Field Guide · concept

A logic gate is a basic building block of digital circuits that computes a simple Boolean function — such as AND, OR, or NOT — on one or more binary inputs.1

AND OR NOT A B → Y 0 0 → 0 0 1 → 0 1 0 → 0 1 1 → 1 A B → Y 0 0 → 0 0 1 → 1 1 0 → 1 1 1 → 1 A → Y 0 → 1 1 → 0 built from transistor switches · NAND and NOR alone can build any of them
Each gate maps binary inputs to an output by a fixed truth table: AND needs all inputs high, OR needs any, NOT inverts. Physically they're a handful of transistor switches — and from these primitives (especially the universal NAND and NOR) you can build adders, memory, and ultimately a whole processor.

Overview

Each gate takes inputs that are either 0 or 1 and produces an output defined by a truth table: an AND gate outputs 1 only when all inputs are 1, an OR when any is 1, a NOT inverts its input. Physically, gates are built from a handful of transistors acting as switches. From these primitives — especially the universal NAND and NOR — you can construct adders, memory cells, and ultimately every part of a processor.

Where it fits

Logic gates are the bridge from raw bits to computation: arrange enough of them and you get the arithmetic and control circuits inside a CPU, all fabricated together on an integrated circuit. The whole von Neumann machine — fetch, decode, execute — reduces, at the bottom, to networks of gates switching in step with the clock.

Sources

  1. Logic gate — Wikipedia, on logic gates, Boolean functions, and truth tables. ↩

See also