Jawaharlal Nehru Technological University Hyderabad B.Tech III COMPUTER ORGANIZATION Previous Question Paper (February - 2022 - R16)

A microprogram sequencer is a functional unit in a control unit that determines the address of the next microinstruction to be executed. It manages the flow of control within the microprogram by selecting the next address from various sources based on the current microinstruction, status flags, and external inputs.

The sequence of operations to generate the next address is as follows:

• Incrementer: The current address in the Control Address Register (CAR) is incremented by 1 to point to the next sequential microinstruction.
• Branch Address: A field within the current microinstruction may contain a branch address, which is loaded into the CAR if a jump condition is met.
• Subroutine Return: A Subroutine Register (SBR) stores the return address when a micro-subroutine is called, allowing the sequencer to jump back to the main microprogram.
• External Mapping: For initial instruction execution, an external mapping logic converts the opcode of the machine instruction into a starting microprogram address.

The block diagram consists of the following components:

• Control Address Register (CAR): Holds the address of the microinstruction currently being read from the Control Memory.
• Control Memory: Stores the microprograms.
• Multiplexer: Selects the next address from the incrementer, the branch address field, the SBR, or the mapping logic based on the control signals from the instruction register and status flags.
• Subroutine Register (SBR): Used to save the return address during a micro-subroutine call.

The sequencer uses a multiplexer controlled by the status bits and the microinstruction's branch control field to determine which of these sources will provide the next address to the CAR. This mechanism allows for conditional branching, looping, and subroutine calls within the microprogram, effectively controlling the execution of machine-level instructions.