Microprogrammed Control Unit


Microprogrammed control unit is implemented by writing microprograms in the control memory. These microprograms are a set of microinstructions required for executing the machine instruction. Each microinstruction somehow(implicitly or explicitly ) also specify the address of next microinstruction to be fetched, thus sequencing the microoperations. The following terms must be understood before we go for the details on microprogrammed control unit.

  1. Microprogram: The set of related microinstructions forms a microprogram. The assembly instructions are implemented by writing a microprogram. These microprograms can be changed easily, hence microprogramming yields control units that are more flexible than hardwired control unit.
  2. Microinstructions: microinstruction : the control signals to be activated at any time for executing a microoperation is specified by a microinstructin.
  3. Microoperations: These are the operations performed on the contents of the registers. each microoperation is associatd with a group of control lines that must be activted in a prescribed sequence to trigger the microoperations.

Block diagram of MCU

Coponents of the microprogram control unit:

  • Control memory

  • Address Sequencer

  • Micro-Instruction Register

  • Micro-Instruction Decoder

Control Memory

Control memory is in the form of read only memory. Control memory stores the microinstructions. The format of the microinstruction is:

Control field

Address field to point to next microinstruction

The control field part specify the control signals to be activated in response to the assembly instruction and the address field contains the address of the next microinstruction in control memory to be executed. The control memory unit also has a current and the next memory address.

As indicated in the figure the control memory address register (CMAR) is connected to the 2×1 MUX which gets the input from the 2nd field of the microinstructioncan (address part) or from the external source of address. The instructions opcode after suitable encoding, provides the starting address of its microprogram.

The external source of address usually denote the starting address of the microprogram for a particular instruction. The control unit also respond to external signal or external conditions which allow the current microinstruction to choose or select one of the two possible address fields

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