By Max the Magnifi cent
Max’s Cool Beans
Fig.1. The virtual printer (left) and 4-Bit HRRG Emulator (right).
I
have to say that I’m super
excited at the moment, because a
rather cool hobby project that has
been dormant for over ten years recently
sprung back to life. This is something
I’m working on with my chum, Joe Farr,
who is a whizz at both hardware and
software design.
The project in question involves de-
signing and building a 4-bit computer
from the ground up. Seriously, we’re
going to document how to implement
this design using only primitive logic
functions (eg, AND, OR, NAND, NOT),
if anyone were to desire to go this far.
But this is not just any old 4-bit com-
puter. No, indeedy-doody. This is the
Maxfi eld-Farr 4-bit HRRG Computer,
where HRRG stands for Heath Robin-
son and Rube Goldberg. As you are no-
doubt aware, William Heath Robinson
(1872-1944) and Reuben Lucius Goldberg
(1883-1970) were both famous for creat-
ing illustrations of machines that were
intended to perform relatively simple
tasks, but whose implementations were
incredibly complex such that they per-
formed their chores in exceedingly con-
voluted and indirect ways.
So, why do we qualify our computer
as being ‘HRRG’? Well, let me elucidate
(don’t worry, I’m a professional).
Mixing technologies
This all started deep in the mists of time
when I ran across a mega-cool electro-
mechanical relay-based computer that
was created by Harry Porter (no relation
to Harry Potter; http://bit.ly/pe-may19-
cb1). Harry’s machine is gloriously
presented in a series of glass-fronted
wooden cabinets mounted to the wall.
I’ve long wanted to build a relay-based
computer of my own, but I don’t want to
simply replicate something that someone
else has already done. Thus was born the
idea of a mixed-technology computer, in
which the contents of each of the cabi-
nets is implemented using a different
technology, including (but not limited
to) relays, vacuum tubes, discrete tran-
sistors, 74-series TTL logic, along with
mechanical, magnetic, pneumatic, and
hydraulic logic.
One thing with which I’m not enam-
oured about Harry’s implementation is
that his cabinets are linked by great big
bundles of wires. I can’t see my wife
(Gina the Gorgeous) going for that, so
the cabinets in our realisation will com-
municate via a wireless mesh network.
Now, you may be saying to yourself, ‘If
this doesn’t count as HRRG, I don’t know
what does,’ to which I would reply, ‘You
ain’t seen nothing yet!’Decisions... decisions...
I tell you, you have no idea how many
decisions are involved in this sort of
project until you start writing things
down. Fortunately, things are a lot easier
where there are two of you to bounceideas off each other. On the one hand,
Joe and I were tempted to build an 8-bit
computer, but we eventually opted for
the 4-bit version for a number of reasons- not least because we want it to play
an educational role, and – in some re-
spects – a 4-bit machine will be easier for
beginners to wrap their brains around.
Another consideration is that we
fi nd it fun to work around the limita-
tions imposed by a 4-bit architecture,
such as the fact that we have only 2^4 =
16 instructions to play with. Similarly,
the fact that we have opted for a 12-bit
address bus that supports only 2^12 =
4,096 memory locations adds to the fun
and frivolity.
Some of the early microprocessors
had a single accumulator as their main
internal data register. Others sported
two accumulators or a bunch of general-
purpose registers. Also, there is a range
of possibilities when it comes to the ad-
dressing modes one decides to support,
because each mode adds to the com-
plexity of the instruction decode and
addressing logic. Happily, after count-
less hours of discussion, we eventually
tied everything down, now all we have
to do is build the beast.
Returning to Earth
I’m not as stupid as I look, but there
again, who could be? Remember that
we want our computer to be educa-
tional, but even I realise that buildingThe glorious Maxfi eld-Farr 4-bit HRRG Computer