A-Level Computer Science Flashcards
What is the SDLC?
A project management model that allows the project to be on time and in budget
What does SDLC stand for?
System Development Life Cycle
What does the SDLC do?
Defines the stages involved in bringing a project from start to end
How many stages are on the SDLC?
7
How is each stage planned and carried out?
They are planned and carried out in a time frame, depending on the size of a problem
What happens if the software/hardware is out of date?
If the software/hardware is out of date, the system analyst may create an entire new system
What are the 7 stages of the SDLC?
- Feasibility Study
- Analysis
- Design
- Create
- Test
- Changeover
- Maintenence
What happens in the Feasibility Study stage of the SDLC?
Understand the problem the client has given you
What happens at the Analysis stage of the SDLC?
Analyse the problem given to you, breaking it down
What happens at the Design stage of the SDLC?
Produce designs for the problem
What happens at the Create stage of the SDLC?
Programming the design
What happens at the Test stage of the SDLC?
Make sure the code is bug free
What happens at the Changeover stage of the SDLC?
The code is installed into the new system
What happens at the Maintenence stage of the SDLC?
Maintaining the code making sure there are no errors
What are methodologies/system approaches?
Methods to ensure the project is on time and in budget
What are the 2 main methodologies?
Agile, Waterfall
What are some similarities and differences between the 2 methodologies?
Both follow the same 7 stages of the SDLC, however they have different approaches
What are some features of the Waterfall methodologies?
Fixed time and budget- no flexibility
Problems can be difficult and timely
Previous stage MUST be completed before going to next
Large amounts of documentation
Rigid
Needs to be clear on its responsibility
Suited for small projects
What are some features of the Agile methodologies?
Adaptable, focuses on customer’s needs
Several variations of Agile (Kanban, Scrum, Extreme Programming/XP)
Allows for feedback
Flexible
Suited for indecisive client
Can go between stages- does NOT have to be in order
Looping back may take time
Rapid method
Lack of emphasis on necessary designing and feedback
What are some features of the Feasibility Study?
Takes months
Has to decide if current hardware is good to use or should start from scratch
Has to decide what type of project it is
What is the acronym for the aspects Feasibility Study?
S- Schedule Feasibility
T- Technical Feasibility
O-Operational Feasibility
L-Legal Feasibility
E-Economic Feasibility
What would be considered under Schedule Feasibility?
Can it be done in a reasonable time?
What should be considered under Technical Feasibility?
Use current hardware or start from scratch?
What should be considered under Operational Feasibility?
Will there be extra work? Is anyone going to need to be trained?
What should be considered under Legal Feasibility?
Will there be any legal issues?
What should be considered under Economic Feasibility?
How much will it cost? Is it worth spending the money?
What is the Analysis stage of the SDLC?
Finding out exactly what needs to be done to solve the problem using various fact finding methods
What are the 4 main fact finding methods?
Observation
Questionnaire
Document Collecting
Interview
What is Observation?
The process of gaining data by observing events and behaviour in a systematic manner
Who is observed during an Observation study?
The End Users
What are some advantages and disadvantages of Observation?
Advantages: Accurate, used on End Users
Disadvantages: Time consuming, behaviour may change
What is Questionnaire?
Creating valid and reliable questions that relate to the assessment objectives
What are some advantages and disadvantages of Questionnaire?
Advantages: Cheap, accurate (used on End Users)
Disadvantages: May lack responses, may be time consuming
What is Document Collecting?
Collecting a set of documents that specify what the client needs
What are some advantages and disadvantages of Document Collecting?
Advantages: Easily shared with others working on the project, low cost
Disadvantages: May contain errors, might be time consuming to go through
What is Interview?
A one-on-one with the client to specify what they need
What are some advantages and disadvantages of Interviews?
Advantages: Personalised, allows the client to be specific
Disadvantages: Time consuming, expensive
What is Decomposition?
Breaks down a large problem into several smaller problems that are easier to tackle and work with
What is Abstarction?
Removes all specific details and only keeps the facts needed. Creates a general idea of what the problem is and how to solve it
How might a System Analyst represent data?
A Data Flow Diagram (DFD)
What does a D(ata) F(low) D(iagram) show?
