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

[BruceH]

Not sure if this has been mentioned before but it's a nice write-up.
http://www.righto.com/2017/03/inside-vin...ow-it.html

Quote: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.

[toml_12953]

(06-19-2019 09:32 PM)BruceH Wrote:  Not sure if this has been mentioned before but it's a nice write-up.
http://www.righto.com/2017/03/inside-vin...ow-it.html

I built a logic trainer around one based on plans in Popular Electronics. It was a cool looking box with four lights, toggle switches and one rotary switch that chose which of the 16 functions to display on the lights (before LEDs were common). Also the first computer I ever used that I actually saw was the Wang 3300 (Pre-2200 and quite rare!) and that computer was 74181-based.