The machine moved to the museum
over a year ago now and has been running ever since ! Hardly any
problems. In fact it has just celebrated it's second birthday and
I went in to say hello.
During the move there were a couple of issues, it really doesn't
like being moved. In the early days it had to be shuffled around a
bit as the place was being decorated and usually one or other of
the inter-frame cables worked loose doing this but that was easy
enough to fix. The biggest troubles were caused, ironically, when
fitting protective perspex panels.
We slipped when trying to fit the panel to the memory frame and
trashed one of the memory boards...
And unfortunately a transistor died somewhere in the state machine
at much the same time. I've not yet managed to find it. It's
causing strange voltage levels to propagate around so the logic
handling iterative instructions doesn't work. So no
multiplication, division, square root or shifting. Fortunately we
don't need these for Tetris, so its not urgent. But I would like
to fix it.
The grand opening was on 24th November 2017 and was good fun.
During the school holidays the museum put on one or two sessions
where I go in for an hour or two to talk/demonstrate to anyone who
Another highlight was getting an "official" world record from
Slightly contrived category but really quite chuffed nevertheless.
And also we had another little burst of publicity when Jason, the
director at the museum, was interviews for Computerphile and
did a pretty good intro to the machine.
A slight irritation during the year was some miscreant
continuously hacking the website to try and sell dodgy watches.
After much huffing and puffing the site host finally let slip that
I was on an ancient legacy system and perhaps I should move, for a
fee, to one of their more modern offerings. I did move, but to
somewhere else entirely and things seem a lot better now.
Well last week the team from the museum
turned up (L to R: me, Steve, Bryan, Chris) and we took
apart and wrapped up the machine...
packed it into a van...
drove across Cambridge ...
and unloaded it....
I then recreated my test system and spent the next week
All in all things went very well. Only three solder joints failed.
Much more surprising was that I found about ten joints I had not
soldered in the first place !
It's now up and running ....
but we've not yet completed the installation. There's a bit more
testing (I have my suspicions about the memory frame, there have
been anomalies in the past but it decided to behave itself when
under test); we need to properly fix it in place (to the wall,
sort out windows, add protection etc); and then stitch it into the
rest of the museum. Maybe a few weeks to go, but the hard bit
looks like its done. It has also now run a whole day kind of on
display without me having to nurse it along.
The Megaprocessor has a new home. The Centre
for Computing History based in Cambridge. The move will be
in a few weeks time, middle of October. It's going to be
"interesting", I suspect a few solder joints will fail and we'll
find out just how good my test system is.
So I need to finish my set of spares (nearly have) and write some
kind of Owners Manual. And finish testing some of its function (it
has a UART I've never used). And it would be nice if it had an
audio beep function. And ...
Now the build has finished the pace has slackened somewhat but
stuff is still happening.
The next Open Day is 10th Sep. I've had several so far which I've
really enjoyed. The beast has mostly behaved itself.
It's been a couple of months since the build finished so we can
maybe start to make a judgement with regards to reliability. The
impression I have so far is that if you don't move (or shake) it
then everything will be fine. Unfortunately I have had to move the
memory frame around a few times and it's developed a couple of
faults. Currently these are benign in the sense that they don't
break any of the programs I currently have. My inclination at the
moment is to leave well alone and wait till it breaks properly and
fix things then. There's also a loose connection somewhere in the
control logic for the PC mux. This revealed itself when I had a
large group of people here and things were being shaken a little,
but disappeared before I could properly locate and fix it.
All of this means that when the Megaprocessor is moved to its
final home I can expect a bunch of problems. I'm (currently)
relatively sanguine about this, I think I have a good set of tools
to find and fix these kind of faults. As a backup I'm also
building up a small kit of spares (the soldering has begun again
A few people have sent me (or brought in) programs they've
written. I've filmed them, or at least their output on the Memory
frame and uploaded them to this YouTube
Something that has caught my interest is looking at the website
traffic and trying to work out what is happening and what it
means. I don't know yet but my first observations are here.
25th June 2016
Several people have remarked that the videos can be slow to
download so I've uploaded them all to YouTube at https://www.youtube.com/c/Megaprocessor
I've created two playlists. One is a collection of those created
during test and integration. The second is the set of videos for
the Stepping Stones page.
22nd June 2016
It's built ! And it works !!
Here is a video of the complete machine: tour_video.
There's a few imperfections:
a couple of display LEDs aren't quite right - I think
there's either some marginal build quality or a transistor is
in distress. We'll find out which when it goes pop.
the variety of multiplexor boards with their different
layouts is unhelpful
it doesn't go as fast as it should - there's a bit more I
need to understand
But it works, so I'm declaring version 1.0. From now on its
tinkering rather than building.