The input and output of data, what processes were carried out and what data was sorted
What does this symbol represent?:
—————->
Flow of Data
What does this symbol represent?:
|
———————–
Storage
What does the phone symbol represent?
|
Process
What does this represent?:
---------- | | ---------- | | ----------
External Entities
What are the 2 programming languages?
Low level languages, High level languages
What is a Low level language?
Low level languages have certain code words instead of long Binary numbers. Converted into machine code
What is Machine Code?
The language the Computer understands
What is a High level language?
Coded in a way so humans can understand it. Close to the human language. Also converts into machine code
Which converts faster, and why?: Low level languages, High level languages
Low level languages convert faster than high level languages because Low level languages converts into High level languages
What is a Procedural Language?
Structured in sequence and order. It is an example of a Programming paradigm.
What are some features of Procedural Languages
Uses step-by-step instructions
Uses functions and procedures
Obeys instructions
Carries out actions/calculations
Programmer defines precisely each step when performing a task
Very versatile (covers a lot of the basic programming needs)
Many programmers learn to code with Procedural Languages
What is a Programming Paradigm?
A fundamental approach and structure of a programming language. Can be thought of as different categories of High level languages
What is the Programming Stage?
The Programmer replicates the design so you can have it on a computer
What’s the 5th stage of the SDLC?
Testing Stage
What is the Testing Stage?
The System Analyst ensures the system is useable and has no bugs before the program is installed
Why is the Testing Stage important?
To make sure the program is bug free
Why is a program being bug free important?
If there are many bugs, the system can keep crashing
Can be harmful to company, may lose customers
What are the 3 types of testing?
Alpha, Beta, User Acceptance
What is Alpha testing?
Still at development stage and is tested IN HOUSE
Changes may still be made to the system
Large companies use seperate software testing engineers
What is Beta testing?
Software is very close to completion
Given to users who have a general interest in the product
Used in ways software engineers don’t consider
What is User Acceptance testing?
When users buy the final product/staff use the new system
Tested in the ‘Real World’
Can still be updated to fix bugs or fix any weakpoints in the system
What is Continuity of Service?
Service offered by businesses has to be constantly offered. Can’t have the service drop
What are the 4 types of changeover?
Direct changeover
Pilot changeover
Phased changeover
Parallel changeover
What is Direct changeover?
Turn off the old system, turn on the new system
What are some advantages of Direct changeover?
New system is available immediately
Easiest method of changeover
Cheapest method of changeover
Doesn’t need multiple sets of data
What are some disadvantages of Direct changeover?
Riskiest method (if new system fails there is no other system to fall back on)
All data has to be moved to new system for old one to shut off
Period of downtime is created
Upheaval for staff who haven’t used the new system
What does ‘upheaval’ mean?
Making you use something you aren’t familiar with
What is Pilot changeover?
Some branches try the new system while the other branches keep the old system
What are some advantages of Pilot changeover?
Only a small part of the system is affected
Problems in the new system can be identified without affecting the whole company
Staff from the pilot branches can train other staff who haven’t used the new system
What are some disadvantages of Pilot changeover?
Chosen branches experience same disadvantages as other branches for a ‘direct changeover’
Staff on new system might find it hard to share data with staff on old system
Extra work for IT, have to manage 2 systems
What is Phased changeover?
The new system gets integrated with the old system, and the old system is slowly replaced by the new system
What are some advantages of Phased changeover?
Less risks of the system going wrong
Only effects one specific part
Staff are introduced to system in small stages
What are some disadvantages of Phased changeover?
Can take a long period of time
Users asking for changes holds up next phase of installation
Difficult to integrate old and new system
What is Parallel changeover?
Old and new system run at the same time
Once the new system is okay, it can be changed
If new system fails, old one is there
What are some advantages of Parallel changeover?
Less risk than over methods- if the old system fails, the old one is still available
Less stress for staff, still have security of old system
Staff can take their time learning the new system
What are some disadvantages of Parallel changeover?
Time consuming- data has to be entered on 2 systems
One system becomes out of sync with the other
Maintaining multiple sets of data can lead to errors
Extra cost of running and maintaining 2 systems
What is Documentation?
Documents that are produced throughout the stages of the SDLC
Includes design documents, diagrams, requirement specifications and reports
Who are the documents in the interest of?
All documents are of interest to the development team
When and why are documents important?
