L04 - CPU Performance

Fetch-Decode-Execute Cycle

• Overview: The fetch-decode-execute cycle is fundamental to the operation of a CPU, allowing it to process instructions. It consists of several stages that allow the CPU to execute programs.

Stages of the Cycle

1. Increment the Program Counter (PC): The address held in the Program Counter is copied to the Memory Address Register (MAR) to fetch the instruction.

2. Fetch the Instruction: The instruction is retrieved from memory and placed in the Memory Data Register (MDR).

3. Copy the Instruction: The instruction is then moved from the MDR to the Current Instruction Register (CIR).

4. Decode the Instruction: The CPU decodes the instruction to understand what action is required.

5. Execute the Instruction: The CPU executes the instruction as decoded.

6. Update the PC: The PC is incremented to point to the next instruction in sequence.

7. Cycle Repeats: The cycle then continues with these steps as new instructions are fetched.

CPU Performance Factors

• Moore's Law: Proposed by Gordon Moore in 1965, suggests that the number of transistors in a processor doubles every two years, leading to better performance.

• Benefits of More Transistors: Increased transistor count allows more complex processing capabilities such as better ALUs (Arithmetic Logic Units), larger registers, and more memory addresses available for RAM.

Clock Speed

• Function: The clock sends a timing signal to the Control Unit, measured in Hertz (Hz).

• Effect: A higher clock speed enables more operations per second, enhancing performance.

Limitations of CPU Performance

• Some operations take longer to complete, and data travel time over buses must be considered.

• Factors like power consumption and heat generation also limit performance.

Cache Memory

• Refers to a smaller, faster memory located within the CPU that stores frequently accessed data and instructions.

• Speeds up access times compared to standard RAM by preloading sequential memory addresses.

Advanced CPU Components

• Cores: More cores on a CPU allow parallel processing, sustaining or increasing performance without boosting clock speeds.

• Limitations of Cores: Not all programs can effectively utilize multiple cores; management of processes can introduce delays and data transfer bottlenecks.

Transistor Miniaturization

• Transistors on chips are measured in nanometers (nm); smaller sizes generally lead to better performance.

• Innovations have led to increases in transistor capacity, such as IBM's 2nm process accommodating up to 50 billion transistors in a chip the size of a fingernail.