Control Unit

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A control unit is a composite component built for the express purpose of controlling another circuit.

In general, a control unit can do 2 things:

1. provide a secured interface that prevents illegal control signal combinations from being generated.
2. produce a sequence of control signal combinations over time.

In the context of computers, a control unit focuses more on this second behavior.

Computer Control Units Abstract

All computers, no matter how complicated, are built on a single action: transfer state from one place to another. Sequence different forms of this action in different ways and in different combinations, and you can perform any computation.

All transfers require 1 read signal and 1 write signal. If you want to transfer the state of one Register to another, you would need to produce a signal that first enabled the read signal on one register, then enabled the write signal on the other.

A control unit is what would produce those signals in that order.

Likewise, if you wish to perform more state transfers, modify the state at some point, and use it to do something like activate a Display, a control unit would be used to produce those signals in that order too.

Computer control units can often also produce different sequences of control signals depending on its inputs.

These inputs are usually connected to various parts of the computer such as the Instruction Register, Status Register, or other flag signals around the system.

This set up allows the control unit to "execute an instruction" by looking at the instruction register and modifying the sequence of control signals accordingly.

A control unit can also modify the state of an instruction register by bringing a new state in from somewhere else. A sequence often referred to as an instruction fetch.

In computers, every control unit will include in its sequences of operations a special sequence that first performs an instruction fetch, then queries the instruction register to determine what sequence it should produce next.

This process is referred to as the fetch, decode, execute cycle.

Typically, timing is regulated through the use of a Clock.

Computer Control Units Implementation

For Pipeline Computers, the sequence of control signals are produced with a chain of shift registers. Each one passing its state to the next one at the rising edge of the clock.

Each register represents one stage in the pipeline, and each stage produces its control signals by passing the stages state into a Decoder, Lookup Table, Programmable Logic Array, or ROM.

As new states are shifted into the stage, the control signals for that stage will change accordingly.

For all other computers, a more complex solution is often in order.

While you can get away with using a Counter and a ROM, more complex sequences involving looping sequences and conditional checks is more often achieved with the use of a Finite State Machine.