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Microprogrammed Control Unit

 Microprogrammed Control Unit (MCU) is a Control Unit of a Computer. The  design of MCU is Program based. Such a Control Unit is a programmed one. The Programming activity that is deployed is called  Microprogramming. A Microprogram is designed performing Microprogramming action.

In fact, Microprogramming is the task undertaken for designing and building  the Control Unit of a Computer . Such a Control Unit is called Microprogrammed Control Unit (MCU). For building the MCU, an additional memory called Microprogram Store is used.

MCU is responsible for controlling the activities inside a Computer System. Generally, the directives issued to the Computer System at the Hardware level are controlled and coordinated by the MCU. Hardware level activities performed using MCU affect data processing facilitated by a Computer.

MCU performs like a conventional Control Unit. The difference is that, instead of being hardwired, MCU is designed to be programmed (or Microprogrammed to be precise).

In a Computer Program, we have instructions. In the same way, a Microprogram is composed of microinstructions. Several microinstructions are required to execute a machine instruction (an instruction given to a Computer). 

A Microinstruction performs several Control Functions. These Control Functions are Microoperations. A Microperation is signified by a binary digit(bit).


The instructions that we normally encounter are machine instructions. Machine instructions are used to instruct the machines, or Computers. 

Microinstructions are used to perform control activities. Execution of an instruction given to a Computer is carried out by several Microinstructions.

The activities that are related to Microprogrammed Control impact the Computer System very effectively. The effectiveness of control action directed by the MCU is a decisive one.

Let us detail the structure of a Microinstruction. Microinstruction is  composed of several binary digits ( bits ). A Microinstruction, being a multi bit entity, performs several  Control Functions. One bit means, one Control Function. So, several bits mean several Control Functions. Control Functions are visible in the form of Microperations.

Yes, the structural constituents of Microinstructions are binary digits. The bits are, of course, indivisibly small. 

There are several examples of MCU driven  Microoperations. Microoperations have to be ordered, or sequenced.

Execution of Microoperations is sequenced by Microprogram Sequencer. Microprogram Sequencer is associated with the Microprogram Store. Such Sequencer can be compared to Program Counter. We note that Program Counter is a part of Computer Architecture.

We can have three Microperations for a Computer Instruction that requires transfer of data from CPU to the Main Memory of a Computer. Let us suppose that we require such a transfer.

The Microoperations that are performed see the following activities:

( 1 ) A Microoperation may transfer the contents of the Program Counter to Memory Address Register (MAR).

( 2 ) Incrementing the contents of Program Counter may require a Microperation. 

( 3 ) Performing the transfer of the contents of the Accumulator to the Memory Buffer Register (MBR) highlights the execution of a Microperation.

////////////////////////////////////////////

Computer Instruction

             ^

             |

MICROPROGRAM

             ^

             |

MICROINSTRUCTIONS 

             ^

             |

MICROINSTRUCTION        

            ^

            |

          BITS

            ^

            |

          BIT

            ^

            |

CONTROL FUNCTION

            ^

            |

Microoperation 

////////////////////////////////////////

Fig: Microprogram

(Structural & Functional Details Given) 

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Goodbye!

Have a great day!!

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