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-wiredlogic 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. (1995-04-23)
