80-Bus News |
July–August 1983 · Volume 2 · Issue 4 |
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Starting off back in the Nascom days, the main expansion memory was the domestic tape recorder. Using this programs and data were saved for reloading later. The data transfer rate varied from about 110 Baud upto 2400 Baud. (A few adventurous souls reached the dizzy heights of 4800 baud). In fact there must be megabytes of data held on the humble audio cassette, which was recorded on ordinary domestic tape recorders. The next step on from this was the arrival of the floppy disk in the form of GM805, followed in a little while by double and quad density disk drives and the various 80-BUS disk controller boards. The disk systems offered far higher performance than the cassette, and after a while those who were using disks began to wonder how on earth they used to manage with cassette tapes. The trouble is that users’ habits change to meet the capabilities of the equipment they use. With the faster access time of disks they started using multipass compilers, linking loaders, and a lot of other software that uses disk for the intermediate storage of data. So after a while the cry goes out again for higher performance. The next step on is the Winchester disk. This offers high capacity, (you don’t have to keep changing disks), together with faster access times. But unfortunately it is expensive, requires a specialised controller (which isn’t exactly cheap either!), and is still a mechanical device with the attendent constraints on access times. By now some programs are manipulating large amounts of data, and as the system memory is finite (64k), most of this manipulation is done via intermediate storage on disk. Life would be far better if more memory was available.
Various approaches have been used to expand the address space of the Z80. The two that 80-BUS readers will be familiar with are the original Nascom “page-mode”, where individual memory boards are paged into and out of the system, and the Gemini memory mapping scheme implemented on the GM813 CPU/Memory board.
Any method of expanding the address space of the Z80 suffers from the problem that it tends to be unique to a particular system, and no standard software will support it. Only software written specifically for the system would be any use, and it is likely that software like that would be extremely thin on the ground. So the approach being taken now, by a large number of manufacturers, is to use the extra memory in conjunction with disk operating systems (DOSs), and to make the memory appear as a disk. This solves virtually all the problems. The DOS handles the management problem of organising data in the extra memory, and all that has to be added to the BIOS of the disk system, is a simple driver to convert a disk track/sector read/write request into a read/write request to a specific area of this extra memory. All the standard disk software still runs perfectly under the DOS, but any read/write request to the “Memory disk” will result in an immediate response. This is because it only takes a few microseconds to locate the wanted “track” and “sector” on the memory drive, as opposed to tens or hundreds of milliseconds on a mechanical drive.
Let’s now consider the various ways of implementing the memory drive.
The disadvantage of the RAMB/GM802 page mode is that it switches an entire memory board in and out of the system. Considering the case of GM802 (a full 64k board) this means that when a board is paged out, the running program
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