Microprogrammed Control Unit in Computer Organization

A microprogrammed control unit is a type of control unit that uses a set of stored instructions known as a microprogram to generate control signals. When compared to hardwired control units, which are designed using combinational logic circuits, a microprogrammed control unit utilizes a memory-based approach. This means that the control signals are retrieved from the memory as sequences of microinstructions that form a part of the machine's control memory.

The control memory holds microprograms that guide the control unit in executing various machine operations. The microinstructions are fetched and executed successively, allowing the computer to follow complex instruction cycles. Each microinstruction in the microprogram corresponds to a unique set of control signals that drive the system to perform actions like register transfers, ALU operations, and data transmissions.

Here are some key characteristics of a microprogrammed control unit:

• The microprogrammed control unit relies on control memory, typically a ROM (Read-Only Memory), to store microprograms.
• Each microinstruction specifies one or more control signals that guide the execution of micro-operations.
• This holds the address of the current microinstruction.
• It is known as the microprogram sequencer, which determines the address of the next microinstruction based on the current instruction or condition flags.
• Microinstructions execute various micro-operations (e.g., register transfers) by activating corresponding control signals.

There are two main types of microprogrammed control units:

• Horizontal Microprogramming: In this type, each microinstruction controls several operations simultaneously. It is more efficient in terms of the number of control signals required but often requires larger memory.
• Vertical Microprogramming: Here, each microinstruction controls a single operation, which makes it more compact and easier to manage but requires more microinstructions to complete complex tasks.

The microprogrammed control unit offers several advantages over other control mechanisms:

• Changing a microprogram is much simpler than altering a hardwired control unit.
• It reduces the complexity of designing control circuits and can lower hardware costs.
• A microprogrammed control unit can be easily adapted to support more complex instruction sets or future expansions of the system.
• It uses a predefined set of instructions to resolve errors and minimize miscommunication between circuits.

Some limitations of the microprogrammed control unit are:

• Memory access can slow down the overall execution of instructions compared to the faster-hardwired control units.
• A large amount of control memory is needed to store the microprogram, which increases the memory requirements.
• While flexible, the process of fetching and decoding microinstructions can increase the overall complexity of execution when compared to hardwired control units.

In conclusion, the microprogrammed control unit is a significant component in computer organization that offers flexibility and ease of modification, making it suitable for a variety of computer systems. Though it has certain limitations like slower performance and larger memory requirements, it provides an excellent alternative to hardwired control units, especially in situations where system changes or updates are frequent. With the advent of modern processors and systems, the microprogrammed control unit continues to evolve, balancing the trade-off between performance and flexibility.

1. What are the types of control units in COA?

The Control Unit (CU) is responsible for directing the operations of the processor by generating control signals. The two main types of control units in COA are:

• Hardwired Control Unit
• Microprogrammed Control Unit

2. What is the purpose of the control unit in computer organization?

The Control Unit (CU) in a computer organization is responsible for directing the operation of the processor. The primary purpose is to decode the instructions fetched from memory to determine which operation needs to be performed.

3. What is the difference between microprogrammed and hardwired control?

Here is the comparison of microprogrammed control and hardwired control:

Microprogrammed Control Unit

• Control signals are generated by reading microinstructions from memory.
• It is easy to modify; changing a microinstruction can modify the control sequence.

Hardwired Control Unit

• Control signals are generated using fixed combinational logic circuits.
• Difficult to modify; any change requires redesigning the hardware.

4. What is Wilkes' microprogrammed control unit?

Wilkes' microprogrammed control unit refers to the early development of microprogrammed control units by M.V Wilkes. His work in 1951 laid the foundation for using stored programs to control machine operations in modern processors.