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Cambridge IGCSE Computer Science · 0478

Topic 3: Hardware — Part 1

Computer Architecture

The Central Processing Unit

Role of the CPU
The CPU processes instructions and data that are input into the computer so that the result can be output.
Microprocessor
A microprocessor is a type of integrated circuit on a single chip that contains the CPU.
Instruction sets
An instruction set is a complete list of all the machine code commands that a particular CPU can understand and execute. It is built into the CPU's hardware and can vary between different processors.

Von Neumann architecture

Arithmetic Logic Unit (ALU)
Performs mathematical calculations and logical operations.
Control Unit (CU)
Coordinates the operation of the Fetch-Decode-Execute (FDE) cycle, decodes instructions, and sends commands to other components.

CPU registers

Small, fast storage locations used temporarily by the CPU during processing.

Register Full Name Function
PC Program Counter Stores the address of the next instruction to be fetched. It increments during every cycle.
MAR Memory Address Register Stores the address of the data/instruction currently being fetched or where data is to be stored.
MDR Memory Data Register Stores the actual data or instruction fetched from or being written to memory. Acts as a buffer.
CIR Current Instruction Register Stores the instruction currently being decoded and executed.
ACC Accumulator Stores the results of calculations or operations carried out by the ALU.

Internal buses

A collection of wires used to transmit signals between components.

Bus Direction Function
Address bus Unidirectional Carries memory addresses from the CPU to components like RAM or I/O devices.
Data bus Bidirectional Transmits the actual data or instructions between the CPU and other components.
Control bus Bidirectional Sends control signals (e.g., read/write) and timing information to synchronise components.

Fetch-Decode-Execute cycle

  1. Fetch
    • The address in the PC is copied to the MAR.
    • The address bus carries this address from the MAR to RAM.
    • The instruction is fetched from RAM and sent to the MDR via the data bus.
    • The PC increments by 1 to point to the next instruction.
  2. Decode
    • The instruction in the MDR is copied to the CIR.
    • The CU reads the instruction from the CIR and decodes it to determine the required action.
  3. Execute
    • The CPU carries out the decoded instruction (e.g., ALU performs calculations or data is moved between registers).
    • Results are typically stored in the ACC or written back to RAM.
    • The cycle repeats.
Von Neumann architecture diagram showing the CPU containing ALU, CU, and registers connected to RAM via address, data, and control buses
Diagram 1: Von Neumann architecture showing the CPU containing units and registers, connected to RAM via the address bus (one-way), data bus (two-way), and control bus (two-way).

CPU performance factors

Clock speed
The frequency of electrical pulses used to synchronise components; a greater clock speed allows more instructions to be executed per second.
Cache size
Small, high-speed memory on the CPU that stores frequently used data and instructions. Larger cache reduces the need for the CPU to access slower RAM.
Number of cores
Individual processing units. More cores allow processing more instructions at once (multitasking), provided the program is designed for it.

Embedded systems

Definition
A computer system designed to perform a dedicated function within a larger mechanical or electronic system.
Characteristics
Designed for one task, usually has minimal/no user interface, typically low power, small, and optimised for reliability.
Examples
Domestic appliances (washing machines), cars (braking systems), security systems, vending machines.
Contrast with general-purpose computers
Unlike general-purpose computers (PCs/laptops) that perform many different functions.

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