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An instruction set where each field (a bit or group of bits) in an instruction word controls some functional unit or gate directly, as opposed to vertical encoding where instruction fields are decoded (by hard-wired logic or microcode) to produce the control signals. Horizontal encoding allows all possible combinations of control signals (and therefore operations) to be expressed as instructions whereas vertical encoding uses a shorter instruction word but can only encode those combinations of operations built into the decoding logic.
An instruction set may use a mixture of horizontal and vertical encoding within each instruction. Because an architecture using horizontal encoding typically requires more instruction word bits it is sometimes known as a very long instruction word (VLIW) architecture.