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

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 bizar ...

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Reverse-engineering the surprisingly advanced ALU of the 8008 microprocessor

A computer's arithmetic-logic unit (ALU) is the heart of the processor, performing arithmetic and logic operations on data. If you've studied digital logic, you've probably learned how to combine simple binary adder circuits to build an ALU. However, the 8008's ALU uses clever logic circuits that can perform multiple operations efficiently. And unlike most 1970's microprocessors, the 8008 uses a complex car ...

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Inside the 74181 ALU chip: die photos and reverse engineering

What's inside a TTL chip? To find out, I opened up a 74181 ALU chip, took high-resolution die photos, and reverse-engineered the chip.1 Inside I found several types of gates, implemented with interesting circuitry and unusual transistors. The 74181 was a popular chip in the 1970s used to perform calculations in the arithmetic-logic unit (ALU) of minicomputers. It is a moderately complex chip containing abou ...

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