Important in the development cycle
Important for customers and end users
What is a user guide?
Guides the user on how to use the system
Used as training
What are some features of a user guide?
Instillation guide
Glossary
Material must be suited for novices
Frequently Asked Questions (FAQs) that help with common problems (trouble shooting)
Reference manual
What is a Technical guide?
Designed to help IT literate users maintain and potentially improve the system
What are some features of a Technical guide?
Algorithmic designs (pseudocode, flowcharts)
Annotated code listings (that abide by coding standards set by the company)
Variable lists
Data dictionary
Configuration guide and options
What is maintenance?
Monitoring the system to make sure there are no bugs
What are some features of maintenance?
Rarely free
Expensive
Needs monitoring
Have to pay someone with experience
What are the 3 types of maintenance?
Corrective maintenance
Adaptive maintenance
Perfective maintenance
What is Corrective maintenance?
Finding bugs in the system
What is Adaptive maintenance?
Making changes to the system due to external influence
What is perfective maintenance?
Minor improvements to systems that have been around for a while
What is the name for planning for data loss?
Disaster Planning
How can data be destroyed?
Fire
Theft
Disk failure
Hacking
What is a backup?
Keeping a copy of software and data so they can be recovered should there be a loss
Why is regular backup crucial for organisations?
The data they have is more valuable than anything else
What are periodic backups?
Backing up data on a regular basis, can differ from person to person/business to business
Name the 3 main backup methods
SSD
The Cloud
USB
What are SSD’s?
A storage medium that uses interconnected flash memories to store data without power
What are some advantages of SSD’s?
Fast read-write time
Energy efficient
Compact
What are some disadvantages of SSD’s?
Expensive
Less available
Read-write cycle concerns
What is The Cloud?
Storage medium that can be accessed by anyone with Wi-Fi
What are some advantages of The Cloud?
High speed
Reliable
Mobile
What are some disadvantages of The Cloud?
Security threats
Downtime
Lower Bandwidth
What is a USB?
Small storage medium that can be carried with you
What are some advantages of a USB?
Portable
Cheap
Secure
What are some disadvantages of a USB?
Risk of malware
Not durable
Used to steal information
What should every company have, in terms of backups?
Every company should have a Backup Policy
What is a backup?
The process of copying files from main storage to a seperate area
Why should the backup files always be tested?
So the backup data is ready for use should it be needed for use
Why are backups important?
If a file gets deleted, the backup file can be collected for use
What should the backup policy consider?
Where will the backup be stored?
What will the backup be stored on?
How often will the backup occur?
How long will the backup be kept?
What should be considered when planning a backup procedure?
Content and type (What will be backed up?)
Timing (When will the best time for a backup be?)
Frequency (How often will a backup occur?)
Backup media (Which media should be used for the backup?)
Location (Where will the backup be kept?)
Responsibility (Whose responsibility will it be to ensure the backups are kept?)
What is a full back up?
Copying all the files on the system to a seperate area
What type of business would use a full backup?
Small businesses that don’t have a lot of data
What are some advantages of full backups?
Ultimate protection
System can be restored at any time
Most efficient system (If files haven’t changed, the same files will be getting copied)
What are some disadvantages of full backups?
As business grows, the time to back up all the data will grow
If the backups get stolen, the thief has access to all the company’s data
What is a differential backup?
Only backing up the files that have been changed since the last full backup
How does a differential backup compare to a full backup?
Faster than full backups
Data getting backed up grows until next full backup
More flexible, but unwieldly to do more than once a day
What is incremental backup?
Only backing up data that has been changed since the last incremental backup
What is recovery?
Taking the backup and copying the data to wherever the data is needed
What is an algorithm?
A list of instructions carried out in order
What is syntax?
The laws of the language that allow everything to run
What is a Syntax Error?
Breaking the laws of the language
What are some contemporary methods of HCI?
Voice inputs
Touch screen
Force feedback
Virtual reality
Augmented reality
How is interaction between humans and computers evolving?
Moving away from interfaces like keyboards and mice, to more tactile and intuitive methods
What is voice input?
Using the human voice to control computing systems
What is an advantage of voice input?
Useful for those who can’t physically type
What is touch screen?
Allows for gestures- such as swipes- to interact with computing systems
What is force/haptic feedback?
