Computer Science - J277

Fetch-Decode-Execute

fetch the next instruction, decode the instruction, execute the instruction.

control unit

controls execution
decodes
regulates and controls system clock
sends and recieves contol signals

ALU

logical / arithmetic / shift operations

special purpose registers

special very fast memory locations within the CPU with specific purposes in the process of executing instructions.

MAR

holds address of data to be fetched

MDR

temporarily holds data that is being transferred to or from memory

PC

hold address of next data

ACC

holds results of operations from ALU

factors affecting CPU performance

clock speed
number of cores
cache size

cache memory

type of memory used to temporarily store frequently used data or programs for quick access; similar to RAM but faster.

embedded system

- small computer system with a specific purpose that is built into a larger device

- reliability / minimal resources

virtual memory

space on a hard disk or other storage device that simulates random access memory when it is full

RAM

volatile
stores programs currently being used
memory can be written to or read from

ROM

non-volatile
store BIOS
can only be read from

features for choosing secondary storage

capacity
speed
portability
durability
reliability
cost

magnetic devices

magnetic disks are read and written to with a moving head inside the disk drive
they often contain moving parts and are susceptible to damage
magnetic devices can be either internal or portable

SSD

more reliable
lighter
faster
lower power consumption
thin - no moving parts

optical devices

lasers write data to disc and read data from it

images

(image width x image height x colour depth) / 8

how is an analogue signal converted

- analogue recieved by microphone
- convert to electrical analogue signal
- signal amplitude measured at regular intervals
- values round to a level
- values stored as series of binary value

what is sample rate

The number of audio samples captured every second

what is sample resolution

The number of bits used to store each sample

what is the sample size

The number of seconds over which the sample was taken

calculate sample size

( sample rate x sample resolution x seconds ) / 8

What is an IP address?

a sequence of numbers that uniquely identifies each computer or device connected to the Internet

how is DNS used

- URL sent to DNS where it is translated
- if IP address is found it is transferred back to user
- if IP address not found it is passed to next DNS before returning error message

what does each packet contain

- IP address its going to
- IP address its come from
- sequence number of packet
- number of packets in whole communication
- error checking data

advantages and disadvantages of star topologies

adv :
- if one cable fails rest of devices still work
- consistent performance
disadv :
- costly
- if server fails then whole network fails

What is symmetric encryption?

An encryption method in which the same key is used to encrypt and decrypt a message

what is asymmetric encryption

uses 2 different keys - public + private

what is a wireless access point

This is the point which a wireless enabled device connects to a network

what is a router

A piece of hardware that forwards packets across a WAN

what is a switch

component of LAN that learns MAC of each individual device connected to it
- froward inbound packets to intended recipient

what is a NIC

a communications device that enables a computer or device that does not have built-in networking capability to access a network

describe ethernet

family of protocols which describe how devices should format data ready for transmission.

describe wifi

family of protocols that make wireless networking run smoothly

describe Bluetooth

It has a short range radio frequency and is used for personalised networks

what is meant by hosting

company that stores files for you and make them available for you

What is cloud computing?

where processing and storage are performed elsewhere over a network connection rather than locally

factors affecting network performance

bandwidth -
number of devices connected - as devices increase bandwidth get used up
latency - time delay between first transmission of first byte and when its received
errors in transmission - due to noise or distortion
interference - any electromagnetic field can weaken wifi connection eg. micro waves / phones

protocols - TCP / IP

TCP - defines how packets are broken up and reassembled
IP - identifies location of devices + packets

protocols - HTTP / HTTPS

HTTP - access and receive web pages in form of HTML
HTTPS - encrypts info so it cant be understood if hacked

protocols - FTP

used when transferring computer files between client and server on a network

protocols - POP

receives and holds email until individual picks it up
- sent email is stored locally
- works best with one computer

protocols - IMAP

stores email messages on server
- different devices

protocols - SMTP

send emails between servers
- necessary if two different email service providers

describe layers

- break up the sending of messages into separate components and activities
- easily adapted by new suppliers
- application layer , transport layer, internet layer , link layer

application layer

Encodes the data being sent so that it will be understandable by the recipient.

transport layer

splits data into packets and adds packet information

internet layer

responsible for addressing, routing, and packaging data packets

link layer

attatches MAC address of sender and recipient

how are computers used in the modern world

communication
employment
shopping
manufacturing

Recent developments in software

DeepMind AlphaGo program
- split itself in half and played millions of matches against itself
- learnt from each victory and loss

environmental impact of computers

- carbon footprint
- waste products
- means people drive less

who is a stakeholder / key stakeholder

- anyone affected by the company's actions or policies
- owner of company / community / supplier

data protection act 2018

key principals when holding personal data
- fair / lawful / transparent process
- purpose limitation
- data minimisation
- accuarcy
- data security

computer misuse act 1990

prevent unauthorised access / hacking
- unauthorised access to computer material
- unauthorised modification

copyright design and patents act 1998

protect creators from having their work illegally copied

what is abstraction?

- removing unnecessary details from a problem in order to solve it

what is decomposition?

- breaking down a complex problem into smaller problems and solving each one individually

linear search

start at beginning of unsorted list and stop when number found

binary search

sorted list keeps halving until number is found

bubble sort

compare adjacent element and if in wrong order then swap

insertion sort

one data item placed in correct location at a time

merge sort

continuously divide list in half until each sub-list is one then each sub-list sorted

what is casting

changing one variable type to another

what is concatenation?

the joining of two or more strings

to get length of string

stringname.length

to get a substring

stringname.substring(startingposition, numberofcharacters)

to convert cases

stringname.upper
stringname.lower

procedure vs functions

procedure doesnt return a value however a function does

different types of validation

range / presence / format / length / type

ways to make program understandable

leave white space between codes
add comments
use indentation
use subprograms to make it modular
sensible names for variables

what is a syntax error

grammatical mistake in a program

what data should test tables include

normal - typical data designed for program
boundary - both ends of range and data just outside this
invalid - outside limits of valid data
erroneous - different data type

what is a logic error

when program doesn't run the same way as expected