I've also started a new page, Stepping
Stones, to try and show how one gets from a single
transistor to a whole microprocessor. This could take a while to
23rd May 2016
Progress is very good. The five frames that make up the body of
the processor itself have been integrated together. They ran for
3 solid hours executing about 10,000,000 cycles without fault
(the speed, of about 1kHz, is limited by my test system). This
is a key stage. It looks like there is now a real chance of
success. One real practical benefit of this is that now I don't
need all round access to the frames I can mount them hard
against the walls and reclaim some of my living room for living
in. Here is a little video of them under test.
I can't begin to describe the sense of relief I felt when those
three hours ended, unreliable operation would be a nightmare.
Ten million cycles without fault. This is good.
Once everything was in place it took about two days to stitch
in the fifth frame:
There were two bugs in the test software:
the test starts by generating an initialisation sequence
to coerce the wannabe Megaprocessor into a known state
before the test proper. It sorted out the PC, R0, R1, R2, R3
& SP registers. But forgot to do the PS register. Doh !
the other bug is too horribly detailed to describe but
meant that the test program (but not the hardware) got the
wrong answer to division under some conditions
There was some dodgy soldering on one of the connectors.
Everything looked fine at a first, second or third glance. But
the problem was easy to spot once you'd dismantled the right
board. And looked under a microscope. From the right angle.
And a new failure mode.. a mere whisker of a short.
My earlier testing did not pick up on these. My guess is that
they had not manifested at that time. I suspect there are a few
more of these faults waiting to occur. Each time I move the
frames around one or two more will reveal themselves. I'm
actually quite relaxed about these. They are finite in number.
And my test strategy will localise them tolerably quickly when
they reveal themselves. The nightmare scenario was random
transistor failure. So far, since the ban on Henry,
there has been no evidence of this. The other big fears I had at
the start were:
badness with my clocks. That happened and is now (I
believe) under control
badness with my power distribution and/or grounds. I
currently believe my approach has been satisfactory
The current situation is that the seven frames are now
integrated as 5+2. The 5 are as good as a I can test for now.
The 2 have some issues and I will now turn my attention back to
them. When the 2 (these are the control and memory frames) work
properly together then I can join the two "halves"
together and we'll have (give or take a few more problems) a
working Megaprocessor. We're really close now.
Here's a question for you.... when you think of the
Megaprocessor do you think of food ? No ? Me neither. Try typing
into Google "hostess wow your dinner guests". At the time of
writing (this'll no doubt fade) the Megaprocessor is ♯1 ! Has
been for two weeks now. I check every day and it still tickles
me. It's courtesy of a rather jolly headline from The
Verge. They picked up the Hackaday
article and added a twist of their own.
Quite a bit of the last three weeks or so has been trying to round
out the web site. The architecture page(s)
is now going in what I hope is the right direction. It's at least
better. And I've uploaded my current simulator and assembler to
the programming page. With user
Those of a similar vintage to myself might be interested in this project
building a 6502 from discrete components. Looks like a very fine
board (could do with a few more LEDs).
[I grew up with the BBC micro and spent hours of my youth
programming 6502 assembler. The memories of that time are why I
wouldn't bring another 6502 into this world.]
1st May 2016
We now have the four right hand frames running together which
are Input & Decoding, Special Purpose Registers, ALU,
General Purpose Registers. With the current state of the
living room I can't really show them in action. You can get some
feel for the conditions from this picture:
Top left you can see the armchair which is all the space that's
left for me. On the left are the two front end frames being
integrated together, and on the right are the five frames of the
processor proper. When integration is complete I'll be able to
rearrange things and liberate some space but for now it's rather
Still going fairly well but this frame was a bit harder. Had a
new failure mode. Fortunately very simple to fix, but had me
confused for a day or two. Some of the connectors weren't
I was pacing up and down in front of the stalled machine when
suddenly some lights flickered. Ah hah! dodgy connection. After
seeing the above I went round all the ribbons and find 11 that
weren't completely pushed home. Most of them, and all the worst,
were on this fourth frame. Strange.
I also had a couple more bad solder joints. So far these have all
been when soldering wires onto terminals rather than on the PCBs
themselves. Even under a magnifying glass its not obvious to my
eyes there's a problem. It does become clearer under a microscope.
