Control Unit

• The control unit is responsible for directing and coordinating most of the computer system activities.
• It does not execute instructions itself, but it tells other parts of the computer system what to do.
•  It determines the movement of electronic signals between the main memory and the arithmetic and logic unit.
• It also controls the signals between the CPU and input/output devices.
• It consists of registers to hold the address of the current instructions being executed, current instruction itself and the address of the current instruction being executed and take necessary action.
• The control unit consists of several registers like address registers, instruction registers, sequence registers, decoder etc.
• When program is executed, a number of steps are to be followed by the computer system.
• The instruction is selected by the sequenced resisters and sent to the instruction register.
• The operation part of the instruction is sent to the decoder and address part to the address register.
• The control unit issues order to extract the contents from the address and transfer them to ALU. Also, the instruction like multiply or divide also goes to ALU.
• The sequence register moves on to the next instruction.

The process of fetching the instruction and sending its part to appropriate registers is
called fetch cycle. The calculation of the result after performing the required arithmetic or logic operation is called execution cycle.