How Computers Calculate - the ALU: Crash Course Computer Science #5

Today we're going to talk about a fundamental part of all modern computers. The thing that basically everything else uses - the Arithmetic and Logic Unit (or the ALU). The ALU may not have to most exciting name, but it is the mathematical brain of a computer and is responsible for all the calculations your computer does! And it's actually not that complicated. So today we're going to use the binary and logic gates we learned in previous episodes to build one from scratch, and then we'll use our newly minted ALU when we construct the heart of a computer, the CPU, in episode 7.

*CORRECTION*

We got our wires crossed with the Intel 4004, which we discuss later. The 74181 was introduced by Texas Instruments in 1970 but appeared in technical manuals around 1969. The design of the 74181, like most of the 74xx/74xxx series, was an open design which was manufactured by many other companies - Fairchild was one such manufacturer. They produced a chip, the Fairchild 9341, which was pin-for-pin compatible with the 74181.

Fairchild was the first to prototype an ALU, building the Fairchild 4711 in 1968 - a one-off device not optimized for scale manufacturing. In 1969, Signetics came out with the 8260, which they marketed in a very limited sense (it was attached, AFAICT, to one particular computer, the Data General SUPERNOVA). TI follows afterwards (March 1970) with the 74181, coupled with the 9341 from Fairchild. The 74181 became the standard number for this part, and was available from many manufacturers (back in those days, chip makers cross-licensed designs all over the place in order to provide assurance that their part could be sourced from multiple manufacturers).

Produced in collaboration with PBS Digital Studios: http://youtube.com/pbsdigitalstudios

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