Thankfully they're not too painful to track down. When a
problem occurs I rerun the test to the cycle just before it went
wrong. I can then compare the signals of the hardware just before
the point of error with what the software simulation says they
should be and, so far, the offending signal is easily found. I've
extended the growing list of build
And Hackaday have done an article on the Megaprocessor here.
26th April 2016
I've just reformatted the web pages so that hopefully they
might render in a sensible fashion on smaller screens.
Whilst sorting out a few things I came across some of my
earlier ideas of how the Megaprocessor might look (lot smaller !
and cheaper !!) and how it developed. I've started to write it
I've also filled out the pages on construction and finished
uploading pictures of all the boards, modules and frames. Might
have to redo a few, but at least there is now a complete set to
21st April 2016
I guess I'm nominally about half way through the integration
plan now. Things are generally going well, certainly a lot
better than I feared. But maybe this has been the easy half and
trouble lies ahead. And my living room is getting pretty crowded
The Megaprocessor has seven frames. I've just today had the
right hand three (Special Registers, ALU, General Registers)
working together. A week or two back I had the two left hand
frames (Control and Memory) talking to each other. Here's a
couple of videos:
Hardware wise its all been pretty good so far. A couple of
transposed cables. Just one, that's right, just one, failed bad
solder joint. I have to say that once you know roughly where the
problem is the LEDs can make it fairly obvious.
No transistor mortality as yet, about which I'm enormously
There has been a couple of significant problems.
There was a bug in my interrupt handling which my modelling showed
but my tests didn't pick up. When an interrupt occurs the procesor
is supposed to store the address of the next instruction so that
it can pick up where it left off once its processed the interrupt.
Unfortunately I forgot that beause it would have tried to prefetch
the next instruction the PC was pointing to the next but one
instruction. I thought the fix was fairly straightforward ... but
as I write this I'm thinking I may not have solved it after all. A
few more tests are called for.
The other main issue has been my clock. I shall write up all the
gory details later but essentially my slow (GBU_Speed) driver circuits meant that
my clock got salami sliced to nothing as it fought its way from
one frame to the next. To be fair to myself I had anticipated this
a while back and had planned to swap the resistor values for the
clock drivers to speed them up. Its just that I forgot to do so so
I had a bit of an initial disappointment when things didn't work
to begin with.
18th March 2016
Just completed step 1 of the integration plan! This was
essentially a first go at getting all the parts of the Control
& I/O frame to work together.
The test was to download a program into the chip RAM and then have
the processor (the FPGA based emulator) boot and run it to output
a counting pattern on the output port. This doesn't test much of
the true Megaprocessor itself (only the external interface) but
it's a big step forward. A lot of things have to be right to get
this lot to run together. A key part of this is that it is the
first time a version of the Megaprocessor has run on
hardware in the physical world rather than software simulation.
True it's on an FPGA rather than on all those beautiful
transistors but it is progress.
It was fairly hard work. At step 1 everything is buggy and nothing
can be trusted so there was some horrible interactions between
bugs so once the easy ones were fixed I had a couple of days of
bewilderment at the end until I was able to untangle what was
going on. So far it's just small errors (missing connection,
disagreements over signal polarity, another bad ribbon cable)
nothing architectural as yet.
The next few steps on the plan (there's about 15 or so to go) are
just working through all the different peripherals (timers,
interrupts, UART and the Venom joystick). The next big landmark
will be connecting the Control & I/O frame to the Memory
I've added a couple of new pages. Various people ask how much
the project has cost and how much "stuff" I've used. The new
& Materials has some answers. The terrifying fact I
learned when writing it is that the Megaprocessor has well over
a quarter of a million solder joint. Only needs one to break...
But one good bit of news is that I've been underestimating the
number of LEDs. It seems the total is just over 10,000. Double
what I thought before.
I've also added a few more entries to the Good, Bad & Ugly series.
However most of the last few weeks have been sorting out the
tools I need for the final integration. Firstly is a much larger
I/O expander. This'll let me connect up to 48 of my 16 bit buses
which will let my test program drive whole frames (and sets of
And the other key tool is a hardware simulator of the
Megaprocessor for which I used a little FPGA board.
The whole Megaprocessor fits in a small corner of the square chip.
The chips I've added round it are for the 3V / 5V level
Which means I think I now have all the tools I need. So
integration starts tomorrow !
I've treated myself to a sneak preview of what the Megaprocessor
might look like when fully deployed. Wildly distorted by the
panorama stitcher, I'll try and do better next time.
I've been sorting out some of my software and tools ready for
the final push. As part of this I now have my launch programs
ready to go; Tetris, noughts and crosses (tic-tac-toe) and Life.
