Microprogrammed Control Unit Address Sequencing: Techniques and Mechanisms

Explore the essential mechanisms for address sequencing in microprogrammed control units. This guide details various techniques for determining the next microinstruction address, including sequential execution, conditional branching, and subroutine calls, providing a comprehensive understanding of microprogram control flow.

Address Sequencing in Microprogrammed Control Units

Microprogrammed Control Units

A microprogrammed control unit uses a microprogram (a sequence of microinstructions) stored in a control memory (also known as control store) to generate control signals that direct the operation of a computer's hardware components. Each microinstruction in this microprogram typically specifies one or more micro-operations and contains information to determine the next microinstruction to be executed.

Address Sequencing in Control Memory

The control unit needs a mechanism to determine the address of the next microinstruction to be executed. This address sequencing capability is crucial for implementing instruction execution and handling branching. The address of the next microinstruction can be determined in several ways:

• Incrementing the current address.
• Conditional branching (based on status bits).
• Unconditional branching (jumping to a specified address).
• Subroutine calls and returns.

Block Diagram of Address Sequencing

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Conditional Branching

Conditional branching allows the next microinstruction's address to depend on status bits representing various conditions (like zero, carry, overflow, negative flags, mode bits, and I/O status). The current microinstruction specifies a branch address, and the branch logic determines whether to jump to that address or proceed to the next sequential address based on the status bits. A multiplexer selects the next address based on the conditional status.

Unconditional Branching

In an unconditional branch, the next instruction's address is directly specified in the current microinstruction. The control address register is loaded with this address from the control memory, and execution continues from there.

Instruction Mapping

The control unit needs a way to map instructions to their corresponding microroutines (sequences of microinstructions) in the control memory. This mapping is often done using the operation code (opcode) of the instruction.

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Subroutines in Microprogramming

Subroutines are used to implement common sequences of microinstructions. They save space and improve code organization. A subroutine register holds the return address for the calling routine. The register file where the subroutine addresses are stored often uses a Last-In, First-Out (LIFO) stack structure.

Conclusion

Address sequencing mechanisms are crucial for controlling the execution flow within microprogrammed control units. Understanding conditional and unconditional branching, along with instruction mapping and subroutine calls, is essential for designing efficient and functional control units.