horizontal encoding

processor 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 verticalencoding 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 longinstruction word (VLIW) architecture.

(1995-04-23)