GCSE AQA Computer Science Paper 2

computer

machine that processes data

computer system

consists of hardware and software that work together to process data / complete tasks.

hardware

physical components of a computer e.g. (cpu, motherboard, monitor)

software

programs that a computer system runs e.g. (word processors, web browsers, email clients, games)

embedded systems

computer systems built into other devices

what are embedded systems often used as?

they are used as control systems. they monitor and control variables to achieve a desired results.

embedded system properties

• easy to design - cheap to produce - more efficient and doing their task than a general purpose PC

computer components

• power supply - case cooling fan - cpu heat sink and cooling fan - cpu - optical drive - ram sticks - motherboard - hard disk drive

CPU

Central processing unit

Arithmetic Logic Unit

• does all the calculations - performs logical operations and binary shifts - results are stored in registers

cache

• very fast memory in the CPU - slower than registers, but faster than RAM - stores regularly used data so CPU can access it quicker next time - cache have a very low capacity and are expensive

clock

• sends out a signal between 1 and 0 at a constant rate - signals are used to sync when instructions will be carried out

clock speed

number of clock cycles per second

buses

• collection of wires that are used to transmit data between components of CPU to other parts in the system

fetch instruction

• control unit reads the memory address of the next CPU instructions - instruction in the address is copied from memory to a register

decode instruction

• instruction that was copied is decoded by the control unit

execute instruction

• instruction is performed by the ALU

main feature of von neumann

• uses one memory for both data and instructions

volatile memory

• temporary memory - requires power to retain data

non-volatile memory

• permanent memory - keeps its contents with no power

CPU main parts

• Control Unit - Arithmetic Logic Unit - Cache - Clock - Buses

control unit

• overall control of the CPU - main job is to manage and follow the fetch-decode-execute cycle - controls the flow of data inside and outside the CPU

ROM

• read only memory- tells the CPU how to boot up - contains all the instructions a systems needs to boot up (BIOS)

boot up

• cpu reads the instructions from ROM - tells the CPU to perform self checks and set up the system

embedded system ram/rom

• have more ROM than RAM because they don't write much data to memory - don't have secondary storage so ROM is used - ROM and RAM are stored on the same chip to reduce cost

non-embedded systems ram/rom

• more RAM than ROM as they need to write more data to memory - ROM is typically used for the BIOS, which doesn't need much memory - ROM and RAM stored on motherboard, away from the CPU

cpu performance factors

• clock speed - CPU cores - cache size - cache type

cpu cores

• each core can process data independently of the rest - the more cores, the more instructions it can carry out at once

cache size

• is data storage inside the CPU that is much faster than RAM - larger cache gives CPU faster access to more data it needs to process

cache type

• different levels (L1, L2, L3) - higher the level, the more it can hold, but the slower it is - cache speed is based on how far it is from the CPU - L1 is quick as it's on the CPU, L3 is on motherboard so it's slower

GPUs

• graphical processing unit - they handle graphic and image processing - they relieve the processing load on the CPU - computers have basic GPUs installed, but they can be upgraded to improve performance

what happens if there's not enough ram?

• it may not be able to keep all app data loaded at once, slowing the system down

what happens if there's more ram?

• the more RAM, the more apps or more memory-intensive applications it can smoothly run, making it faster overall

what happens if there's more ram than needed?

• if a computer already has plenty of RAM to run everything, increasing RAM may make no difference to performance

tiers of storage

• primary: volatile (registers, cache, ROM, RAM) - secondary: non volatile (app, user files, OS)

magnetic hard disk characteristics

• high capacity - reliable storage

magnetic hard disks

• made up of stacked magnetised metal disks - data is stored magnetically in small areas called sectors - read/write heads are used to access sectors on the disk

solid state drives characteristics

• fast - reliable - light - quiet - expensive

solid state drive

• storage device with no moving parts - most use a type of flash memory

ssd characteristics 2

• faster read/write times than HDDs - gives much quicker times for booting up and opening programs and files

