Computer Systems (without Logic) AQA

Hardware

Hardware

The physical components that make up a computer system, including processors, memory, storage and input + output devices

Software

The programs that run on a computer, including operating systems, utility programs, library programs and application software (which includes mobile apps)

System software

Programs that are needed for effective communication with the hardware, and for launching application software

Examples: Operating systems (e.g. Windows, Android) and utility software (e.g. browsers and word processing packages)

Operating system (OS)

A piece of system software that acts as an interface between the user and the hardware, managing all hardware and all other software

What is managed by the operating system?

Processors, Memory, Input/Output devices, Applications, Security

Processors

The operating system decides:

• which processes will be carried out by which processors

• if multiples processes are running, which one the processor should handle next

• how long a time slice the process is given, i.e. how long before the processor's attention switches to the next process

Memory

The operating system

• loads programs and data from backup store to main memory

• removes programs that are not needed to make room for more

• manages virtual memory

Input/Output devices

The operating system

• acts as a go-between between application software and I/O devices

• manages device drivers, which are programs telling the operating system how to communicate with attached I/O devices

Applications

The operating system

• communicates between application software and hardware

• processes requests from application software for resources, such as a network connection or a remotely stored file

Security

The operating system can

• manage multiple user accounts, keeping users' data seperate

• automatically back up data, increasing security

• handle usernames and passwords to prevent unauthorised access

• recognise a user as administrator, giving them greater access rights

Utility software

Programs that keep the computer functioning efficiently

Compression

Reducing the size of a file so that it can be stored using less space or transmitted more quickly

Defragmentation

Moving seperate parts of a file physically together, to speed up disk access

Backing up

Creating a copy of files, either on the same disk, on a backup device or in the cloud

Encryption

Allowing for data to be scrambled in order to prevent unauthorised individuals from understanding any files that they see. This might be for secure storage or secure transmission

Von Neumann Architecture

Data is input, processed by the CPU, and outputted. There is also memory, allowing data to be stored

CPU

Central Processing Unit. Executes program instructions, performing calculations and comparisons, as well as coordinating the behaviour of other hardware.

Arithmetic Logic Unit (ALU)

Performs various operations:

• Arithmetic operations (+ - * /)

• Relational operations (< > =)

• Logic operations (AND, OR, NOT)

Control Unit (CU)

Manages the execution of instructions by coordinating the activities of the other hardware

Buses

Collections of wires that transmit data between computer components

Data bus

Moves back and forth between the CPU and memory

Address bus

Transmits memory locations. Any data retrieved from or placed into memory belongs in a specific address or memory location

Control bus

Transmits commands to other components, such as read or write

Registers

Short-term storage for small, specific pieces of data, within the CPU itself

Clock

Synchronises activities within the CPU. Every action performed by the CPU must begin during the clock pulse.

Main memory

The main working area for data currently being used and programs currently running

RAM (Random Access Memory)

Smaller but faster than a hard disk. A programs data and instructions are loaded from a computer's hard disk into RAM. It is volatile. More RAM means that the computer can run more applications at the same time

ROM (Read Only Memory)

The content is not volatile - it cannot be edited or deleted. ROM stores data or instructions that will not need to be updated, such as bootstrapping instructions

Cache

Stores copies of data or instructions from RAM that are regularly accessed. These can be accessed very quickly from cache, although a computer's cache memory is usually very small.

Factors affecting CPU performance

• Number of cores

• Clock speed

• Size of cache

• Type of cache

Core

A single unit, comprising an ALU and a control unit, which can execute instructions. More cores means that more instructions per second can be processed

Clock speed

The number of clock pulses per seond, typically measured in gigahertz (GHz)

Fetch-execute cycle

A continual sequence of tasks that results in instructions being fetched from main memory, decoded so that the CPU knows what to do with them, and then executed

Stage 1 - Fetch

• The clock pulses

• One of the registers (program counter) contains the location in memory where the next instruction is to be found

• This location (address) is transmitted along the address bus to memory

• Memeory sends the content of the address along the data bus back to the processor. The instruction is stored in a register (Instruction Register)

Stage 2 - Decode

The instructions is read by the control unit, which prepares the registers for whatever date they will be storing.

Stage 3 - Execute

The instrction is carried out. The program counter is updated because the current instrction has been executed, and the cycle begins again

Secondary Storage

Long-term storage in a computer system. It is necessary because main memory is volatile (loses its contents when powered down) and will also run out of storage space

Optical Storage

Written to and read from using lasers - the disks spin to allow the laser to read the data from the correct location. The surface of these disks have billions of pits and lands to represent either 0s or 1s

Examples: CDs, DVDs, Blu-Ray disks

Advantages of Optical Storage

• One optical disk is very cheap

• Is often read-only, so it is difficult to accidentally overwrite data

Disadvantages of Optical Storage

• An unprotected disk is vulnerable to being scratched

• Low data capacity

Magnetic Storage

Round disks divided into tracks and sectors. The disk spins to allow a read-write head to access the data in a particular segment. Contains magnetic particles that are either magnetised (1) or not magnetised (0)

Advantages of Magnetic Storage

• Cheapest storage medium per magabyte

• Less cumbersome than multiple optical disks

Disadvantages of Magnetic Storage

• Slower access speeds than solid state

• The data is all on one device, which could be lost

Solid State Drives (SSDs)

Use electronic circuits to store data electronically, and can retain data without a power supply

Advantages of SSDs

• Faster acess speeds than both optical and magnetic

• Harder to damage because there are no moving parts

Disadvantages of SSDs

• Most expensive form of storage per megabyte

• Limited number of times each bit can be written to

Cloud storage

Involves storage on remote computers, managed by other organisations. When a file is saved or loaded , it is transmitted across the internet. Multiple backups exist across the world. Uses magnetic and SSD storage

Advantage of Cloud Storage

• Less likely to be damaged or misplaced

• Capacity on your local machine is freed, giving you more storage space

Disadvantages of Cloud Storage

• Often comes with a subscription fee

• Limited by your internet connection

Embedded System

A computer that exists within a larger mechanical or electrical device

Examples of embedded systems

• ATMs

• Printers

• Utility smart meters

• Petrol pumps

Examples of non-embedded systems

• Personal computers

• Laptops

• Tablets

• Smartphones

Low Level Language

• close to binary code

Machine Code

• low level language

• easily executed by computers

• each processor has unique code so not portable

• high risk of human error

Assembly Language

• low level language

• one to one machine code relationship

• direct manipulation of registers

• no compiler, converted by assembler to object code

Object Code

A program translated into machine code

High Level Language

• resembles human language

• one to many machine code relationship

• needs translation

• portable and easy to maintain

Imperative (procedural) language

• lists of instructions

• subroutines or procedures

• eg. C

Object Oriented Language

created objects that contain instructions, eg. C++

Declarative Language

describes what the program needs to accomplish, eg. SQL

Logic Programming

• facts and rules interrogate data

• commonly used in AI

• eg. Prolog

Functional Lanuage

treats procedures like mathematical functions, eg. Haskell

Interpreter

• reads source code statement and carries out immediately

• every iteration the statement is read again

• used will developing a program

• source code must be given to users

• computers must use the same interpreter

Compiler

• converts source code to object code

• unless changed, source code no longer needed

• code will run on computer with same platform

• can be translated