Mind map

AS/A2 Computer Science OCR: Unit 1.2

Sam Gue English

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SDLC
- System Design Life Cycle
  - Spiral
    - Cycle is repeated until perfected
      - Each cycle the product is improved
        Best for long tasks and high risk projects
    - Risk analysis is a specialised skill and is expensive
  - Waterfall
    - Each step is signed off
      - Customer then cannot claim it was not done to specification
        Best for low risk tasks
        Easy to manage
    - Carries a high level of risk
  - RAD
    - Rapid Application Development
      - Constant customer contact
        Customer has all say in what the product will be
        Large amount of testing and constructive criticism
      - Customer may not have time to stay in contact with the company
        Process then must be halted until they can provide
        Waste of time
  - XP
    - Extreme Programming
      - Two programmers working on the same project
        One programs while the other checks the code
        Increases vigilance and reduces mistakes
        Swap around every so often to keep themselves "fresh"
        Make fewer mistakes
        Faster Production
        Good for long, complex, high risk tasks
      - Little user involvement
        A customer representative is attached to the project on the customer's behalf
  - Agile
    - Early and continuous delivery of working programs
      - Allowed change of requirements from anywhere in the process
        Face to face communication with customer
        Customer knows what they are getting
    - Customer must maintain contact
      - Like RAD
      - No necessity for immediate accuracy as one can go back on oneself
- Stages of the SDLC
  - Problem Definition
    - Customer and analyst confer as to what the customer wants
    - A list of specifications is usually drawn up
    - Can help determine an estimated price before the project is started. Leads into Feasibility Study
  - Feasibility Study
    - TELOS
      - Technological
        Is the technology needed to do the task available?
      - Economical
        Is it economically viable? Profits VS Costs
      - Legal
        Is the project legal?
      - Operational
        What are the environmental effects?
        Are there enough people to run it once it is completed?
        What are the social effects?
        What is the effect on the customer?
      - Schedule
        Can it be completed within the time-frame?
  - Data Collection and Analysis
    - What input and output devices may be needed?
      - Find out how people would use the product
        Questionnaires
        Simple Questions
        Wide influence
        Interviewing
        Complex Questions
        Questions based on previous responses
        Observation
        Watching people in the workplace and how the operate the system
        Studying Documents e.g.
        User manuals/guidelines
        Codes of conduct
        Paper files
        invoices and receipts
        Notes on the wall
    - Analysis is:
      - Understanding of the current system
      - Understanding of requirements
      - Gathering user requirements
      - Proposing the new system
      - Producing a new specification
      - Data
        What data needs to be stored?
        How is that data collected?
        Manually
        Data capture forms
        Automatically
        Bar Codes; OMR etc.
      - Input
        How is data input?
        Keyed In?
        Automatic methods?
      - Processing
        What processing must be performed on the data?
  - Design
    - Inputs
      - Development of data validation methods
    - Outputs
      - Screen designs for displaying outputs
      - Design of printed reports
    - Type of system
      - Batch
      - Real-Time
    - Software
      - What software?
      - What type of HCI? (Human Computer Interface)
        GUI
        Command Line
        Menus
        Speech
    - Processing
    - Design Specification
      - Details what the system will do. May include:
        Algorithms
        Pseudocode
        DFDs (Data Flow Diagrams)
        Screen Layouts
        Designs
        How the data will be stored
        ERDs (Entity Relationship Diagrams)
  - Implementation
    - Application of design and information into the final product ready for evauluation
  - Evaluation
    - Testing the product to ensure it runs as intended
    - Product must be rigorously tested
    - A hindsight look on the project
    - Proof of functionality
    - Allows the company to get paid
  - Installation
    - How will the design be integrated into the environment?
      - Purchase of hardware
      - Staff training
      - Creation of data files for storage
      - Identification of an appropriate changeover method
      - An idea of the maintenance service that will be provided in the future
  - Maintenance
    - What type of maintenance?
      - Perfective
        Maintaining the system to meet the requirements
      - Adaptive
        Maintaining the system to meet the changes
      - Corrective
        Regular bug fixing
- Conversion
  - Direct Changeover
    - Immediately swap systems
      - Unseen errors may arise leading to down-time
  - Parallel Conversion
    - Run both systems in parallel
      - Slowly phase old one out
        Unseen errors are found before whole new system is implemented
  - Phased Conversion
    - Splitting new system into parts
      - Replacing parts from the old system instead of all at once
        Errors seen and can be rectified by temporarily changing back to the old system
  - Prototyping
    - Using working models to identify errors before implementation
MEMORY MANAGEMENT
- Paging
  - Method of data storage where parts of a program are stored in equally sized sections within RAM
    - Some code may be chopped off within logical strings of code
      - More branching is then needed which slows down the process
    - Some sections of code do not need the maximum space required leading to large amounts of empty space which the computer cannot fill