Doing these programs drove me to finally sort out what I would
actually use for user I/O. I dithered about building various
things but in the end I modded a Venom Arcade Stick so we have a
joystick and a few buttons.
Normally it has a USB interface and expects to be connected to
a Playstation. It was a very civilized thing to mod.
To develop/test the programs I extended my simulation to
include a model of the peripherals, here are some screen shots.
Now that the simulator is up to scratch the next step is to
upgrade the hardware test tools which will take the next couple of
weeks. Then, finally, integration
After having used so many transistors I thought it would be nice
to see one in the flesh. So I got hold of a few larger ones and
sawed the top off to have a look inside.
The first time round I thought I'd destroyed it because I couldn't
find anything. So I chopped up another one being careful to remove
as little as possible. Still nothing. Well actually it was there,
just tiny tiny tiny. Treated myself to a new microscope to have a
You can just make out the tiny gold wires connecting two of the
legs to a tiny little square in the middle. That little square is
the transistor. Tiny. (The third leg connects to the metal case
which is in contact with the bottom of the transistor).
In more detail:
I've built the Controller & I/O frame !
This combines the Igor and I/O frames of my initial sketch. Igor
turned out to be somewhat smaller than I initially thought.
[You'll see some chips in this picture. I'm allowed them. They're
not part of the processor proper but its peripherals. I did
initially intend to do the peripherals with transistors only, and
I might still do that. But I'm rather keen to see the processor
run sooner rather than later so thought I'd allow myself a
shortcut for now. The double line you might be able to see
meandering across the middle of the module marks the boundary of
the processor (above) and peripherals (below).]
It went together reasonably easily, there was a final sting in the
tail from the great RS transistor disaster which meant I had to
build a couple of extra boards. Most of the time was spent writing
programs for the FPGAs and PICs.
With this seventh frame we now have enough to construct a
This means that the build phase is over !
the soldering is done !!
the soldering is
the soldering is
There's still work to get to the end but the long long slog of
endless soldering has ended. Still can't quite believe it.
So what remains to be done -
Having finished the frames I need to go back over the
earlier ones for some remedial work. Fitting the faster adders
I've mentioned in a previous entry, I've noticed various
inconsistencies in signal labeling, and a few other little
I also need to sort out the software/documentation side of
the project. During the long march to get here things have got
a bit tatty and need to be trimmed and tidied up to make sure
everything is clean and consistent.
I need to uprate my test tools, both hardware and software,
to support the integration phase
Then its integration, wire the frames together in stages
testing as I go. And make it work. (How hard can that be ?) As
each frame gets its final signoff there'll be a bit of
cosmetic work as everything gets fixed down for good.
Write some programs.
And finally order the champagne and canapes and have a party
There's still scope for a few problems to emerge from somewhere
but surely now the end has begun.
Here is how the 7 frames are now stacked up in my living room.
The sofa had to move out at Christmas time, but I brought back one
of the armchairs so there is still a little comfort to be had.
That'll have to go when integration starts.
On a historical note whilst sorting things out I came across my
very first prototype gate.
I've completed the memory frame !! Can't really believe it,
it's been an epic amount of work. Here's a little video of it in
As well as the boards trashed with the wrong transistors I also
had to build another 4 of the memory boards to replace those I
couldn't get working. If a single bit cell was broken then I
could generally debug and fix it. But when the failure was such
as to take out a byte or a column of bits then it usually proved
beyond me, or at least it was taking longer to work out what was
wrong than build a new board. There are 32 memory boards on the
frame (8 bytes each). Out of the first 32 I built only 11 worked
first time. Just 1 in 3. So there was a lot of debugging. I'm
not sure but I think the main issue was solder bridges. On a
couple of occasions when I had chopped off what I thought were
the problem transistors I could see something like the picture
I don't think these were caused when the transistor itself was
being soldered in, rather it's the mess I created when soldering
the resistors. Occasionally when trying to solder a resistor I
would miss the lead and hit a pad meant for a transistor and
fill it up with solder. I then used a solder sucker to clear out
the blocked hole. I took care to clear up on the side I was
working (the solder side) but forgot to look on the component side
where it turns out the solder sucker was creating flashes of
solder which went on to form the bridges. Once I worked this out I
was a lot more scrupulous about clearing up both sides. For what
its worth the extra set of 4 boards that I did at the end all
worked first time, not a brilliant statistical example but does
provide some confirmation.
The memory frame differs from the others in that it has boards on
both sides. The boards on the front implement the actual RAM, here
they are shown part way through the assembly.
And then there are a set of boards on the reverse which multiplex
the 8B boards together to form the 256B.
