Block 3 Principles of Computation

CPU

Central Processing Unit; executes instructions and processes data

Von Neumann Architecture

System where data and instructions share the same memory

Input → CPU → Output

Basic data flow in a computer system

CPU Components

Control Unit (CU), Arithmetic Logic Unit (ALU), Registers

Control Unit (CU)

Manages and coordinates CPU operations and sends control signals

Control Unit role

Fetches, decodes, and executes instructions

Control Unit interrupts

Handles hardware interrupts from other devices

Control Unit importance

Considered the “powerhouse” managing CPU processes

Arithmetic Logic Unit (ALU)

Performs arithmetic and logical operations on data

ALU arithmetic operations

Addition, subtraction (and other calculations)

ALU logic operations

AND, OR, NOT

ALU comparisons

Can compare numbers against 0

ALU equality check

Can test if two numbers are equal

ALU implementation

Uses logic gates combined to perform operations

Registers

Small, extremely fast storage locations inside CPU

Register speed

Faster than RAM

Instruction Register (IR)

Holds the current instruction being executed

Program Counter (PC)

Stores the address of the next instruction

Accumulator

Stores results of ALU operations

General Purpose Registers

Temporary storage for data being processed

Clock

Synchronises CPU operations

Clock speed

Measured in Hertz (Hz), cycles per second

1 Hz

One cycle per second

500 Hz

500 cycles per second

3 GHz

3 billion cycles per second

Clock tick

One cycle allowing part of the fetch-decode-execute cycle

Higher clock speed

More instructions processed per second

Pipelining

Technique where multiple instructions are processed simultaneously

Pipelining benefit

Keeps CPU components busy, improves performance

Pipeline condition

If one instruction is fetched every clock cycle

Bus

Pathway connecting CPU to other components

Bus function

Transfers data, addresses, and control signals

Data Bus

Transfers actual data between components

Data bus direction

Bi-directional

Data bus purpose

Used for both reading and writing data

Control Bus

Carries control and status signals

Control bus direction

Bi-directional

Control bus role

Sends commands and receives status signals

Address Bus

Carries memory addresses

Address bus direction

Uni-directional

Address bus purpose

CPU specifies location in memory or device

Address bus size

Determines maximum memory addressable

RAM

Random Access Memory; primary storage

Primary storage

Located close to CPU for fast access

RAM type

Volatile memory

Volatile memory

Data lost when power is off

RAM purpose

Stores currently running programs and data

Secondary Storage

Long-term storage for data

Purpose

Retains data when power is off

Requirement

Must allow data retrieval after restart

Storage Types

Magnetic, Optical, Solid State

Magnetic storage

Uses magnetised material to store data

How it works (magnetic)

Magnetises regions as north/south poles

Binary representation (magnetic)

North/South = 1s and 0s

Examples (magnetic)

Hard disk drive (HDD), Magnetic tape

Magnetic advantages

High capacity, low cost per GB, moderate speed

Magnetic disadvantages

Easily damaged, heavy, prone to mechanical failure

Optical storage

Uses laser to read/write data on reflective surface

Structure (optical)

Flat disc with reflective layer

Pits and lands

Pits = burned areas, Lands = unburned areas

Binary interpretation (optical)

Reflection = land, no reflection = pit

Examples (optical)

DVD, CD

Optical advantages

Low cost, no physical contact, portable, silent

Optical disadvantages

Very low capacity, slow speeds, prone to scratches

Solid state storage

Uses NAND flash memory (no moving parts)

Material (SSD)

Silicon chips

How it works (SSD)

Electrons trapped in “pools”

Binary representation (SSD)

Full pool = 0, empty pool = 1

Examples (SSD)

SSD, USB flash drive

Solid state advantages

Fast read/write, durable, portable, silent

Solid state disadvantages

High cost per GB, limited write cycles

Primary vs Secondary Storage

RAM is volatile; secondary storage is permanent

Register vs RAM

Registers are faster but much smaller

Clock speed vs performance

Higher speed = more cycles = faster processing

Address bus vs memory

Wider bus = more memory accessible

Pipelining impact

Improves throughput, not necessarily single instruction speed