How Computers Work: The Basic Principles of Computers For Everyone by Wells James

Author:Wells, James [Wells, James]
Language: eng
Format: azw3, epub
Published: 2017-10-15T04:00:00+00:00

It does this using a bunch of OR gates to see if any of the bits are 1. Even if one single bit is 1, we know the number can’t be zero and then we use a final NOT gate to flip this input so the output is 1 only if the input number is 0. So that’s a high level overview of what makes up an ALU. We even built several of the main components from scratch, like our ripple adder. As you saw, it’s just a big bunch of logic gates connected in clever ways. This brings us back to that ALU you admired so much at the beginning of the chapter. The Intel 74181. Unlike the 8-bit ALU we made today, the 74181 could only handle 4-bit inputs, which means YOU BUILT AN ALU THAT’S LIKE TWICE AS GOOD AS THAT SUPER FAMOUS ONE. WITH YOUR MIND! Well… sort of. We didn’t build the whole thing… but you get the idea. The 74181 used about 70 logic gates, and it couldn’t multiply or divide. But it was a huge step forward in miniaturization, opening the doors to more capable and less expensive computers. This 4-bit ALU circuit is already a lot to take in, but our 8-bit ALU would require hundreds of logic gates to fully build and engineers don’t want to see all that complexity when using an ALU, so they came up with a special symbol to wrap it all up, which looks like a bit “V”.

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