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An instruction is, at the highest level of abstraction, the smallest unit of computation which a programmer can direct the CPU to perform.

In more practical terms, the CPU operates by fetching items from main memory; each item contains a single directive for the CPU (e.g. add the contents of one register to another, or move a unit of data from a register, to main memory) - those items are instructions. (An instruction is usually a word, but on some CPUs, multiple instructions could be packed in each word, while on others, some instructions took multiple words.)

Some early computers used the alternative term order for instructions.

See also