Inside the vintage 74181 ALU chip: how it works and why it’s so strange

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Ken Shirriff writes:

The 74181 ALU (arithmetic/logic unit) chip powered many of the minicomputers of the 1970s: it provided fast 4-bit arithmetic and logic functions, and could be combined to handle larger words, making it a key part of many CPUs. But if you look at the chip more closely, there are a few mysteries. It implements addition, subtraction, and the Boolean functions you’d expect, but why does it provide several bizarre functions such as “A plus (A and not B)”? And if you look at the circuit diagram (below), why does it look like a random pile of gates rather than being built from standard full adder circuits. In this article, I explain that the 74181’s set of functions isn’t arbitrary but has a logical explanation. And I show how the 74181 implements carry lookahead for high speed, resulting in its complex gate structure.

More details at┬áKen Shirriff’s blog.

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3 Comments

  1. Hey, look – an article that isn’t conveniently obfuscated into a video! Something I can actually, you know, _read_!

  2. Interesting to read about how they did things in the early days, with space and budget constraints. They really had to try hard to make it all fit and work. Today’s engineers have become lazy with all the headroom they have.

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