Physical sense that tells you an action has occurred through vibrations
What is virtual reality?
User is transported to a different world using technology
What is one effect of virtual reality?
Takes over the user’s senses
What is augmented reality?
Lays digital information over the real world
What is needed to use AR (augmented reality)?
Special lense/phone camera
What does HCI stand for?
Human Computer Interface
What is Design Validation?
A check done after the new system has been designed to ensure the design meets original specifications set during the analysis stage
Why is Design Validation important?
It makes sure there are no errors within the design, and makes sure any errors that are found within the design are rectified
What are some aspects that are checked within Design Validation?
Checks the design has met the objectives
Checks the design corresponds with the specification set during the analysis stage
Checks the most appropriate techniques have been used
Checks the user interface is appropriate for the audience
What is Design Evaluation?
When the development company and the client evaluate if the project has been successful
How does design evaluation effect the developers?
If the design evaluation is unsuccessful for them, then they might not get paid
What are the 5 key factors that are checked during design evaluation?
Requirements- has the project met the clear expectations set by the client?
Performance- is the performance optimal?
Robustness- is the server likely to crash/have bugs?
Cost- has the project ended in budget? (Little bit of tolerance offered)
Usability- how user friendly is the server?
What makes up the processor?
Registers, the CU and the ALU
What makes up the processor?
Registers, the CU and the ALU
What are the 2 tasks of components in a computer system?
Adding
Storing
What are the functions of the processors?
The fetch, decode, execute cycle
What is a register?
A temporary data store that holds the current instruction to be executed having been fetched from memory
What is the ALU?
The process and manipulation of data. Normally consists of arithmetic operations or logical comparisons that allows a program to make a decision
What is the CU?
Fetches each instruction in sequence, decodes it and then synchronises it before executing it. Sends control signals to other parts of the computer
What is the Von Neumann Bottleneck?
Limitation between the CPU and RAM is lower than the speed the CPU can process data
What is the Von Neumann Architecture?
When the data and the software are stored in RAM. The task being carried out by the computer can be changed by loading a different piece of memory
What has been developed against the Von Neumann Bottleneck?
Cache memory- temporary data store that is very close to CPU
Multithreading- allows multiple threads to be processed by the CPU simultaneously
Parallel processing- using multiple processors to handle different tasks at the same time
When is the Bottleneck created?
When the instructions are fetched at the same time as data operation on the same memory and bus system
What is the first step in the Fetch-Decode-Execute cycle?
The PC displays the next address in RAM of the next instruction to be processed
This value is copied into the MAR
What is the second step in the Fetch-Decode-Execute cycle?
The PC Register increases by 1
This prepares the CPU for the next instruction to be fetched
What is the third step in the Fetch-Decode-Execute cycle?
The CPU checks the address in RAM which matches the address held in the MAR
What is the fourth step in the Fetch-Decode-Execute cycle?
The instruction in RAM is transferred to the MDR
What is the fifth step in the Fetch-Decode-Execute cycle?
The instruction in the MDR is copied into the CIR
What is the sixth step in the Fetch-Decode-Execute cycle?
The instruction in the CIR is decoded and executed
Any result of the execution is stored in the ACC
What is Disk Fragmentation?
Files are split up and stored on different parts of the disc
What are some of the effects of Disk Fragmentation?
Increases disk head movement which disk access speed
What is Serial Computing?
Problem broken down into a series of instructions
Instructions are executed sequentially
Executed on a single core (one at a time)
What is parallel computing?
Problem broken down into a series of instructions that can be solved concurrently (at the same time)
Each part is broken down further into a series of instructions
Instruction are executed simultaneously on different processors
What are some limits of Parallel Processing?
Latency, Bandwidth
What is latency?
The amount of time it takes for data to travel from a device to the server, and back again
What is latency measured in?
Milliseconds (ms)
What is Bandwidth?
The capacity at which a network can transmit data
What is Clock Speed?
a measure of how many clock cycles a CPU can perform
What is Clock Speed measured in?
Gigahertz (GHz)
What is the Data Bus?
Any data from any component travels down the data bus
For example, data from RAM travels down the data bus to a register
What is the Address Bus?
Picks up the address (location) of data stored in RAM
Once an address is located, the data will travel down the data bus to the register
What is the Control Bus?
