Beyond The 64K Barrier

by Chris Blackmore

Anyone who has owned a Nascom for any great length of time will tell you that, unlike systems that arrive in ready-made plastic cases, Nascoms tend to expand almost indefinitely. The abilty to expand was designed into the system from the very beginning, and it shows. When you start out, you think that a 32K RAM board will never be too small. There is left over space in the memory map, and a great many of the expansion boards that are available are memory mapped, which means that they appear to the CPU to be memory, and they take up memory space. Then one day you upgrade your system to 64K of RAM, or perhaps even more than that, because the Nascom will allow you to have enormous amounts of RAM (unlike the aforementioned plastic boxes…). Now your memory mapped sound generator board, programmable character generator, and home made digital clock card become nuisances, as they overlap some of your nice new memory.

So you have to find a way of preventing the waste of memory, and the Nascom provides it in the form of memory paging. Your main RAM board will be on page 0, and all your memory mapped ‘extras’ (unless you have more than 64K of them) will be on page 1, 2 or 3.

So this means that you will have to fit a paging circuit to each of these boards, doesn’t it? No, it doesn’t – one will do! There are some spare lines on the bus, called NDEF1 and NDEF2 in the Gemini 80 specification, and shown as ‘reserved’ in the Nasbus specification, which can be used in more or less any way you wish. I suggest that they should be used to carry the read and write signals to all boards that are on memory page 3.

Only a simple modification is needed on each of these boards, instead of the quite complex task of adding a paging circuit to each of them. The single paging circuit can be built on a prototyping board, of which there tends to be at least one in any system that has reached this stage of development.

Modify each board that is to appear on page 3 as shown in diagram 2, or fit the board with equivalent switching if you want the board to be usable on systems without the page 3 signals, or with software that has not allowed for the use of paged addressing.

The circuit to provide the page 3 read and write signals is shown in diagram 1. It will not take up much space on the prototyping board, as it consists of very few components. It could even be fitted to the end of the mother board if you are that short of space. A header should be made up with links from pins 1 to 16 and 5 to 12 for page 3 operation; different header connections will allow for different page selections it is even possible to have the read signal on one page and the