A1.1.1 Functions and Interactions of the Main CPU Components Notes

What is a CPU?

1. Stands for the "Central Processing Unit".
2. The part of the computer that is responsible for "thinking", "deciding" or "Controlling" what the computer does.

Core Components of the CPU

The CPU consists of several key components, each with a specific role:

1. Registers: small, fast memory storage locations for holding current data and instructions being processed
2. Units: Responsible for executing the algorithm steps
   1. Arithmetic Logic Unit (ALU): arithmetic and logical operations.
   2. Control Unit (CU): Generates control signals to activate hardware units.
3. Busses: Responsible for carrying data and instructions from one component to another

Arithmetic Logic Unit (ALU)

1. It does all the thinking!
2. Performs arithmetic (add, subtract) and logical (AND, OR, NOT) operations.
3. Works with data from registers.

Control Unit (CU)

1. The boss or controller of the CPU.
2. Orchestrates the fetch-decode-execute cycle.
3. Decodes instructions and directs data flow between CPU, memory, and peripherals.
4. Sends control signals via the control bus.

Registers: The CPU's Fastest Memory

They temporarily hold data and instructions during execution.

1. Instruction Register (IR)
   1. Holds the current instruction being executed.
2. Program Counter (PC)
   1. Stores the address of the next instruction to be executed.
3. Memory Address Register (MAR)
   1. Holds the address of the memory location to be accessed.
4. Memory Data Register (MDR)
   1. Stores data being transferred to or from memory.
5. Accumulator (AC)
   1. Holds intermediate results of arithmetic and logical operations.

Buses: The Communication Pathways

1. Address Bus
   1. Carries memory addresses from the CPU to specify where data should be read or written.
   2. This is unidirectional and only goes from the CPU to the memory
2. Data Bus
   1. Transfers actual data between the CPU, memory, and other components.
   2. Bidirectional, allowing data flow in both directions.
3. Control Bus
   1. Carries control signals from the CU to manage actions and timing.

Cache

1. A small, high-speed memory located inside or very close to the CPU.
2. Purpose: Stores frequently used data and instructions to speed up access and reduce the need to fetch from slower main memory (RAM).
3. Levels:
   1. L1 Cache: Smallest and fastest, closest to the CPU core.
   2. L2 Cache: Larger but slightly slower.
   3. L3 Cache: Even larger, shared across cores in multi-core CPUs.
4. How it works: When the CPU needs data, it first checks the cache. If the data is there (cache hit), it is retrieved quickly. If not (cache miss), the CPU fetches it from RAM.
5. Benefit: Improves processing speed and overall system performance by reducing latency.

Processors: Single-Core, Multi-Core, and Co-Processors

1. Single-Core Processors
   1. One processing unit handles one instruction at a time.
   2. Limitations: Struggle with multitasking and parallel processing.
2. Multi-Core Processors
   1. Multiple independent cores execute instructions simultaneously.
   2. Benefits: Improved performance for multitasking and parallel processing.
3. Co-Processors
   1. Specialized processors that offload specific tasks from the CPU.

Interactions Between CPU Components

The CPU components work together seamlessly to execute instructions:

1. Fetch:
   1. The PC sends the address of the next instruction to the MAR.
   2. The instruction is fetched from memory and stored in the IR.
2. Decode:
   1. The CU decodes the instruction in the IR, generating control signals to direct the ALU and other components.
3. Execute:
   1. The ALU performs the required operation, using data from registers like the AC.
   2. Results are stored back in the AC or written to memory via the MDR.

Clock

1. The clock is a vital component of the CPU that generates a regular sequence of electrical pulses.
2. These pulses synchronize all operations in the CPU, ensuring that instructions are executed in an orderly manner.
3. Fetch-Decode-Execute Cycle: The CPU repeatedly fetches instructions from memory, decodes them, and executes them.
4. The clock acts like a metronome, providing timing signals so that different parts of the CPU work together smoothly.

Functions of the clock:

1. Controls the pace at which instructions are carried out.
2. Coordinates the activities of the ALU, CU, registers, and buses.

Clock Speed

1. Clock speed determines how many clock cycles occur per second.
2. Measured in Hertz (Hz)
3. 1 Hz = 1 cycle per second.
4. Modern CPUs are usually measured in Gigahertz (GHz) (1 GHz = 1 billion cycles per second).

Impact of clock speed:

1. A higher clock speed means the CPU can process more instructions per second → generally leads to faster performance.
2. Example: A 3.5 GHz CPU can perform 3.5 billion cycles per second.

The Significance of Understanding CPU Components

1. Efficiency: Knowing how the CPU works helps optimize software for better performance.
2. Troubleshooting: Understanding CPU interactions aids in diagnosing hardware and software issues.
3. Innovation: Insights into CPU architecture drive advancements in computing technology.