The arrangement is easier to see here:
It took about 50 ribbon cables to connect it all together. And
that was another source of woe. For some reason this time I made a
lot of bad cables. 8 of them had problems, took a while to
untangle that mess.
I've had a small disaster on the soldering front. The last
batch of 4000 transistors were wrong. I ordered the right ones,
the packing list said they were the ones I ordered, and they
looked like the ones I wanted. I didn't check further than that.
But they were a completely different type. So I soldered them
into the boards. 2 weeks time spent carefully and
accurately crafting complete junk. That knocked the morale a
But I have now finished the frame for Special Purpose
A few problems on this frame.
The main mistake was another bad ribbon connector where bits
2&3 of a bus were shorted together at one end, but at the
other end bit 3 was floating. That took some hours to sort out.
The other big issue was a mismatch between my simulation and the
hardware. On this frame there's a few byte rather than word values
being moved around but I still use 16 bit buses. In software land
the MS bits were zero but in hardware land they were floating and
read as ones. So my tests all failed. It is possible that it
doesn't matter. But it was easier to do the correct thing and fix
the hardware to set the MS bits to zero than changing the
simulation to see if I could get away with not doing so.
All of which means that I have finished the 5 frames that make up
the processor itself !!!
In principle I could wire them together and have a processor, but
we need something to connect them to. Memory frame next and then
I've taken a bit of time out of soldering to start padding out
this website and have added a page where I try and answer a few
questions and add a bit of detail, see here.
I've mad a lot of progress over the last month or so on several
Firstly I've finally broken the back of the soldering. Still a
few weeks to go but I can now see the end. Huzzaah !
The second chunk of work has been to complete all the metalwork
and carpentry for all the frames. This does mean the living room
is getting rather full now.
I've had to lose the armchairs, but am still clinging onto the
sofa and TV. The upside is that I can now walk down the hallway
without having to thread my way through stored materials. From the
left you can see the four completed frames, the frame for RAM on
its side, special registers ready to start assembly, and then the
frame for I/O and Igor.
The third chunk of work has been to sort out what I'm doing for
the non-processor frames. In my initial sketch there were four of
these (RAM, ROM, I/O, Igor). For "Release 1" there will be only
two. Critically this means that I can set up the beast in my
living room, or at least that's what the plan claims.
You'll notice that ROM has disappeared. One of the peripherals I'm
adding is a 32k RAM chip so I can run some bigger programs. I've
dual ported this and put a micro on the other side so that I can
download programs/data from a host PC. And one of the features of
the Megaprocessor is that you can switch the reset boot vector
between 0x0000 and 0xFFF0. So to begin with it'll boot from the
big RAM chip, and then if/when I do the ROM it can boot from that
Igor was a place holder for the control station. I didn't quite
know what I wanted at the start. Certainly an ability to control
the clock speed, reset and maybe run/halt, but I wondered if I
would have some status display or something, In the end I'm not
doing much and what I am doing doesn't take much space so I've
merged it with the I/O.
I've decided on the peripherals I'm adding on the I/O frame. These
a 32K RAM chip
I'm implementing these with some FPGAs and micro controllers. I
was/am intending to do the first three of those the same way as
the processor itself, i.e. from discrete components. But I really
would like to see the processor run sooner rather than later and
I'm right at the end stop with regards to space so I'm doing them
the easy/small way for now.
I've prototyped all the bits I didn't know how to do so I think
I'm ready for the final sprint/slog to the end.
I also went back and had another look at the processor speed. I
now know what's going on and I've been a twit.
When I was first wondering how quickly the Megaprocessor would run
I looked at the datasheet for the transistor (2N7000) that I'm
using and saw:
that it had switch times of about 10ns. Pretty fast. My 16 bit
ripple adder means that data must propogate through about 32 gates
for the adder alone. There's other logic to get through and you
could imaging that the sheer physical size of the circuit (there's
yards and yards of cabling) might slug the signals by quite a bit
but I thought a fantasy of a MHz not impossible and certainly
100kHz should be achievable. And that would be entirely
satisfactory. So I was happy. I did have a little look at the
first working gates and it all seemed ok. But I didn't look
closely enough. I also had not looked at the data sheet properly.
There's more numbers there.
You get the 10ns speed when you have a "low"load resistor, and the
transistor is carrying half an amp. With 14,000 transistors that
would mean I'd need a 7000A power supply. Which is tricky.
I'm using 10kΩ pull up resistors so my speed's a lot lower. As it
happened I had not built all my adders, so I built the last one
using 1kΩ pull up resistors. And guess what, it's about ten times
faster. (Note these were 8 bit adders).
