MICROPROGRAM SEQUENCER ORGANISATION

A microprogram control unit consists of two parts :

1) Control Memory that stores the microinstructions

2) Associated circuits that control the generation of the next address .

The address generation part is sometimes called a microprogram sequencer ,since it sequences the microinstructions in control memory .

 Typical sequencer provides the following address-sequencing capabilities :

1.Increments the present address for control memory .

  Sequential access: The content of address register of the control memory (CAR) is    incremented after execution of each micro-instruction .

2. Branches to an address as specified by the address field of the microinstruction.

3 . Branches to a given address if a specified status bit equal to 1.

4.Transfer control to a new address as specified by an external source .

 The external address transfers control to the first microinstruction in a microprogram .

5 .Support subroutine call and returns.

    placing return address into a stack and then branching to the beginning of the subroutine .After the execution of a subroutine the return address can be loaded to CAR from stack top and return the control to main program .