other types of flash storage

• USB pen drive and memory cards - much slower than SSDs and have shorter read/write life - used to expand the storage of small devices (cameras)

advantages of HDDs vs SSDs

• cheaper - both are high capacity, but HDDs are higher - HDDs have a longer read/write life (SSDs can only be written a certain number of times before they start to deteriorate)

advantages of SSDs vs HDDs

• faster - don't need defragmenting - more shock-proof - HDDs make noise, SSDs are silent

optical disks characteristics

• cheap - robust

optical disk storage capacity

• CDs can hold 700MB - DVD can hold 4.7GB - Blue ray can hold 25GB

optical disk types

• read-only (CD-ROM) - write-once (CD-R) - rewritable (CD-RW)

optical disk advantages

• cheap - portable - won't be damaged by water or shocks - easily scratched (disadvantage)

cloud storage

• service where files can be uploaded via the internet to a remove server

cloud storage pros

• can access files from any device - files can be shared with others - easy to increase storage - no need to buy expensive hardware to store data - no need to pay IT staff to manage data - cloud host provides security and backups for you - can be cheap/free if not much storage needed

cloud storage cons

• need connection to the internet - upload/download speed depends on bandwidth - depends on host for security and backups - stored data can be vulnerable to hackers - unclear who has ownership over cloud data - subscriptions fees can be expensive in the long term

main functions of an OS

• communication with I/O devices via drivers - platform for applications to run on, and User Interface - memory management - process management - file and disk management - system security and user accounts

device drivers

• operating systems use device drivers to communicate with I/O devices connected to the system

multi tasking

• OS provide a platform for apps to run on, manage system resources to allow systems to run multiple apps at once

memory management

• when an app is opened, OS copies the important parts to memory - OS manages how much RAM a program has access to - when running multiple apps at once, OS makes sure they don't interfere with each other

process management

• when apps are launched, they create multiple processes - each process has instructions that it needs the CPU to execute

scheduling

"- OS uses scheduling to determine the most efficient order to execute instructions - each process is allocated a ""priority"" by the OS - OS may interrupt the current process if a higher priority process is free - for multitasking, the CPU switches between different processes very rapidly"

file and disk management

• computers store data as files - OS manages this data (along with movement, editing and deletion)

utility software

• OS organises and maintains the hard disk with utility softwares like a defragmentation software

system security

• user account controll - different permissions for different users to store their own data and files - OS has anti theft measures to prevent other users from accesing locked devices

utilities

utility software helpts to maintain a computer

disk health

• disk healthy utilities scan the drive for problems and fix issues where possible

system cleanup

• system cleanup utilities go through and get rid of temporary files that take up space unnecessarily

compression

• reduces the size of files so they take up less disk space - it's used on the Internet to make files quicker to download

backup

• is a copy of a computer systems files and settings stored externally that can be recovered in an event of data loss

data loss reasons

can happen for many reasons: - fire - theft - flood - malware - hardware failure

encryption

• scrambles data to stop third parties from accessing it - data can be decrypted using a special key

virus scanners

• these inspect each file on your computer looking for viruses - they usually have a list of known viruses to check for (need to be updated constantly so they don't miss anything)

LAN

local area network

LAN properties

• covers an area locate on a single site - LANs can be wired or wireless

WAN properties

• wide area network- connects LANs in different locations - more expensive to setup than LANs - connected using telephone lines, satellite links or radio links - Internet is one big WAN

PAN

• personal area network - centred around a single user - use Bluetooth to connect devices - short range but strong signal - don't require any additional hardware, just the device itself

fibre optic cables

• transmit data as light - high performance - expensive cables - don't suffer interference - can transmit over long distances at high bandwidth without losing signal quality

CAT 5e and CAT 6

• common type of Ethernet cable - contain pairs of copper wires which are twisted to reduce internal interference - cheaper than fibre optic cables - decent bandwidth - commonly used in homes and LANs

coaxial cables

• made of a single copper wire surrounded by a plastic layer for insulation - has a metal mesh which provides shielding from outside interference - very cheap - low bandwidth

network interface card (NIC)