Sends and receives signals from all parts from all parts of the computer which makes sure all processes take place at the right time in the right order
What is an input device?
A device that gives instructions to the computer
What is an output device?
A device that carries out the instructions given to it
What is a hybrid device?
A device that is both input and output
Name some examples of input devices
Joystick, microphone, controller
Name some examples of output devices
Monitor, speaker, printer
Name some examples of hybrid devices
Mobile phone, laptop, iPad
What is Optical Character Recognition?
Converts an image or text into a format computers can understand
The computer saves the scan as an image file
What is Optical Mark Recognition?
Recognises marks made on certain paper documents in a certain colour
Used for school examples, etc.
What does OCR stand for?
Optical Character Recognition
What does OMR stand for?
Optical Mark Recognition
What is Magnetic Ink Character Recognition?
Prints characters on a document using a special ink that contains magnetic particles
Used in the banking industry
What does MICR stand for?
Magnetic Ink Character Recognition
What is Voiceprint Recognition?
Recognising the voicewaves in your voice so the software knows it’s you
What is Voice Dictation?
When what you say gets typed up
How is voiceprint recognition useful, in terms of data protection?
It will only understand a certain voice- the voice won’t be able to be faked
How are most computers connected?
Through wired/wireless links
What is a computer network?
Interconnected computers that have the ability to communicate with one another
What does a network ned to work?
Switch/router and an ethernet cable
How do computers transmit data?
Through Wi-Fi
What is an NIC?
Allows data packets to travel between computers and network
Each NIC has its own unique 48 bit code called a MAC address
What is a MAC address?
a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment
What is a Router?
Forwards data when it arrives from one network to another
What steps happen when data arrives at a router?
Reads packet’s destination address
Looks at all the paths to the address
Checks which path will be quickest
Sends packet down fastest path
What is a HUB?
Connects multiple computers together
Allows them to share data packets
How do computers communicate together?
Using a Protocol Suite (TCP/IP protocol)
How many layers is the Protocol Suite made up of?
4
What is the role of the layers within the Protocol Suite?
Each layer depicts what task is carried out by which protocol
Each layer has more than one protocol
What is the Application layer?
Allows the user to create messages that can be sent across the internet using a number of protocols
Any application will need the data to be formatted
What is the Transport layer?
Controls the flow of data across the internet
Breaks down messages into packets and labels them ready to be sent to other computers
Checks the packets have arrived safely
What is the Internet layer?
Delivers messages based on the address
Routes data between networks
Ensures relevant information is added to each packet (Destination Address, Source Address, Packet Number etc)
What is the Network Interface layer?
Physical connection that allows packets to be sent from one network interface to another
Determines how data is physically transmitted along a network
What protocols fall into the Application layer?
HTTP
FTP
POP
IMAP
SMTP
SNMP
What protocols fall into the Transport layer?
ICP
UDP
What protocols fall into the Internet layer?
IP
ICMP
What protocols fall into the Network Interface layer?
MAC
Ethernet
What is TCP (Transmission Control Protocol)?
Establishes connections between devices on a network, guarantees packets reach their destination
What is UPD (User Data Protocol)?
Sets up networks like a TCP, however doesn’t guarantee the arrival of the packet
“Fire and forget”
Faster than a TCP
What is IP (Internet Protocol)?
Determines fields of information to be sent in packets (source address, destination address, packet number)
What is SMTP (Simple Mail Transfer Protocol)?
Mail server which stores sent mail until the user is online
What is POP3 (Post Office Protocol)?
Allows users to receive mail from the mail server
Used to download the email from the mail server
What is FTP (File Transfer Protocol)?
Controls how files are sent over the network
What is HTTP (Hyper Text transfer Protocol)?
Determines how the files that make up webpages are exchanged
What is SNMP (Simple Network Management Protocol)?
Allows certain administrative devices to see information about the devices connected to a network
What is ICMP (Internet Control Message Protocol)?
Handles transmission errors, alerts devices to any errors that have occurred and specifies what the issue was
What is a Network Standard?
A set of rules that enable hardware to be compatible with each other and communicate when sending and receiving data, using different hardware/software brands
What would happen without network standards?
