Computer Architecture (Copy)

Role of the Central Processing Unit (CPU)

• The CPU is the ‘brain’ of the computer, responsible for processing all instructions from programs and carrying out calculations.
• Functions include:
  • Fetching instructions from memory
  • Decoding instructions into understandable signals for execution
  • Executing the instructions (e.g. performing calculations, moving data)
  • Managing the flow of data between different parts of the computer
• CPU performance directly affects the overall speed and efficiency of a computer.
• The CPU operates using the fetch–decode–execute cycle.

What is a Microprocessor

• A microprocessor is a single integrated circuit (IC) containing all the essential CPU components.
• It is a miniaturized version of a CPU designed for compact systems.
• Found in:
  • Personal computers (PCs)
  • Embedded systems (washing machines, microwaves)
  • Mobile devices
• Advantages:
  • Small size → portable devices possible
  • Low power consumption
  • High speed for specific tasks

Von Neumann Architecture

• A computer architecture model where data and instructions share the same memory and pathways.
• Components:
  1. Arithmetic Logic Unit (ALU) – Performs all arithmetic (addition, subtraction, etc.) and logic (AND, OR, NOT) operations.
  2. Control Unit (CU) – Directs the flow of instructions and data within the CPU, controls other components.
  3. Registers – Small, high-speed storage locations inside the CPU:
     • Program Counter (PC) – Holds the address of the next instruction to fetch.
     • Memory Address Register (MAR) – Holds the address in memory where data/instructions will be fetched from or written to.
     • Memory Data Register (MDR) – Holds the actual data/instruction fetched from or written to memory.
     • Current Instruction Register (CIR) – Holds the current instruction being decoded/executed.
     • Accumulator (ACC) – Temporarily stores the results of calculations from the ALU.
  4. Buses – Pathways for transmitting data and signals:
     • Address Bus – Carries memory addresses (one way – CPU → RAM).
     • Data Bus – Carries data between CPU, memory, and input/output devices.
     • Control Bus – Carries control signals (e.g., Read/Write, clock signals).

The Fetch–Decode–Execute (FDE) Cycle

1. Fetch:

• The Program Counter (PC) sends the address of the next instruction to the MAR.
• The Address Bus sends this address to RAM.
• The CPU sends a Read signal via the Control Bus.
• The instruction is fetched from RAM into the MDR.
• The instruction is then moved from the MDR to the CIR.
• The PC is incremented to point to the next instruction.

2. Decode:

• The Control Unit decodes the instruction in the CIR into a series of control signals.
• Determines which parts of the CPU and memory will be used.

3. Execute:

• The appropriate components (ALU, registers, memory) carry out the instruction.
• If it’s a calculation, the ALU processes it and stores the result in the ACC.
• If it’s a data transfer, the Data Bus is used to move data to/from memory.

Cores, Cache, and Clock Speed

Cores:

• A core is an independent processing unit inside a CPU.
• A single-core CPU can only process one instruction at a time, while multi-core CPUs can handle multiple instructions simultaneously.
• More cores = potential for better multitasking and faster execution of parallel tasks.

Cache:

• Small, very fast memory inside the CPU used to store frequently accessed instructions/data.
• Levels:
  • L1 Cache – Smallest but fastest, directly built into the core.
  • L2 Cache – Larger but slightly slower, may be shared between cores.
  • L3 Cache – Larger and slower than L1/L2, shared between all cores.
• Larger cache → fewer accesses to slower main memory → improved performance.

Clock Speed:

• Measured in GHz (gigahertz).
• Indicates how many clock cycles the CPU can complete per second.
• Higher clock speed → more instructions executed per second.
• Limited by heat generation and power consumption.

Instruction Set

• The instruction set is the complete list of commands the CPU can execute.
• Written in machine code (binary) and corresponds to specific operations.
• Examples:
  • Load data from memory
  • Add numbers in two registers
  • Compare two values
• Each CPU has its own instruction set (e.g., x86, ARM).

Embedded Systems

• Definition: A computer system designed for a specific dedicated function.
• Differences from general-purpose computers:
  • Purpose-built for one task rather than multiple tasks.
  • Often smaller, cheaper, and more power-efficient.
• Common examples:
  • Domestic appliances (microwave ovens, washing machines)
  • Cars (engine control units, airbags)
  • Security systems (CCTV, alarms)
  • Lighting control systems
  • Vending machines
• Advantages:
  • Reliable for specific function
  • Low power consumption
  • Smaller and cheaper than general-purpose PCs
• Disadvantages:
  • Cannot easily be reprogrammed for new tasks
  • Limited functionality compared to general-purpose computers