate documentation. Now software has been written for it, it is of much greater use.

… continued by Richard Real:

The MAP 256K card, which I shall refer to as MAP RAM, is the first memory card for the 80-BUS which offers more than 64K. The history of the RAM cards is worth remembering.

First came the Nascom RAM A, which had 32K RAM and 4K ROM, which seemed marvellous at the time. This card was plagued with problems, some not perhaps its own fault, and it took a long time before a definitive set of modifications to make it work perfectly was produced (see INMC News Issue 7). To be fair, the original specification never included operation at 4MHz, which everyone wanted.

This was followed by the Nascom 48K RAM card, which worked perfectly, and had the apparently useless new feature of a page select system allowing any of four 64K pares to be selected. But this gave the first hints of what was to follow.

Gemini then produced a 64K RAM card, which was useful to disk system users, who needed the full RAM memory. It too supported the four page system. Then came the Gemini GM813 CPU and 64K RAM card which puzzled everyone by having memory mapping. This uses a high speed RAM between the CPU address lines and the memory chips, which alters the addresses selected so that many different physical blocks of 4K are mapped onto the 16 4K logical memory areas. This translation process is set up by output of values to port FE.

When MAP decided to produce a 256K card they therefore had a challenge, which was to produce a card which would operate with all the existing systems. They have put a great deal of thought into their product, and have achieved this aim excellently. MAP RAM works with the Nascom 1, Nascom 2 and Gemini GM811 computers using a new 32K paging system, which is very easy to control. It also works with the Gemini GM813 computer, exactly as Gemini intended, by adding more physical 4K pages which are addressed by the memory mapping system, as a logical extension of the GM813 design.

The next important question is, “Does it work?”. The answer is a definite YES. There is no difficulty in getting the card going. If the card is plugged in to a Nascom or GM811 system, it immediately acts as a normal 64K RAM card. The appropriate software can then be used to activate the other memory pages. If the card is to be used with the GM813, then the two header plugs on the MAP board have to be changed, and one link altered. It would be better if this link had been made easier to change. MAP offer to help if you are not confident of making any of these changes yourself. You can have up to four MAP RAM cards in a system,

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