Computer architecture

1. Overview

This topic explores the inner workings of a computer, focusing on the Central Processing Unit (CPU) and its components. Understanding computer architecture is crucial for understanding how software interacts with hardware and how to optimise computer performance.

Key Definitions

• CPU (Central Processing Unit): The "brain" of the computer responsible for processing data and instructions.
• Microprocessor: A CPU contained on a single integrated circuit (chip).
• ALU (Arithmetic Logic Unit): Part of the CPU that performs arithmetic and logical operations.
• Control Unit: Part of the CPU that coordinates all operations by decoding instructions and sending control signals.
• Register: A small, high-speed storage location within the CPU.
• PC (Program Counter): A register that holds the memory address of the next instruction to be executed.
• MAR (Memory Address Register): A register that holds the memory address currently being accessed.
• MDR (Memory Data Register): A register that holds the data being transferred to or from memory.
• CIR (Current Instruction Register): A register that holds the instruction currently being decoded and executed.
• Accumulator: A register used to store the results of calculations performed by the ALU.
• Bus: A set of parallel wires that transmits data between different components.
• Address Bus: A unidirectional bus that carries memory addresses from the CPU to memory.
• Data Bus: A bidirectional bus that carries data between the CPU and memory, or between different components.
• Control Bus: A bus that carries control signals (e.g., read, write) between the CPU and other components.
• Fetch-Decode-Execute Cycle: The fundamental cycle of operations performed by the CPU.
• Core: An independent processing unit within a CPU.
• Cache Memory: Small, fast memory within the CPU used to store frequently accessed data.
• Clock Speed: The rate at which the CPU executes instructions, measured in Hertz (Hz).
• Instruction Set: The complete set of instructions that a CPU can execute.
• Embedded System: A dedicated computer system built into a device for a specific function.

Core Content

Role of the CPU

The CPU is the core component of a computer, responsible for:

• Fetching instructions from memory.
• Decoding instructions.
• Executing instructions.
• Performing arithmetic and logical operations.
• Controlling the flow of data within the computer.

Microprocessor

• A microprocessor is a CPU contained on a single integrated circuit or chip.
• Modern computers, smartphones, and embedded systems use microprocessors.
• They combine the ALU, control unit, and registers on a single chip.

CPU Components and Von Neumann Architecture

The Von Neumann architecture defines the basic structure of a computer, where both instructions and data are stored in the same memory location. The CPU contains the following key components:

• ALU (Arithmetic Logic Unit): Performs calculations and logical operations (e.g., addition, subtraction, AND, OR).
• Control Unit:
  • Fetches instructions from memory.
  • Decodes instructions to determine what operations to perform.
  • Sends control signals to other components to coordinate operations.
• Registers: Small, high-speed storage locations within the CPU. Key registers include:
  • PC (Program Counter): Holds the address of the next instruction to be executed.
  • MAR (Memory Address Register): Holds the address of the memory location being accessed.
  • MDR (Memory Data Register): Holds the data being transferred to or from memory.
  • CIR (Current Instruction Register): Holds the instruction currently being decoded and executed.
  • Accumulator: Stores the results of calculations performed by the ALU.
• Buses:
  • Address Bus: Carries the memory address from the CPU to memory. It is unidirectional (CPU to memory).
  • Data Bus: Carries data between the CPU and memory (or other components). It is bidirectional.
  • Control Bus: Carries control signals (e.g., read, write, interrupt) between the CPU and other components.

Fetch-Decode-Execute (FDE) Cycle

The FDE cycle is the fundamental process by which the CPU executes instructions:

1. FETCH:
   • The PC contains the address of the next instruction.
   • The address in the PC is copied to the MAR.
   • The instruction at that address is fetched from memory and placed in the MDR.
   • The instruction in the MDR is copied to the CIR.
   • The PC is incremented to point to the next instruction.
2. DECODE:
   • The Control Unit decodes the instruction in the CIR.
   • The Control Unit determines what operation needs to be performed.
3. EXECUTE:
   • The Control Unit sends signals to the appropriate components (e.g., ALU, registers) to perform the operation.
   • If the instruction involves a calculation, the ALU performs the calculation.
   • Data is moved between registers and memory as needed.
4. The cycle repeats continuously.

