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Meet the 4-Bit Cambridge-1 CPU

Created by Richard Grafton, this 4-bit 7400-based CPU recently debuted at the Retro Computer Festival, which was held at the Museum of Computing in Cambridge, England.

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A couple of months ago I was introduced to Richard Grafton, who hails from the United Kingdom. Richard currently works as a freelance technology consultant designing and making creative things for his clients. Richard is also the founder of ARITH-MATIC, where he creates and sells some very tasty 4-bit computer subsystem boards based on classic 7400-series integrated circuits.
An assembled S1-AU Mk1 kit (Source: Richard Grafton)
I have to say that I love the “look-and-feel” of Richard’s work, such as the black boards and the red LEDs and the red tactile switches. Although its unnecessary, functionality-wise, I also appreciate the fact that all the resistors are aligned the same way, which is just the way I’d do it. Of course, I couldn’t stop myself from asking, “How about doing a Steampunk version of your boards?” To which Richard replied, “Now, there’s an idea!” Sad to relate, I fear he was just humoring me, which means the only way I’m going to lay my sticky fingers on Steampunk boards is if I build them myself. Earlier this month, Richard emailed me to say that he’d just caught up with my musings on our 4-Bit HRRG Computer project. He noted that he’d found the columns on the Instruction Set and Instruction Tradeoffs to be of particular interest, because he’d just finished building a 4-bit 7400 CPU to take to the Retro Computer Festival, which was to be held the Museum of Computing in Cambridge that coming weekend. As Richard said, “It’s indeed an interesting challenge cramming everything into 16 instructions, although I’ve seen some quite creative variations on this!” The reason for my meandering musings here is that Richard just emailed me to say that he’s uploaded this video of his little beauty, which he calls the Cambridge-1, to YouTube.  
Richard also tells me that he’s started writing up this project, with details of the CPU now appearing on his Cambridge-1 Github page. Also, he’s posted this blog on his ARITH-MATIC site covering the genesis of the Cambridge-1 and its trip to the Retro Computer Festival. As you may know, I bounce over to England once or twice a year to visit my dear old mom. On the last Friday before I return home, a bunch of my techno-geek chums travel from around the country to congregate at my brother’s house. We spend the day showing off our latest creations while my mother provides appropriate ‘Ooh’ and ‘Aah’ sound effects as required, and then we spend the evening quaffing beer furiously (quaffing is like regular drinking, except you tend to spill more down your chest). Happily, Richard has agreed to join our merry band on my next trip. I’m sure he’ll be bringing the Cambridge-1 with him, and I’m equally sure that we’ll all spend an inordinate amount of time watching it perform its magic. Yes, of course I will take pictures and write it all up in a blog — it will be just as good as being there (unless you are one of the ones who is actually there, of course).

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Aubrey Kagan

The 10K pull up resistors bring back memories. In the mid 70s there was a shortage of 10K resistors (I have no clue as to why and if it was only in South Africa) so I converted everything to 12K pull-ups, and I seem to have stuck with that to this day, albeit in SMD.

Ian Johns

As I remember it happened in Australia as well. 10K was a typical value suggested by the chip makers in their data sheets, so everyone used it. As a pullup resistor on a chip function eg a logic gate, the value was low enough to not produce a voltage drop due to the input leakage current, & was high enough to keep power consumption down if say the gate input was being driven by a switch to 0V.

So when confronted by the long lead times for 10K resistors, the natural move was to 12K, as we all did.

Aubrey Kagan

There is also the Four Yorkshiremen sketch

https://www.youtube.com/watch?v=VKHFZBUTA4k

Interesting to see young Marty Feldman, John Cleese and Graham Chapman

Remade by Monty Python
https://www.youtube.com/watch?v=ue7wM0QC5LE

Aubrey Kagan

Well there was certainly the great i2716 (Intel’s 2Kx8 5V EPROM) shortage of ‘78

Thomas J Burke

Snoopy is a world-renowned quaffer of root beer.
https://www.gocomics.com/peanuts/1973/01/13

Thomas J Burke

Those immortal words spawned a contest:
https://en.wikipedia.org/wiki/Bulwer-Lytton_Fiction_Contest

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