Microprogrammed Control : Microinstruction Execution

The microinstruction cycle is the basic event on a microprogrammed processor. Each cycle is made up the two parts: fetch and execute. The effect of the execution of a microinstruction is to generate control signals for both the internal control to processor and the external control to processor. The organization of a control unit is shown in below figure :

At the above figure, the control memory is made up a program that explain the behaviour of the control unit. It follows that we could carry out the control unit by only executing that program. The above figure shows the important elements of such an implementation. The set of microinstructions is stored in the control memory. The Control Address Register possess the address of the next microinstruction to be read. When a microinstruction is read from the control memory, it is moved to a control buffer register. The left-hand portion of that register ties to the control lines. So, reading a microinstruction from the control memory is similar as executing that microinstruction. The third element present in the figure is a register and produces a read command. Now, lets examine this in detail.

As represented in the above figure, we can see that the control unit still has the same inputs ( IR, ALU  flags, clock ) and outputs (Control signals). the control unit works as follows :

  1. To execute an instruction, the sequencing logic unit produces a READ command to the control  memory.
  2. The word whose address is denoted in the control address register is read into the control buffer register.
  3. The content of the control buffer register produces control signals and next address information for the sequencing logic unit.
  4. The sequencing Logic unit loads a new address into the contorl address register according to the next-address information from the control buffer register and the ALU flags.

All this action performed during single clock pulse. On the completion of each microinstruction, the sequencing logic unit loads a new address into the control address register.

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