Computer Science: Computer Architecture, I/O, Data Storage, and Networking

The CPU & Microprocessors

Purpose of the CPU

The Central Processing Unit (CPU) fetches, decodes, and executes instructions. It is the "brain" of the
computer, taking input, processing data, and producing output.

Microprocessors

A microprocessor is an integrated circuit on a single chip that contains the CPU and other components,
such as:

• Memory controllers

• I/O interfaces

The CPU is a component, while the microprocessor is the chip that may include the CPU and other parts.

Von Neumann Architecture

Definition

The Von Neumann architecture is a CPU design where the computer memory, input/output devices, and
processor all work together. It consists of:

• Control unit (CU)

• Arithmetic logic unit (ALU)

• Registers

• Buses

Components and Functions

• Arithmetic Logic Unit (ALU): Performs arithmetic operations and logical decisions (e.g., IF X > 5
THEN DO...).

• Control Unit (CU): Coordinates data movement within the CPU by sending signals. It also
decodes instructions fetched from memory.

• Registers: Extremely small, fast memory located in the CPU that holds small amounts of data
needed during the fetch-execute cycle.

Key Registers

• Program Counter (PC): Holds the memory address of the next instruction to be executed. It
increments by 1 as the fetch-decode-execute cycle runs.

• Memory Address Register (MAR): Holds the memory address of where data or instructions are
to be fetched from memory.
 • Memory Data Register (MDR): Stores the data or instruction that has been fetched from
memory.

• Current Instruction Register (CIR): Stores the instruction the CPU is currently decoding or
executing.

• Accumulator (ACC): Stores the results of calculations from the ALU.

Buses

Buses are sets of parallel wires that transmit data/signals between components.

• Address Bus: Unidirectional; carries memory location data (addresses) for reading/writing.

• Data Bus: Bidirectional; carries data or instructions.

• Control Bus: Bidirectional; carries commands and control signals.

Fetch-Decode-Execute Cycle (FDE)

Definition

The Fetch-Decode-Execute Cycle is the process the CPU runs through to execute instructions.

Stages

1. Fetch:

• The PC holds the address of the next instruction.

• The address is copied into the MAR.

• The address is sent along the address bus.

• The CU sends a signal along the control bus.

• Data/instructions are fetched from main memory to the MDR via the data bus.

• A copy of the instruction/data is stored in the CIR.

• The PC increments by 1.

2. Decode:

• The CPU determines what is required from the instruction.

• The instruction is split into:

o Opcode: The operation to be performed.

o Operand: What to do it to (data or address).

3. Execute:

• The CPU carries out the instruction.

 • Examples: performing a calculation, storing a result, fetching data from a different
location.

Characteristics of the CPU

The common characteristics are Clock Speed, Cache Size, and Number of Cores.

Clock Speed

Measured in Hertz (Hz). It represents the number of fetch-decode-execute cycles per second. A faster
clock speed means more instructions can be executed per second. Modern computers use GHz (billions
of cycles per second).

Cache Size

Cache is small, fast memory on or near the CPU. It stores frequently used instructions/data for quick
access. A larger cache size means more frequently used data can be stored, reducing the need to fetch
from RAM.

Number of Cores

A core acts like its own CPU. Multiple cores allow multiple instructions to be fetched, decoded, and
executed simultaneously. For example, a quad-core CPU (4 cores) running at 3 GHz can
execute 4×3=124×3=12 billion instructions per second.

CPU Instruction Sets

An instruction set is a list of all commands a CPU can process. Each command has a binary code called
machine code. The opcode is the operation, and the operand is the location.

Instruction Mnemonic Binary Code Command

Add ADD 10100001 Adds a value to the value currently stored in the accumulator (ACC)

Subtract SUB 00100010 Subtracts a value from the values stored in the accumulator

Load LDA 10111111 Loads the value stored in a memory location into the accumulator

Store STA 01100000 Stores the value in the accumulator in a specific location in memory

Stop HLT 00000000 Stops the program

Embedded Systems

An embedded system is a computer system performing a dedicated function within a larger mechanical
unit. Examples: thermostats, washing machines, satellite navigation systems, factory equipment,
security systems, traffic lights.

