<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 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.
