Control Unit (CU)

Control Unit (CU)

A control unit is circuitry that directs operations within the computer's processor by directing the input and output of a computer system. The processor then controls how the rest of the computer operates (giving directions to the other parts and systems). A control unit works by gathering input through a series of commands it receives from instructions in a running programs and then outputs those commands into control signals that the computer and other hardware attached to the computer carry out.

The control unit is basically circuitry inside the CPU, controlling the operations inside the CPU and "directing traffic" in a sense. The functions a control unit performs can depend on the type of CPU, since the varying degrees of architecture between all the different CPUs determine the functions of the control unit.

A CU takes its input from the instruction and status registers. Its rules of operation, or microprogram, are encoded in a programmable logic array (PLA), random logic or read-only memory (ROM).

CU functions are as follows:

• Controls sequential instruction execution
• Interprets instructions
• Guides data flow through different computer areas
• Regulates and controls processor timing
• Sends and receives control signals from other computer devices
• Handles multiple tasks, such as fetching, decoding, execution handling and storing results

CUs are designed in two ways:

• Hardwired control: Design is based on a fixed architecture. The CU is made up of flip-flops, logic gates, digital circuits and encoder and decoder circuits that are wired in a specific and fixed way. When instruction set changes are required, wiring and circuit changes must be made. This is preferred in a reduced instruction set computing (RISC) architecture, which only has a small number of instructions.
• Microprogram control: Microprograms are stored in a special control memory and are based on flowcharts. They are replaceable and ideal because of their simplicity.

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