Technology developed by different companies wouldn’t work with each other, unless everyone used the same brand
We wouldn’t be able to send/view different formats (sending a PDF to someone without a PDF reader)
What do protocols allow?
Protocols allow different devices to communicate with each other (computer to printer)
What is the Operating System?
Interacts with the users and applications, including the Kernel
What is the Kernel?
Manages system resources like the CPU, memory, I/O devices
Interacts with the hardware
Links the user, applications and hardware
What are the roles of the Operating System?
User interface- provides a general purpose interface to interact with applications
Security- deals with user access rights, typically through a password, device becomes a Multi-User OS
Backing store (secondary software)- The storage of the data on the hard disk
Utility software- Has other utility software involved, helps maintain software, increases life expectancy. File managers, firewalls, network managers, task managers ,etc.
What are the roles of the Kernel?
Peripheral management- oversees system devices which include I/O devices and storage devices. Ensures data exchanges between devices and applications, handling information flow
Memory management (RAM)- Kernel allocates and deallocates memory for processes by keeping track of the sections of memory. Stores active processes in memory, releases memory when processing ends
Process management (CPU)- Kernel allocates time to each process. After a process completes its execution, the Kernel starts the next one and assesses it’s state (ready, running, blocked)
Interrupt handling (CPU instructions)- Kernel manages task priorities, allowing high-priority tasks to take precedence. Handles system calls (essentially software interrupts)
What is the Ready State?
A state in which a process is ready and waiting for its execution
What is the Blocked State?
A state in which a process doesn’t execute unless a process event occurs (completion of an input/output operation)
What is the Running State?
A state in which a process is currently executing
What is Time Slicing?
Allowing each process a short interval of processing time on the CPU
What is Paging?
Enables processes to access more memory than is physically available
What is Segmentation?
Organises and allocates memory for processes in flexible and module manner
What are Interruptions?
Stops a program running. Can be a computer stopping a program (closing a window) or a virus
What is a High Level Interrupt?
When interrupting a program can damage the software
What is a Low Level Interrupt?
When interrupting a program has no effect on the software
What is Data Transmission?
The way in which data can be transferred between devices within a computer network and between devices connected to it
What are the two ways data can be sent?
Parallel, Serial
What is Serial Transmission?
Data is split up into packets and sent one by one across a single wire
What are some features of Serial Transmission?
Used over long distances (e.g. computer and printer), slower than parallel
What is Parallel Transmission?
Data is sent over multiple wires, bit by bit, that are parallel to each other
What are some features of Parallel Transmission?
Data is sent over small distances (e.g. within a computer system), faster than parallel, has the skew effect
Suitable for scenarios which need a high transfer rate
Needs more advanced hardware and cabling
What is the skew effect?
Time distance between multiple pieces of data being transferred. Millisecond difference, not noticed by humans
What are some advantages of serial transmission?
Low power consumption
Low cost
Simple hardware and cabling
Suitable for long-distance communication
What are some disadvantages of serial transmission?
Not suitable for large amounts of data
Slow data transfer rate
Limited bandwidth
What are some advantages of parallel transmission?
High transfer rate
Suitable for large amounts of data
What are some disadvantages of parallel transmission?
Higher power consumption
Higher cost
Limited maximum cable length
Complex hardware and cabling
What is multiplexing?
Managing various signals between devices
What are the three common methods of multiplexing?
Simplex, half-duplex, full-duplex
What is simplex?
Data can be sent in one direction and one direction only, e.g. a speaker
What is half-duplex?
Data can be sent in both directions but only one at a time, e.g. a walkie talkie
What is full-duplex?
Data can be sent in both directions at the same time, e.g. a mobile phone
What is switching?
The process of transferring data from one device to another
What are 2 common methods of switching?
Packet switching, circuit switching
What is packet switching?
Breaking down data into packets, sending them across a network and then reassembling them at their destination address
What is circuit switching?
A dedicated communication path between two devices for communication
What are the 3 parts to a TCP packet?
Trailer, payload, header
What is the purpose of the Trailer?
Informs recipient’s device that the packet has been received
What is in the Payload?
The data being transferred
What is in the Header?
Source Address (allows tracing of sender)
Checksum (calculation that checks for errors)
Packet number (allows packet to be reassembled correctly at end)
Packet address (allows packet to be routed to destinsation)
What is a network collision?