I also did some simulations (using LTSpice) of the transistor
circuits and those results agreed. Yes I should probably have done
those simulations at the start, but the circuits were easy and
seemed to work so I didn't bother. As I said, I'm a twit.
There is no way I can change the pullups on the boards I've built,
it would take forever and I'd probably do horrendous damage. And
the boards are now nearly all built (! yes that's right) and I'm
not starting over. But we do have a way we can can claw back some
performance. It is the (slow) adders that dominate the speed of
the processor. If I replace them with "fast" adders then their
time will become "negligible" and instead the speed will be
determined by the "slow" logic. There's "only" four adders to
build. They're big boards but it won't be too painful so I'm going
to do that. How fast will the processor be then ? Good question.
It depends on what the longest logic chain is which I have to
confess isn't particularly obvious at the moment. My current guess
is that its about 12-15 gates so I'm now hoping the processor will
be able to run > 50kHz.
Just finished the Decoder frame.
So we're now at 4 frames. My original sketch shows us needing 9
frames but I think we can now get away with 7. The I/O and Igor
frames are merging, and to kick off we don't need the ROM as we'll
be able to download programs into the RAM. So the build is over
half complete !
Still no more transistor death so the ban on Henry seems to be
working. In fact not many problems this time though had a couple
of exciting new failure modes, both involving short circuits.
The first was with a ribbon cable. Took me a while to track down
as for the first few hours I assumed it was either my soldering or
the return of transistor death. Once I had narrowed down the
problem to the cable I had to dismantle the connector and look at
it under a microscope to find out what had happened.
It looked to me like there was some extra metal on the tooth
piercing the brown conductor and it was also slicing the red to
give the short circuit. But then I noticed there was only a single
tooth going through the red, where was the other one ? Removing
the ribbon cable revealed what had happened.
I must have bent the tooth destined for the red conductor whilst
making up the cable and it had distorted enough to hit the brown.
The second failure was with this board....
It presented as if the left most input signal (orange wire) was
always seen as 0 so I initially blamed the source which led me
astray for a while. After staring at the board I couldn't see
anything wrong so I tried poking it and it flickered briefly into
obedience. It turns out that the terminal is shorting on the
mounting screw. That shouldn't matter as the boards are mounted on
plywood. But in this particular case the board is mounted over one
of the aluminum bars of the frame, the screw is just long enough
to pierce through the plywood and short to the frame.
One of the most important components of the project is of course
the LEDs and in particular their flashing. Here is a little video
showing them in action.
The next week or two will be a consolidation phase I think. I have
a bunch of loose ends to tidy up. Various bits of software have
fallen behind and need to be brought up to date and I need to nail
down the I/O+Igor frame. And of course some more soldering. Always
6th July 2015
Some people also asked if I could an RSS feed so I've done
that. It's relatively easy once you know the answer.
1st July 2015
The big excitement during the last week or so was the articles
appearing in The
Register and on the BBC News
web site. I still don't know what the trigger was but I really
enjoyed my five minutes of fame. I've had loads of encouraging
emails which has been a big morale boost. However I was badly
caught on the hop so this website was nowhere near ready.
Quite a few people asked for a way to get updates so I'm
creating a Facebook
page where I'll post something whenever the website is
changed. I'll also start trying to fill out the pages with
the information (and schematics) I was planning to add and also
answer the various questions I've been asked. This will likely
be a slow process as I'll be spending most of my time on the
It was Henry !!
I believe I now know the culprit for transistor death. And it is
Henry. So he's banned. This may be a vile slander but I'm pretty
sure it's him. He can try suing if he wants but it doesn't look
like he cares. What was baffling me was that the transistors only
died when mounted on the frames; they survived my soldering, they
survived storage, they survived the single board testing, and then
just gave up the ghost during the relatively mild module test. But
then did not continue to fail over time.
The obvious candidate was static discharge, but improving my work
practices in that respect made no difference.
The other candidate was surges/glitches on the power supply but
turning that on and off a few hundred times did not lead to
further transistor death.
However, eventually, I noticed that I sometimes got little shocks
off Henry when doing the housework. Then I realised that as I was
also in the habit of hoovering the Megaprocessor it might be Henry
that was doing the damage. So I didn't use Henry when building the
General Purpose Registers frame, and there was no transistor
death. So life for Henry is now just carpets, no electronics.
You might be wondering why I hoover electronics. If you look at a
picture of one of the completed frames you might be able to see
that some of the ribbon cables are tied in place to stop them
flopping about. I have to drill the holes for the string after
I've mounted the electronics. This covers everything in dust, so I
was hoovering to clean up the sawdust.
