Control Unit Design: Hardwired vs. Microprogrammed Control
Explore the fundamental design approaches for Control Units (CUs) in computer architecture: hardwired and microprogrammed control. This guide compares and contrasts these methods, detailing their components, advantages, and disadvantages, providing a clear understanding of how CUs direct CPU operations.
Design of a Control Unit
The Control Unit (CU) is a crucial part of a computer's central processing unit (CPU). It directs the operation of the CPU by fetching instructions from memory and decoding them to control the execution of micro-operations. There are two main approaches to designing a control unit: hardwired control and microprogrammed control.
Hardwired Control
In hardwired control, the control logic is implemented directly using digital logic gates, flip-flops, and other components. The design is essentially a complex circuit specifically tailored to the instruction set architecture (ISA).
(A block diagram of a hardwired control organization would be included here in a complete document).
Key components include:
- Instruction Register (IR): Holds the currently executing instruction.
- Decoders: Interpret parts of the instruction to generate control signals.
- Logic Gates: Combine signals to generate the appropriate control signals for various parts of the CPU.
- Sequence Counter (SC): Tracks the order of instructions.
The operation code in the instruction dictates which control signals are activated, triggering the execution of the corresponding micro-operations.
Microprogrammed Control
Microprogrammed control uses a different approach. Instead of complex logic gates, it employs a program (microprogram) stored in a special memory called control memory (often ROM). Each instruction in the microprogram is a micro-instruction that specifies a sequence of micro-operations.
(A block diagram of a microprogrammed control organization would be included here in a complete document).
Key components include:
- Control Memory Address Register: Points to the address of the next microinstruction.
- Control Memory (ROM): Stores the microprogram.
- Control Register: Holds the fetched microinstruction.
- Next Address Generator (Microprogram Sequencer): Determines the sequence of microinstructions to be executed.
The control unit fetches microinstructions from the control memory, and each microinstruction directly controls the micro-operations within the data path. The next address generator ensures the correct order of execution of micro-instructions.