Central Processing Unit

The Central Processing Unit (CPU) is the core component of a computer system that performs most of the processing operations. Often referred to as the “brain of the computer,” it interprets and executes instructions from software programs and coordinates the activities of all hardware components. Every computational process — from running applications to performing arithmetic — ultimately depends on the CPU’s speed, architecture, and efficiency.

Definition

The Central Processing Unit (CPU) is an integrated electronic circuit that carries out the instructions of a computer program by performing the basic arithmetic, logic, control, and input/output (I/O) operations specified by the instructions.
It is a processing engine that transforms input data into meaningful output through a systematic cycle of operations known as the fetch–decode–execute cycle.

Basic Components of the CPU

A CPU typically consists of three main units:

1. Arithmetic Logic Unit (ALU)

• The ALU is responsible for performing all arithmetic and logical operations.
• Arithmetic functions: Addition, subtraction, multiplication, division, and comparison.
• Logical functions: AND, OR, NOT, XOR, and relational comparisons (greater than, less than, equal to).
• The ALU also plays a role in decision-making processes based on logic operations.

2. Control Unit (CU)

• The Control Unit directs the operation of the processor.
• It fetches instructions from memory, decodes them, and controls the flow of data between the CPU, memory, and I/O devices.
• It manages timing and execution by sending control signals to various parts of the computer.
• Essentially, the CU acts as the “manager” or “supervisor” of all CPU operations.

3. Registers

• Registers are high-speed, small memory units located inside the CPU that temporarily store data and instructions.
• They hold intermediate results, addresses, and control information needed during processing.
• Examples include:
  • Accumulator (ACC): Stores arithmetic results.
  • Program Counter (PC): Holds the address of the next instruction to be executed.
  • Instruction Register (IR): Contains the current instruction being decoded.
  • Memory Address Register (MAR): Holds memory locations to be accessed.
  • Memory Data Register (MDR): Temporarily holds data fetched from or to be written to memory.

Working of the CPU: The Fetch–Decode–Execute Cycle

The CPU performs its tasks through a continuous process known as the instruction cycle or fetch–decode–execute cycle.

1. Fetch:
   • The Program Counter (PC) identifies the address of the next instruction.
   • The instruction is fetched from memory and stored in the Instruction Register (IR).
2. Decode:
   • The Control Unit decodes the instruction, identifying the operation to be performed and the operands involved.
3. Execute:
   • The ALU performs the required operation, such as an arithmetic calculation or data transfer.
   • The result is stored in a register or sent back to memory.
4. Store/Next:
   • The CPU updates the Program Counter to point to the next instruction and repeats the cycle.

This continuous loop allows the CPU to process millions or billions of instructions per second.

Types of CPUs

1. Based on Instruction Set Architecture (ISA):

• CISC (Complex Instruction Set Computer):
  • Contains a large set of instructions, allowing complex operations in a single command.
  • Example: Intel x86 processors.
• RISC (Reduced Instruction Set Computer):
  • Utilises a smaller set of simple instructions for faster execution.
  • Example: ARM processors (used in smartphones, tablets).

2. Based on Number of Cores:

• Single-Core CPU: Performs one task at a time; found in older computers.
• Dual-Core / Quad-Core / Octa-Core CPUs: Contain multiple processing cores on one chip, allowing parallel processing and multitasking.

3. Based on Manufacturer or Application:

• Desktop CPUs: Intel Core, AMD Ryzen series.
• Mobile CPUs: ARM-based processors used in mobile and embedded systems.
• Server CPUs: Xeon (Intel), EPYC (AMD) designed for high-performance computing.
• Microcontrollers: Combine CPU with memory and I/O ports for embedded systems (e.g., Arduino, PIC).

Performance Parameters of a CPU

1. Clock Speed:
   • Measured in Hertz (Hz), it represents the number of cycles executed per second.
   • Modern CPUs operate in gigahertz (GHz) range, indicating billions of cycles per second.
2. Instruction Per Cycle (IPC):
   • Denotes how many instructions the CPU can execute per clock cycle; higher IPC improves performance.
3. Cache Memory:
   • Small, high-speed memory inside the CPU used to store frequently accessed data.
   • Divided into levels: L1, L2, and L3 cache — with L1 being the fastest but smallest.
4. Bus Width:
   • The size (in bits) of data the CPU can process at once (e.g., 32-bit or 64-bit).
5. Thermal Design Power (TDP):
   • Indicates heat output and energy consumption; vital for efficiency and cooling design.
6. Architecture Efficiency:
   • Measured by the ability to handle pipelines, branch prediction, and instruction parallelism.

Evolution of the CPU

1. First Generation (1940s–1950s):
   • Vacuum tube-based processors (e.g., ENIAC).
   • Large, slow, and consumed significant power.
2. Second Generation (1950s–1960s):
   • Transistor-based CPUs replaced vacuum tubes.
   • Smaller, faster, and more reliable.
3. Third Generation (1960s–1970s):
   • Use of integrated circuits (ICs).
   • Marked the beginning of minicomputers and mainframes.
4. Fourth Generation (1970s–Present):
   • Development of microprocessors, integrating CPU functions on a single chip.
   • Examples: Intel 4004 (1971), Intel 8086 (1978).
5. Modern Era:
   • Multi-core, hyper-threaded, and energy-efficient processors dominate.
   • CPUs are now integrated with graphics processing units (GPUs) and AI accelerators in System-on-Chip (SoC) designs.

Functions of the CPU

1. Processing: Executes arithmetic, logic, and control operations.
2. Control: Directs and coordinates activities of all hardware components.
3. Storage and Retrieval: Temporarily stores and retrieves data for ongoing operations.
4. Communication: Exchanges data with memory, input/output devices, and secondary storage.
5. Decision-Making: Executes conditional operations, enabling logical flow in program execution.

Role of CPU in Modern Computing

The CPU remains central to computing, though modern systems increasingly distribute processing tasks among specialised units like:

• GPU (Graphics Processing Unit): Handles parallel and graphical computations.
• TPU (Tensor Processing Unit): Designed for machine learning and AI applications.
• FPGA and ASIC: Customised chips for specific tasks.

Despite this diversification, the CPU continues to be the primary controller and coordinator of system operations, executing general-purpose tasks.