<processor> The four steps which the CPU carries out for
each machine language instruction: fetch, decode, execute,
and store. These steps are performed by the control unit,
and may be fixed in the logic of the CPU or may be programmed
as microcode which is itself usually fixed (in ROM) but
may be (partially) modifiable (stored in RAM).
The fetch cycle places the current program counter contents
(the address of the next instruction to execute) on the
address bus and reads in the word at that location into the
instruction register (IR). In RISC CPUs instructions are
usually a single word but in other architectures an
instruction may be several words long, necessitating several
fetches.
The decode cycle uses the contents of the IR to determine
which gates should be opened between the CPU's various
functional units and busses and what operation the ALU(s)
should perform (e.g. add, bitwise and). Each gate allows
data to flow from one unit to another (e.g. from register 0
to ALU input 1) or enables data from one output onto a certain
bus. In the simplest case ("horizontal encoding") each
bit of the instruction register controls a single gate or
several bits may control the ALU operation. This is rarely
used because it requires long instruction words (such an
architecture is sometimes called a very long instruction
word architecture). Commonly, groups of bits from the IR are
fed through decoders to control higher level aspects of the
CPU's operation, e.g. source and destination registers,
addressing mode and ALU operation. This is known as
vertical encoding. One way RISC processors gain their
advantage in speed is by having simple instruction decoding
which can be performed quickly.
The execute cycle occurs when the decoding logic has settled
and entails the passing of values between the various function
units and busses and the operation of the ALU. A simple
instruction will require only a single execute cycle whereas a
complex instruction (e.g. subroutine call or one using memory
indirect addressing) may require three or four.
Instructions in a RISC typically (but not invariably) take
only a single cycle.
The store cycle is when the result of the instruction is
written to its destination, either a register or a memory
location. This is really part of the execute cycle because
some instructions may write to multiple destinations as part
of their execution.
