Hmm, that is really surprising about the (lack of) expansion ability - I would
have thought it's one of the primary reasons in choosing a machine of that
class. I can't imagine doing something like in the ZX series (solder-pad edge
connector) would exactly have put the manufacturing cost up by much!
According to this advert from 1977ish some of the CPU signals go directly to
the edge connector at the back, also note the expansion at the lower right
for an external keyboard..
Typical Sinclair misleading advert, then...
The SC/MP has some I/O lines on the chip. IIRC there are 3 flag outputs
(on/off signals controlled by bits in the CPU status register), 2 sense
inputs (which can be read as bits in the CPU status register, one can
also be used as an interrupt line), and serial I/O lines which go to the
'ends' of the CPU extenstion register (which can be shifted under program
control). These lines _are_ brought out to the connector at the back.
But the CPU buses aren't.
-tony