Logic Gates
How does a computer calculate with 0s and 1s?
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Summary
Logic gates are tiny circuits that take one or two bits and, following their rule, give back a single bit. An AND gate gives 1 when both inputs are 1, an OR gate when either input is 1, and a NOT gate flips its input. Wired together, these simple gates make circuits that add and compare numbers, and even remember things. Every processor is made of billions of gates.
In depth
In 1854 George Boole wrote logic down as an algebra in which variables can only be "true" or "false". In 1937 Claude Shannon showed in his master's thesis that this algebra could be built with electrical switches: two switches in series behave like AND, two in parallel like OR. Today gates are made of transistors; a NAND gate takes four. NAND and NOR are "universal" gates: every other gate can be built from them alone. Combining gates gives a half adder that adds two bits, a full adder that adds three, and a latch that feeds its output back to remember one bit.
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