The Jupiter ACE looks very similar to the Sinclair ZX 81. Actually, the
designers of this computer, Richard Altwasser and Steven Vickers, played
a major role in creating the Sinclair Spectrum.
The ACE is very similar
to the ZX 81, but has some extra features: it has a user port, and it
can generate sound (the ZX 81 can't). It can also use RAM expansion
modules for the ZX 81 smaller than 64KB via a small adapter.
Its most significant feature is its language: instead of using BASIC
like other computers, it uses the FORTH language (as did the Hector HRX).
FORTH's advantages are that it's comprehensible, small, interactive,
fast and extensible. It centers around writing procedures which operate
on objects on a FIFO stack, and is not as intuitive as BASIC. FORTH was
three to ten times faster than BASIC but was not as easy to learn.
This computer did not achieve significant success, probably due to its
relatively obscure language - as far as the general public was concerned.
Steve Vickers wrote the ZX Spectrum manual - he has been an academic
researcher working most recently (2003) at Imperial College London, and
the Open University.