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Comment by andars | Hacker News Reader

Another interesting fact: If I'm remembering correctly, some models of the PDP8 had only a 1 bit ALU to reduce the purchase price. I thought this was pretty wild when I first heard it, but I guess the performance hit was ok if you could gain access to a computer at all.

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The PDP-8/S was the serial model. Serial processors weren't uncommon in the "old days". Other examples are the F-14 CADC (an early MOS processor) and the Datapoint 2200 (whose architecture was turned into the 8008). Many of the early aerospace computers were serial because of weight constraints, such as the Arma Micro (on the Atlas), IBM ASC-15 (on the Saturn I and Titan) and the Autonetics D-37 (on the Minuteman).

And then there's the Motorola MC14500B, which was genuinely a 1-bit microprocessor. It was used for simple control operations to replace relay logic.

The CDP1802 also used a serial ALU:

http://www.visual6502.org/wiki/index.php?title=RCA_1802E

Each instruction took either 16 or 24 clock cycles due to this, and although other operations were done in parallel and could have been significantly faster, I guess performance was not much of a concern back then as making the microsequencer as simple as possible.

Discrete "bit slice" ALU chips were common in early IC computers, tackling only a few bits of the ALU each. Hardware designers could chain those together to get as wide a hardware ALU as necessary, within the timing constraints.

https://en.wikipedia.org/wiki/74181

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