• piece of hardware inside a device that allows it to connect to networks - exist for both wired and wireless connections

switches

• used to connect devices on LANs

router

• routers transmit data between different networks - used to connect to the internet

networking computers benefits

• sharing files is easier - you can share the same hardware - you can install and update software on all computers at once - you can communicate across a network cheaply and easily - user accounts can be stored centrally, so users can log in from any device

networking computers drawbacks

• can be expensive to setup (need a lot of extra hardware) - can be vulnerable to hacking - malware can easily spread between networked computers - some networks are dependent on one or more servers - large networks are difficult to manage and may need special maintaining

wireless network pros

• they are convenient as device automatically connects to the network and you can move around with it - cheaper and better for the environment as no wires - easy to add more users (no extra setup or wires)

wireless network cons

• generally less secure than wired networks - access points are visible to all devices, which can allow hackers to gain access - distance from the WAP, interference from other networks and physical obstructions can reduce signal strength - this means there's a limit to how far the network can reach - lower bandwidth and are less reliable than wired networks

topology

• the layout of the network

types of topologys

• star and bus

star topology

• all devices are connected to a central switch or server that controls the network - allows many devices to access the server simulatenously - may be wired or wireless

star topology pros

• if a device fails or cable disconnects, rest of network is unaffected - simple to add more devices to the network by using a separate cable - tend to have better performance than other setups as all devices transmit data at the same time - fewer data collisions on a star network

star topology cons

• in wired networks, every device needs a cable connected to the central switch - can be expensive - switch itself is an expensive piece of hardware - if there is a problem with the switch, whole network is affected - max number of connections is determined by the switch, might need to buy a new one if you want more connections

bus topology

• uses a single cable, called a bus to connect all the devices

bus topology: terminator

• two terminators are placed at the ends of the bus to stop data reflecting back along the bus - without the terminators, reflected signals would cause interference and make the network unusable

bus topology pros

• network is unaffected if a device fails - aren't dependent on a central switch to keep the whole network running - cheap to setup - total length of wires is much less, and the hardare is cheaper and to maintain

bus topology cons

• data collisions are common on a bus network - when a collision occurs, data must be resent, which slows the network down - more devices you add, the more likely a data collision will occur - this make it unsuitable for large networks - to avoid data collisions, devices must wait for the bus to be available before they send any data, which can slow the network down - if the bus cable gets broken, it splits the network into separate parts - the separated networks don't have terminators at both ends of the bus, so there will be a lot of reflected signals which can shut down the entire network

protocol

• set of rules for how devices communicate and how data is transmitted across a network

what do protocols cover?

• protocols cover how communication between two devices should start and end - how the data should be organised - what the devices should do if the data goes missing

packets

• data sent between networks is split into equal-sized packets - each packet contains extra information (destination and source addresses) and a checksum (used to find errors)

network protocol layers

• a layer is a group of protocols which have similar functions - a layer is self-contained, protocols in each layer do their job without info of what's happening in the other layers - each layer servers the layer above it - it does the hidden work needed for an action on the layer above

application layer (layer 4)

• providing networking services to applications - HTTP, FTP, SMTP

transport layer (layer 3)

• setting up communications between 2 devices, splitting data into packets and checking if they are sent and delivered - TCP, UDP

internet layer (layer 2)

• adding IP addresses to data packets, directing them between devices and handling traffic - used by routers - IP

link layer (layer 1)

• passing data over the physical network - responsible for how data is sent as electrical signals over cables, wireless and other hardware (NICs) - and for interpreting signals using device drivers - e.g. Wi-Fi, Ethernet

pros of using layers

• it breaks network communication into manageable pieces - this helps developors concentrate on only 1 area of the network - they can be changed without affecting the other layers - forces companies to make compatible, universal hardware and software - so each device will always work in the same way

application layer protocols

• HTTP - HTTPS - FTP - IMAP - SMTP