ScuzzBlog: Diaries September 2021

Entry 28th September 2021: Post 1: ZX Spectrum +3 128K - +3DOS.

ZX Spectrum +3 128K - +3DOS.

Continuing on the adventure into early DOS systems, today we look
at +3DOS for the ZX Spectrum +3 model.

With the introduction of the ZX Spectrum +3 in 1988 came the need
to provide the Spectrum with a disk operating system. And so was
born +3DOS. Turns out that +3DOS is a modified version of Amstrad's
PCWDOS used on the PCW computers. Note that we are in the era now
of Amstrad's ownership of the Spectrum. The +3DOS was compatible
with the CP/M disk system, and as such was the first Spectrum to
run CP/M software.

The DOS routines that are embedded into the BASIC system are held
in ROM2 , the +3DOS ROM. There is no provision to run +3DOS in
48K Spectrum mode. The 48K BASIC ROM is held in ROM3. The commands
therefore for +3DOS are held and integrated into the Sinclair
and COPY. Note ROM1 holds the integration code to enable BASIC to
incorporate +3DOS. These first two ROMs were added to the
Spectrum to provide the +3DOS provision.

Production of the ZX Spectrum +3 was ended in December of 1990.
The operating system was maintained until 1992 when all Amstrad
Spectrum models were withdrawn from the market.

It follows therefore that the ZX Spectrum is pretty clever in the
way it handles DOS given that it forms part of the core structure
of BASIC and needs no special syntax or added symbols to access.
This is due in whole to the 3" drive being integrated into the
+3 model and assigned Drive A: as the primary drive. Therefore
when saving on the +3 the computer assumes a SAVE command is going
to be to disk. It recognises all DOS commands from the BASIC

Simply typing CAT on screen with a disk in the drive will display
all current files on the 3" compact disc. The unit uses the same
3" compact discs such as those made by Amsoft that are typical on
the DDI-1, 6128, PCW range. However the computer is not able to
read discs created and written on other systems [ annoying that ].

The power supply for the +3 is a large brick with sufficient power
to drive the disk drive. The computer does not have an on/off switch
and so I have fitted a switch plug to the power supply with neon

The +3 is the best computer of the ZX range. When Amstrad took over
the manufacture of the computer they introduced a whole raft of
upgrades that were common on the CPC range. Like the full size
keyboard and either tape or disk drive built in. The 3" compact
disc became the preferred disk on most home models from Amstrad.
Quite personally I would still have bought a 6128 in the day.

OK that was day three of my DOS review.

ZX Spectrum +3 128K - +3DOS.

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Last updated 28th September 2021

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