Control Unit (CU) Definition/Meaning:
The portion of a central processor
that contains the necessary registers,
counters, and other elements to provide the functionality
required to control the movement of information between the memory,
the arithmetic and logic unit, and
other portions of the machine.
In the simplest form of the classical von Neumann architecture,
the control unit contains a program counter,
an address register, and a register that contains and decodes
the operation code. The latter two
registers are sometimes jointly called the instruction register.
This control unit then operates in a two-step fetch-execute cycle.
In the fetch step the instruction is obtained (fetched) from memory
and the decoder determines the nature of the instruction. If it is a
memory reference instruction the execute step carries out the
necessary operations) and memory reference(s). In some cases, e.g. a nonmemory reference instruction, there
may be no execute step. When the instruction calls for
indirect addressing, an additional
step, usually called "defer", is required to obtain the indirect
address from the memory.
In more complex machines and non von Neumann architectures, the
control unit may contain additional registers such as
index registers, arithmetic units to provide address
modifications, registers, stacks, or
pipelines to contain forthcoming
instructions, and other functional units. At present, most control
units are microprogrammed (see
microprogramming). Some exceptions are found in control units
that have become powerful and complex; in supercomputers, for
example, they may contain specialized hardware that allows for
parallel processing of instructions which are issued sequentially.