I have just finished the metal-bashing and carpentry for the
Decoder frame, shown below. You can see the three built frames
squashed over to the left.
"All" I need to do now is populate and wire it up. I've got the
boards ready to go:
This'll keep me busy for a few weeks.
As well as building up the decoder during the next few weeks I
also need to start looking ahead to the non-processor frames: I/O,
memory and Igor.
21st June 2015
Completed the frame for General Purpose Registers.
So that's three frames so far now ! Half way to something that
might work. Decoder frame next.
I've also been building up some memory and now have the grand
total of 32 Bytes. And they work. Here is a picture of them being
tested. Each of the big squares has 8 Bytes and the thin board in
the middle combines them into a block.
20th May 2015
Completed the ALU !.
Had a few problems in the last couple of months.
Biggest was I found that my multiplexers didn't quite work as
wanted. In particular when no input was selected they were
supposed to output all zeroes. Unfortunately they output all ones.
I thought of all sorts of horrible ways of trying to get round it
but in the end had to respin the board. Which means I now have to
solder a dozen of the new boards. I needed three of the new ones
for the ALU (above), I could use one of the old ones.
The other big issue was trying to sort out inter-frame connections
and try to add some static protection.
Oh, and my simulation model for the adder didn't quite match the
hardware (carry out generation) so I had to tweak the logic of the
ALU a little. As I'd already started constructing it I then had to
dismantle the frame, redesign the artwork and put it back together
again. That was a depressing moment.
Still suffering transistor death, about half a dozen during this
stage. Just to cheer myself up I also botched making up one of the
In the picture you can see I managed to put the cable in
skew-whiff. The connections on the right were ok. Most of the
connections on the left worked most of the time, and some worked
some of the time. What was most amusing is that the dodgy
connections gave rise to symptoms very similar in some ways to
those of transistor death. Drove me absolutely nuts till I managed
to work out what was going on.
With the ALU built I'm able to make a first estimate of the
possible speed of the mega-processor. I think the adders will be
the slowest element as I've done a very simple ripple adder.
They're 16 bits wide so a change at the least significant bit
needs to work its way through about 32 logic gates to affect the
carry out at the end. So I set it up to add 0xFFFF + 0x0000 +
carry_in and looked at the time it took for a change in the
carry_in to propagate through.
You can see it's taking about 40μ s which means our maximum clock
speed will be a bit less than 25kHz.
The other deduction we can make is that the propagation time
through a single logic gate is about 1μs.
20th Feb 2015
Completed the controller module for the status register.
A couple of wiring errors. Just the one transistor death this
time. There was also an error on the schematic so I'm breaking out
the tipex and my felt tip pens to fix it.
Which means we now have our first frame !!
There's a few bits left to do on it mostly relating to the
connectivity with its neighbours. But its a big step forwards.
I'll now start work on the ALU frame. This could be where the
conflict in scale between the Megaprocessor and my living room
starts to escalate.
The other bit of good news is that my test interface boards
daisy-chain quite nicely. I had 3 in a row to test the
status/status controller module.
30th Jan 2015
Just completed the status register module.
Bugs in the software. Bugs in the hardware. All a lot harder than
it should have been. Anyway now to build up the controller for the
state and status registers and then I'll have completed the first
12th Jan 2015
I've built the first module !! And it works !!!The state machine.
Here are some more pictures of its construction. I start by
putting in the wiring:
To help me do this I have a suitably reversed picture on the back
of the module, so it looks like this:
After I've done the wiring I then screw the boards on top:
And then "all" I have to do is connect it up. Took about 3 days.
The kick in the teeth was that at some point during the assembly a
handful of transistors died and that took a bit of debugging. I
need to know what happened before progressing with the rest of the
build. Still, it's started.
16th Dec 2014
Tantalisingly close to being able to actually being able to
start putting things together. But still can't quite start. I
was about to start building the first proper frame to mount the
modules in. Made myself a little jig to help me drill hoes
accurately. Very proud of it.
