memory map, meaning that none of the Nascom software will run without
alteration, but is a de facto professional system, with built in software
compatibility with other CP/M machines and lots of ‘off the shelf’ software. A
lot has been written about CP/M elsewhere.
The next is the Gemini double density system. This uses a NASBUS/
80-BUS compatible 8" x 8" disk controller card which plugs into the bus. The
controller chip is the Western Digital WD1797 and its attendant support chips,
all very new devices. Port decoding is done on board and may be decoded to
either ports starting at C0H or ports starting E0H. The card is capable of
either single or double density operation. The card is called the G809, and is
one of the Gemini ‘Multiboard’ range. Software has been optimised for the same
drives as above, but due to more sophisticated (and expensive) hardware, it is
capable of operating in double density. This gives 350K bytes of formatted
storage in each drive. Other drives, including 8", can be used but software is
only available for the Pertec 5.25" drives. Gemini also supply drive boxes
fitted with one or two Pertec drives and PSU. For those capable of ‘knocking
up’ a PSU and box, the drives and PSU being available separately.
There are two choices of software, POLYDOS 2 for Nascoms, which is
similar to POLYDOS 1 (above) but with different disk controller software for
the G809 card. Then there is CP/M 2.2 in two versions, one for the Nascom and
one for the Gemini ‘Multiboard’. The reason for the two versions is because of
the different video and keyboard requirements and because the I/O support
(serial and parallel input/output) is different between the Nascom and Gemini
machines. When using CP/M, media compatibility between the Henelec system and
the Gemini system is maintained as the Gemini CP/M is ‘auto density seeking’.
Put simply, the Gemini double density system, when presented with a disk it
can not read in drive B, switches automatically to single density to retry
before rejecting a disk as being unreadable. Thus the Gemini G809 system is
capable of reading disks from either system.
The newest system on the market is the Nascom disk system. Originally
dreamed about by Nascom in pre-receivership days and finally got into
production by Lucas two years after the first specifications were drawn up.
Like the Gemini this is a Nasbus 8" x 8" inch card which plugs into the bus.
This card uses the Western Digital WD 1793 controller chip which has been
around some time. The card is capable of either single or double density
operation. Unlike the Henelec or Gemini, the Nascom system uses ‘double track
density’ drives, which write 80 tracks at a density of 96 tracks to the inch.
The drives being only single sided, squeeze roughly the same amount of data on
the one side of the disk as the Pertec squeezes on two sides. The drives are
the TEAC TD-50C single sided drives and are supplied in a box with a built in
PSU. Nascom have exhibited a drive box which matches their new Nascom case,
but to date drives have been supplied in square metal boxes which, if you peel
the labels off the box, you will discover were bought complete from Cumana in
Ilford (they might be cheaper if you tried them direct). There is little
advantage in double track density to the end user, except the drives are
marginally cheaper than double sided drives (to buy in quantity – Nascom
charge the same as Gemini for single drives bought over the counter). At
present double track density causes media compatibility problems between the
competing systems, as no control software has been written (yet) to allow the
double track density drives to ‘double step’ (96 tpi being double 48 tpi) thus
allowing them to read single track density media. Even if double step control
software were written it would not be possible for the Nascom system to write
disks which could be read by single track density drives. No doubt some
enterprising dealer will either tag a Pertec drive on a Nascom system or a
TEAC drive on to a Gemini and write the necessary software to allow ‘cross
fertilization” and charge for the priviledge. A complete Nascom disk system is
almost exactly the same price as the equivalent Gemini system.
There are two DOS’es for the Nascom system. NAS-DOS is the earlier
mentioned DCS-DOS with rewritten disk control software and allows existing
Nascom software to be run. Then there is CP/M 2.2 for the Nascom system.