Core, Cache and Clock

• Core:
  • An independent processing unit within a CPU.
  • Multi-core CPUs (dual-core, quad-core, etc.) can execute multiple instructions simultaneously, improving performance through parallelism.
• Cache:
  • Small, fast memory within the CPU used to store frequently accessed data and instructions.
  • Reduces the need to access slower main memory, improving performance.
  • Levels of cache: L1 (fastest, smallest), L2, L3 (slowest, largest). L1 cache is closest to the core.
• Clock:
  • Generates regular pulses to synchronize operations within the CPU.
  • Clock speed is measured in Hertz (Hz) or Gigahertz (GHz), representing cycles per second.
  • Higher clock speed generally means faster instruction execution. However, other factors like cache size and core architecture also influence performance.

Factors Affecting CPU Performance

Factor	Effect on Performance
Clock Speed	Higher clock speed generally leads to faster instruction execution, but it's not the only factor.
Number of Cores	More cores allow for parallel processing, which can significantly improve performance for tasks that can be divided into smaller subtasks.
Cache Size	Larger cache size reduces the need to access slower main memory, leading to faster access to frequently used data and instructions.
Instruction Set Architecture	Some instruction set architectures (like RISC) are more efficient than others (like CISC), potentially leading to better performance with the same clock speed.

Instruction Set

• The instruction set is the complete set of instructions that a CPU can execute.
• Instructions are typically represented in machine code (binary).
• Examples of instruction types:
  • Data Transfer: LOAD (copy data from memory to a register), STORE (copy data from a register to memory).
  • Arithmetic: ADD (add two values), SUB (subtract two values), MUL (multiply two values), DIV (divide two values).
  • Logical: AND (logical AND), OR (logical OR), NOT (logical NOT).
  • Control Flow: JUMP (unconditional jump to a new address), BRANCH (conditional jump based on a condition), HALT (stop execution).

Embedded Systems

• A dedicated computer system built into a device for a specific function.
• Characteristics:
  • Single purpose (designed for a specific task).
  • Runs continuously.
  • Limited user interface (often no screen or keyboard).
  • Low power consumption (important for battery-powered devices).
  • Real-time operation (must respond quickly to events).
• Examples:
  • Washing machines, microwave ovens, traffic lights, car engine management systems, smart thermostats, medical devices, ATMs.

Exam Focus

• Understand the roles of the CPU components (ALU, Control Unit, Registers, Buses).
• Be able to explain the Fetch-Decode-Execute cycle in detail, including the role of each register.
• Know how clock speed, number of cores, and cache size affect CPU performance.
• Be able to identify examples of embedded systems and their characteristics.
• Use precise terminology when describing CPU components and processes.
• Understand the purpose of the instruction set for a CPU.
• Be able to distinguish between different types of buses (address, data, control).

Common Mistakes to Avoid

• ❌ Wrong: The accumulator stores data being transferred to and from memory. ✓ Right: The MDR stores data being transferred to and from memory; the accumulator stores the result of ALU operations.
• ❌ Wrong: The address bus is bidirectional. ✓ Right: The address bus is unidirectional (CPU to memory).
• ❌ Wrong: The control unit performs arithmetic operations. ✓ Right: The ALU performs arithmetic operations. The control unit coordinates operations.
• ❌ Wrong: Cache memory is the same as main memory. ✓ Right: Cache memory is smaller and faster than main memory, used to store frequently accessed data.

Exam Tips

• When describing the FDE cycle, be specific about the roles of the PC, MAR, MDR, and CIR.
• Use diagrams to help explain complex concepts like the Von Neumann architecture or the FDE cycle (if allowed).
• When discussing CPU performance, consider multiple factors (clock speed, cores, cache) rather than focusing on a single factor.
• Practice defining key terms clearly and concisely.
• Know the characteristics that define an embedded system.