Two devices on the same network attempting to send data at the same time
What happens to packets after they collide?
They get discarded
What are Boolean Laws?
Boolean identities (laws) are the rules that are used to simplify Boolean expressions
What does each Boolean Law have?
Each Boolean law has an AND form and an OR form, depending on what is being used
What is the Commutative Law?
Law that switches order of expression
A.B = B.A
A+B=B+A
What is the Idempotent Law?
Law that removes repetition
A+A=A
A.A+A
What is the complement Law?
Law that inverts something equal to the term (cancelled if there are two NOT’s over a term)
A.¬A=0
A+¬A=1
What is the Identity Law?
Law that simplifies statements down to A, whether a statement is AND’d with a 1 or OR’d with a 0
A.1=A
A+0=A
What is the Annulment Law?
Law that simplifies statements down to a 0, whether it is AND’d with a 0 or OR’d with a 1
A.0=0
A+1=1
What is the Absorption Law?
Law that reduces a bracket to 1 value
A.(A+B)=A
A+(A.B)=A
What is the Association Law?
Law that separates brackets that use the same operator inside and outside the bracket
A.(B.C)=A.B.C
A+(B+C)=A+B+C
What is the Distribution Law?
La that uses the value outside the bracket and multiplies with bow values inside the brackets, forming two new brackets which are linked by the operator formally inside the bracket
A+(B.C)=(A+B).(A+C)
A.(B+C)=(A.B)+(A.C)
What is De Morgan’s Law?
“Split the line, change the sign”
¬(A.B)=¬A+¬B
¬(A+B)=¬A.¬B
What is data stored in?
Files
What happens when a file is stored on a hard-disk?
It is given a name and the OS keeps a record of the address on disk using a File Allocation Table in order to locate the stored file
Why is data within files organised?
Organised so the OS can find these files easier and not abuse the hard disk
What happens every time we create, modify, move or delete a file?
The file info is recorded by the OS into a File Allocation Table (FAT)
What is a FAT?
A method of keeping track of the contents of a hard drive, used by Microsoft in early OS’. Still used today in most embedded systems
What is on the FAT?
Chart of numbers that corresponds to cluster addresses on the hard drive. if a file is large/dispersed across different tracks on a hard disc, these are also recorded
How is data stored on a hard disc?
Stored in ‘sectors’, and these sectors are stored in ‘clusters’
What does each cluster get?
Each cluster gets a number. This number is also stored when data ges saved
What are some advantages of clusters?
Minimises fragmentation, allocates clusters that are contagious allowing for efficient use of disk space
What are some disadvantages of clusters?
Constant altering of files leads to empty clusters, leading to fragmentation
What is a field?
Single data item that can be grouped together or split
What is a record?
Collection of related fields (information about someone)
What is a Fixed Length Record?
Records with the same number of bytes and same number of fields
What are some advantages of Fixed Length Records?
Easier to program
Easy to manage
What are some disadvantages of Fixed Length Records?
May require padding (any blanks are padded out with an empty space)
What is a Variable Length Record?
Records with a different number of bytes and a different number of fields
What is an advantage of Variable Length Records?
No padding required
What is a disadvantage of Variable Length Records?
No padding required
What are the two types of file?
Transaction files
Master files
What are master files?
Principle file that stores crucial information about a business, held by an organisation
What are transaction files?
Contains the day-to-day information generated from business activity. Used to update/process the master file
What are some File Access Methods?
Serial File
Sequential File
Indexed Sequential File
Multilevel Indexed Sequential File
Direct Access (Random) File
What are Serial Files?
Files that store data in the chronological order they have arisen. Needed when data is needed in chronological order
What are some features of Serial Files?
When a specific file is needed- will need to search 1-by-1 through every file
Used as temporary transaction files, never used as a master file
How are records searched for in a Serial File?
Every record is copied in the order it is stored in to a new file. The one you are looking for is NOT copied. The old file is deleted, the new file is updated
What are Sequential Files?
Records are stored in some sort of order using a Primary Key (field with a unique value to that field)
Order is based on a field in the record
What are some features of Sequential Files?
Simple
Efficient
Overall process is slow
What are Indexed Sequential Files?
Same as Sequential File, but you can use an index to search for files (like the encyclopaedia of a book)
