Both hardwired control and microprogrammed control are methods for implementing the control unit (CU) within a central processing unit (CPU). The CU is responsible for fetching, decoding, and executing instructions. Here’s a breakdown of how they differ:

Hardwired Control Unit:

• Design: Relies on physical circuits like logic gates and combinational circuits. Each instruction has a dedicated circuit path to generate the control signals needed for its execution. Imagine a complex network of wires for each instruction.
• Advantages:
  
  
  • Speed: Since the control signals are predetermined by the circuits, execution is fast and efficient.
  • Simplicity: The design is relatively simpler due to the absence of additional memory for control instructions.
• Disadvantages:
  
  
  • Inflexibility: Adding new instructions or modifying existing ones requires physical changes to the circuitry, making it difficult to adapt.
  • Scalability: As the instruction set grows, the control unit becomes more complex and expensive to manufacture.

Microprogrammed Control Unit:

• Design: Utilizes a control store (a small memory) that holds microinstructions, a set of smaller instructions that define how to execute the main instructions.
• Functionality:
  
  
  1. The CU fetches the main instruction from the program memory.
  2. Decodes the instruction and retrieves the corresponding microprogram from the control store.
  3. Executes the sequence of microinstructions, generating control signals for each step of the main instruction.
• Advantages:
  
  
  • Flexibility: New instructions or modifications can be implemented by simply updating the microcode in the control store, making it more adaptable.
  • Scalability: The control unit design remains relatively simple, accommodating a larger instruction set by adding microcode to the control store.
• Disadvantages:
  
  
  • Speed: Extracting microinstructions and generating control signals takes additional time, making it slightly slower than a hardwired approach.
  • Complexity: The introduction of the control store adds complexity to the design.

Choosing Between Them:

• Hardwired control is preferred for simpler CPUs with a fixed instruction set that prioritizes speed. (e.g., embedded systems)
• Microprogrammed control is ideal for complex CPUs with a large instruction set or those requiring flexibility for future upgrades. (e.g., general-purpose processors)

Analogy:

Imagine a recipe book (microcode) for making different dishes (instructions).

• Hardwired control is like having a dedicated kitchen appliance for each recipe in the book. It’s very fast for those specific recipes, but adding new recipes requires buying new appliances.
• Microprogrammed control is like having a basic kitchen setup with a wide range of tools. You can follow any recipe in the book using these tools, but it might take slightly longer to prepare a dish compared to a dedicated appliance.