A one-bit processor explained: reverse-engineering the vintage MC14500B

http://www.righto.com/2021/02/a-one-bit-processor-explained-reverse.html

by _lnwk • 4 years ago

74 31 4 years ago

righto.com

kens10• 4 years ago

I'm not sure why this appeared on HN, but author here if anyone has questions...

bogomipz4• 4 years ago

Hi. What a wonderful post.

I have a question about the 7th image on the page, here:

https://static.righto.com/images/mc14500b/inverter-magnified...

Can you help me understand the relationship between the metal layer, the vias, the gate, in and out? The middle image in the above link makes them seem like they are all connected. Does "in" provide the voltage the that pulls a gate high and turns it on or off? Are the vias and contacts just used to provide connection to a power rail?

Also what governs the fascinating shapes that we find in the two gate outlines such as in the image link above? They looks like and L or P outline. How do they arrive at these? Is it a just a function of floor plan routing and satisfying some minimal surface area needed?

kens10• 4 years ago

The metal wire connects the input to the two gates. The PMOS (upper) transistor turns on if the input is low; it connects the output (right) to the high voltage (left). The NMOS (lower) transistor turns on if the input is high; it connects the output to the low voltage. Each transistor has a contact (via) that connects the transistor to the gate's output. Thus, working together the two transistors implement an inverter. The power connections to the transistor aren't shown in the diagram; they are off to the left.

As for the shapes, they are basically created as a consequence of the routing. The gate is the important part; it needs to be a rectangle with specific dimensions (based on the circuit). The rest of the transistor can meander as needed. Because there's only one metal layer, a lot of routing needs to be done through the silicon.

The cross-section diagrams earlier in the article might help.

elpocko• 4 years ago

It's really hard to tell when it was originally posted. Can you please add a date to your posts?

genocidicbunny• 4 years ago

It's sort of there -- the URL contains the year/mo it was posted in.

kens10• 4 years ago

The chip hasn't changed much since I posted the article.

Dr_Jefyll• 4 years ago

The MC14500B has an unusual architecture, making it more of a building block than a complete microprocessor. [... It] requires multiple external chips to make it usable.

Love the articles you've been posting, Ken! And yes the MC14500 requires multiple external chips to make it usable. In fact, HNers may recall the one-bit machine whose chip-count I optimized by eliminating the MC14500B itself!

One-Bit Computing at 60 Hertz https://news.ycombinator.com/item?id=20565779

timmg• 4 years ago

I remember Connection Machines supposedly had an array (well, hypercube) of “one bit processors”. I never knew what that meant. Is this similar?

kens10• 4 years ago

These are both one-bit processors, but very different. The Connection Machine had a 1-bit ALU and it operated on larger words serially, so to the user it looked like it was doing 32-bit arithmetic (or whatever). The big deal about the Connection Machine is that it had 64K processors operating in parallel, so it formed a massively-parallel supercomputer.

The Motorola chip on the other hand was "genuinely" one bit. It was designed for Boolean control applications with one bit values. (You could do serial arithmetic if necessary, but it was very inconvenient.)

Claude_Shannon• 4 years ago

I was browsing on the processor architectures on Wikipedia and caught my eye on 1-bit processors. I decided to read more upon that, and found your article and found it interesting.

stew-j3• 4 years ago

I'm not sure why this appeared on HN

Because it is more interesting than the use of ML in Bitcoin trading, for example, thanks for sharing it. ;)

I don't know if you are still here answering questions, but I was wondering "What if we sent a musical signal (digital or analog) through the MC14500B (silicon junction or vacuum)?" Could it be used, say, in a programmable guitar effects pedal?

kens10• 4 years ago

The chip doesn't really have the performance for audio processing. It operates at 1 MHz, but since it operates on one bit at a time, you'd probably only be able to perform a couple of additions on each audio sample. So not really enough for effects.

mechagodzilla• 4 years ago

I've always thought it would be a historically interesting 'what if...?' to try to build an entirely mechanical version of something like this controlled by a jacquard-style card-reader mechanism.

sebras• 4 years ago

Usagi Electric has a video series[1] about investigating MC14500B (with help from Ken) and recreating it on a bread board. Later this led to him developing a variation on MC14500B using vacuum tubes[2].

[1] https://www.youtube.com/watch?v=oPA8dHtf_6M&list=PLnw98JPyOb...

[2] https://www.youtube.com/watch?v=y149hLe1zYo&list=PLnw98JPyOb...

Teknoman117• 4 years ago

I really would love to do a project like that, but I'm already trying to justify my "build retro computer using 20 yr old components that are now dirt cheap and a 35-40 yr old CPU projects" - e.g. 68010 and 286 and friends.

I'm way too young to have had the opportunity to experience these machines first hand let alone have any attachment to useful software that may have ran on them. Retro-computing has been a way for me to experience some of computing's past.

stew-j3• 4 years ago

Another link to the vacuum tube project:

https://hackaday.com/2021/12/27/single-bit-computer-from-vac...

avmich• 4 years ago

I'm trying to imagine how a system which includes this processor should look like, and how it should be designed. A detailed example, even a simple one, would probably add significantly to the understanding.

kens10• 4 years ago

The MC14500B Handbook (http://www.bitsavers.org/components/motorola/14500/MC14500B_...) has detailed designs. Page 1 (pdf 7) has a block diagram. A complete schematic of a system is on page 26 (pdf 32).

TedDoesntTalk• 4 years ago

With all of the supporting chips required (instruction counter, etc), why not design something in TTL? What was the advantage of this chip?

bogomipz4• 4 years ago

Would this one bit processor be a precursor to a simple microcontroller today then?

dboreham• 4 years ago

RodgerTheGreat• 4 years ago

Maybe take a look at the TMS1000.

kens10• 4 years ago

Microcontrollers started (oversimplifying a bit) with calculator-based chips: the Intel 4004 (1971), TMS 0100 (1971), TMS 1000 (1974). The Motorola one-bit processor is from 1976, so it's a later chip, more of a strange branch than a precursor.

PaulHoule• 4 years ago

It reminds me of this thing

https://en.wikipedia.org/wiki/PDP-14

which had a CPU that processed individual bits oriented towards boolean equations. Also

https://en.wikipedia.org/wiki/Programmable_logic_controller

A one-bit processor explained: reverse-engineering the vintage MC14500B

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