Properties

• Small in size

• Low power consumption

 • Low cost

Input Devices

Hardware components that allow users to interact with a computer system by inputting data or
commands. Examples: barcode scanners, digital cameras, keyboards, microphones, mice, QR code
scanners, touch screens, 2D/3D scanners.

Output Devices

Hardware components that receive information from a computer system and present it to the user.
Examples: actuators, DLP projectors, inkjet printers, laser printers, LED screens, LCD projectors, LCD
screens, speakers, 3D printers.

Sensors

Input devices that measure a physical property of the environment, such as light levels, temperature, or
movement. Used in monitoring and control systems.

Sensor Type Measures Typical Use

Detecting changes in sound levels of industrial machinery,

Acoustic Sound levels
monitoring noise pollution, security systems

Acceleration rate, tilt, Detecting sudden changes in vehicle movement, mobile

Accelerometer
vibration phone orientation

Rate of gas, liquid, or

Flow Detecting changes in the flow through pipes in water system
powder flow

Detect levels of gas in confined spaces, detect gas levels

Gas Presence of a gas (e.g., CO)
when fixing gas leaks

Humidity Levels of water vapor Monitor humidity in greenhouses

Security systems detecting intruders, measure heat

Infra-red Motion or a heat source
radiation of objects

Detect levels of petrol in a car tank, detect levels of water in

Level Liquid levels
a water tank

Light Light levels Automatically switching on lights when it gets dark

Magnetic field Presence and strength Anti-lock braking system, Monitoring rotating machinery

Presence and levels of Monitoring moisture in the soil, Monitoring dampness in

Moisture
moisture buildings

Monitoring soil to ensure optimum growing conditions,

pH Acidity or alkaline
Monitor ph levels in chemical processes
Sensor Type Measures Typical Use

Gas, liquid, or physical Monitoring tyre pressure, Monitoring pressure in pipes

Pressure
pressure during the manufacturing process.

Monitoring the position of objects in robotics, Used in safety

Proximity Distance
systems to prevent objects from colliding

Temperature (Celsius, Used to maintain temperature in swimming pools, Used to

Temperature
Fahrenheit, Kelvin) control temperature in chemical processes

Data Storage

Primary Storage

Primary storage is directly accessed by the CPU and is faster than secondary storage.

• RAM (Random Access Memory): Volatile memory that holds data and instructions currently in
use.

• ROM (Read-Only Memory): Non-volatile memory that holds the initial startup instructions
(BIOS).

Secondary Storage

Non-volatile storage for long-term storage of programs and data. Types:

• Magnetic: Uses magnets to store data (e.g., hard disk drives).

o High capacity, low cost per gigabyte, moderate speed.

o Prone to mechanical failure, heavy, and noisy.

• Solid-State (Flash Memory): Uses electronic circuits to store data (e.g., SSDs, USB flash drives).

o Very fast read/write access, durable, and silent.

o High cost per gigabyte and limited read/write cycles.

• Optical: Uses lasers to burn the surface of a disk (e.g., CDs, DVDs, Blu-rays).

o Low cost per gigabyte and durable.

o Low capacity, slow read/write access, and prone to scratches.

Virtual Memory

An extension of RAM located on secondary storage. It is used when RAM is full. Data not currently being
executed is transferred to virtual memory, which is much slower than RAM.

Cloud Storage

Long-term storage of data in a remote location, accessible via the internet.

Advantages
 • Accessibility from anywhere.

• Scalability for increasing storage.

• Reliability with automatic backups.

Disadvantages

• Requires a constant internet connection.

• Potential security risks.

• Subscription costs.

Network Hardware

Network Interface Card (NIC)

Required for a computer to connect to a network. It allows the computer to send and receive data.

MAC Addresses & IP Addresses

• MAC (Media Access Control) Address: A unique identifier assigned to devices on a local area
network (LAN) during manufacturing. It is static and cannot change.

• IP (Internet Protocol) Address: A unique identifier assigned to devices communicating over the
Internet (WAN). It can be static or dynamic.

o IPv4: Represented as 4 blocks of denary numbers (e.g., 192.168.1.1).

o IPv6: Represented as 8 blocks of hexadecimal digits (e.g.,

2001:0db8:85a3:0000:0000:8a2e:0370:7334).

Router

Responsible for routing data packets between different networks, connecting LANs to the internet
(WAN). It manages data traffic and assigns IP addresses to devices on the network.