But it turns out I messed up my order for the aluminium. Whilst
I realised I needed two sets of horizontal elements because
there is a top and bottom, I overlooked the fact that I needed
two sets of vertical elements because there is a left and a
right. Bother ! And delivery is 6-10 days and we have Christmas
10th Dec 2014
The last couple of weeks have been reviewing and checking the
mapping between the design and the modules. Not desperately
exciting but necessary. Now I can begin the assembly, once I've
cleared a big enough work space ! The dream is to have the state
machine up and running by Christmas, this might be a bit of a
27th Nov 2014
I now have a first cut on all of the module layouts. Discovered
a bug or two in the process which is quite scary. Next stage
will be a review of a quite anal nature, I really don't want it
to not work once built. I'm quite happy with about the way half
of them look. But there's a couple of the control ones which
don't really tell much of a story. Trouble is the only way I can
think of to make them more explicable is to give them more space
and I'm at the end stop with regards to size. So we'll run with
what we've got.
Finished building up the RAM board. Couple of build errors
(there is over a thousand components !), the transistor fitted
backwards was particularly hard to find and fix. The good news
is that is definitely a memory. Pushed a megabyte of random
read/writes through it without error. The bad news is that it is
also an inverter. Everything comes out upside down. Most vexing.
I know what happened. The thing went through a couple of
re-designs and in one of them it lost an inverting buffer on the
input, which I forgot to compensate for at the output.
So next week or so is more reviewing and tweaking. Then we might
be able to begin assembly !
12th Nov 2014
Starting to make progress on the module layouts. There's a
dozen to do and I have a first cut for 3 of them, I'll not
polish them till I've done a first pass on all.
Built up the RAM board, very nearly. Ran out of bits so got to
put another order into Farnell for a few more thousand
transistors, LEDs and resistors.
Yesterday ordered aluminium for the Mark 2 frame. Also for some
more PCBs; the full number (12) of the register boards I need
now that it's passed it test; a board for connecting between
frames which is simply a bunch of connectors; and also a couple
of simple connectivity things to make the modules a bit tidier
and also (thankfully) a bit smaller. This will all arrive in
about a fortnight.
As always way, not doing enough soldering. But its probably
right to push on with the modules as I have enough boards to
start assembling the modules for the first frame (State &
Status) which will be a major milestone.
2nd Nov 2014
Tested the new register and it seems to work fine. Had it set
up as a shift register and was able to walk a 1 bit through it a
few thousand times with nothing going wrong. The main change
from the first version was to have the master and slave stages
run on separate clocks so I can control the setup and hold times
PCB for RAM has arrived and is waiting for soldering.
Simulator's got far enough to rate a version number. V0.001 or
something like that.
Psyching myself up to ordering the aluminium for a Mark 2
frame. I'll try doing at least the access holes myself, and
possibly some of the tapping as well. Half the cost of the first
set of aluminium was in having them drill/tap the holes for me.
Not done anything like enough soldering. And no real progress
on nailing down the module layouts. Simulator/assembler seems to
have soaked up most of the time.
24th Oct 2014
My Birthday ! Treated myself to finding a way of implementing a
weight instruction and managed to massage one into the shift
The aluminium's arrived and I've built up a frame. 2m looks a
lot bigger than I expected. My initial idea for legs/stabalisers
was bonkers but I think I now have a solution. It's a lot easier
to think how its all going to fit together when you have
something physical. It needs a few tweaks but it's roughly
Done a bit more soldering. Still too scared to test the new
Made quite a bit of progress on the simulator. I've forgotten
so much about how the MFC works, its been years since I last did
any serious windows programming.
12th Oct 2014 - State of the Union
Starting to construct this website ! So it seems like a good
time to take stock of where I am. And the answer is that there's
a shed load of stuff yet to do.
all the PCBs for the logic elements (AND, OR etc) are
designed and manufactured. I've built up a fair bit. Certainly
more than half.
the PCBs for storage elements...not so advanced. I have a
second attempt for the register back ready to build up and
test. PCB for RAM is out for manufacture, PROM is still being
designed (I'll wait to see if the RAM works before doing more
the electronics for the I/O and peripherals ...only just
started, but I think I know how I'm going to do them
I've discovered I need a few more simple boards for handling
I have a gate level model of the hardware, which is what I
use to work out what logic boards I need and how to wire them
I have an abstract model of the processor. I run this on
test data back to back against the gate level model to check
that it still works as I experiment with the architecture.
This abstract model will go into the simulator.
a disassembler. I use this in my back to back model testing.
This will go into the simulator.
I've started to consider the design of the simulator.
I've most of an assembler.
I've yet to start on any programs for the processor itself
I have (finally; this has taken weeks) managed to map the
architecture onto a set of modules which:
might be understandable
avoid an utterly insane amount of interconnection. Though
it's still fairly bonkers.
fit together tolerably well to make up the frame structure
I've barely started designing the panels that will form the
backdrop for each module showing its wiring. These'll take
I've a first design for the frames that will hold the
modules together. Order for the material for first frame is