- Segmentation
  - Method of data storage where parts of a program are stored in sections which are sized in relation to the size of the code called segments
    - Leads to uneven areas of empty space within RAM which leads to clutter and some parts of code being chopped up or having to be stored in VM
    - Splits at logical areas within the code so less branching is required
    - More heavily reliant of VM
- Virtual Memory
  - Volatile method of data storage where data that is normally stored in the RAM is stored in a section of the HDD
    - Slower than RAM as it is further away from the CPU
    - Place where data is stored when there is no more space left in RAM
    - Parts of programs stored here when they are not needed and when they have finished being used
    - Not all of a program can be stored in RAM as there is too little space
      - Most of a program stored in VM
Interrupts
- Types of Scheduling
  - First Come-First Served
    - The first instruction that arrives is done before anything that appears later even if it is not the most important
    - Favours long tasks as the computer spends most of its time on them
  - Round Robin
    - Tasks are given an time slot
      - The time slot is a constant length
        Favours short tasks as long tasks take many time slots to complete and may have to be done in many steps
  - Shortest Job First
    - The shortest instruction is done before the longer instructions
      - Favours short jobs
  - Shortest Remaining Time
    - The queue is sorted on the amount of expected time still to do
      - Favours short jobs
        Long jobs may never be done
  - Multi-Level Feedback Queues
    - Different queues with ranging priorities
      - Jobs can be moved between queues
  - Priority Heirarchy
    - Each task is given a specific priority and the most important jobs are done first
- Halts the processor from just working on the OS
- Types of Interrupts
  - I/O Interrupt
    - Input/Output
      - Keypress
      - Click
      - Microphone
      - Display
  - Software Interrupt
    - An interrupt generated by the processor as it completes an instruction
      - May be a branch/iteration
  - Hardware Interrupt
    - A hardware interrupt is usually caused when necessary hardware like the Hard Disk Drive is damaged and cannot be accessed
      - Usually high priority as it may be necessary to replace the damaged component before the computer can work properly
The Operating System
- Types of OS
  - Distributed
    - Split across multiple machines
      - More power to utilise
        Looks like 1 computer to a user
  - Real Time
    - Carry out 1 function consistently at the same speed
      - Used in production lines
        Needs reliable memory (Not HDD)
        Easily predict outcomes
  - Embedded
    - Specific purpose
      - Limited skill set
        May be used in dedicated systems
  - Multitasking
    - Preemptive
      - Certain amount of time for each task
    - Co-Operative
      - Allows one by one processing
      - Used in most PCs
    - Appearance of running simultaneously
  - Multiuser
    - Used by multiple people at the same time
    - Has to manage work-space and allocate resources for each user
    - Run efficiently, maintain security
THE BIOS
- The Basic Input/Output System
  - Machine code program that can handle the low level input and output systems - e.g. peripherals
    - Provides an interface between the hardware and the OS - Usually found in ROM
      - Conducts the POST (Power On Self Test) and then uses the boot loader (bootstrap) to load the OS's Kernel into memory
        In order to avoid users modifying the OS for different platforms, the BIOS complies with set standards
        BIOS can be updated
        Some changes in the hardware configuration can be coped with manually by altering settings in the BIOS
DEVICE DRIVERS
- A piece of software that tells the OS how to communicate with peripheral devices
VIRTUAL MACHINES
- Software simulating an operating system of another machine to test programs intended for the simulated system
  - Used to take on the function of another machine including executing intermediate code
The Operating System
- The purpose of the OS
  - Device memory management, drawing data into CACHE, RAM and VM when it is needed
  - Processes the UI
  - Provides error messages
  - Sorts interrupts and prioritises them
  - THREE STATES
    - Ready: Ready to be opened Running: On and working Blocked: Halted from working
PROGRAMMING PARADIGMS
- Object Oriented Programming
  - Features
    - CLASSES
      - A template for a set of objects that have a state and behaviour
    - OBJECTS
      - ATTRIBUTES
        Properties of the object
      - METHODS
        Routines, behaviour/actions
      - An instance of a class, a real-world entity, holds attributes and methods
    - INHERITANCE
      - The ability of a derived class to use the same properties of the parent class
    - ENCAPSULATION
      - Methods and properties are made hidden and can only be changed by appropriate methods
    - POLYMORPHISM
      - Creation of general object structures for use with a wide range of data types
- Procedural
  - Excellent for general purpose programming
    - Many books and references available on well tried and tested coding algorithms
      - Good level of control without having to know precise target CPU details
        Portable source code
  - Many procedural languages so people tend to specialise in one
    - Needs to be very precise knowledgeable about programming instructions
      - Not as efficient as hand-crafted source code
        Poor at handling fuzzy conditions e.g. in AI
- Object Oriented
  - Easier to provide working code
    - Classes can be treated as black boxes
      - Many design patterns available
        Code re-use is easy
        Code portability is simple
  - Steep learning curve
    - Complex
      - Not as compact and efficient as low-level languages
- Assembly Code Instructions
  - ADD: add
  - SUB: subtract
  - STA: stores from ACC into MEM
  - LDA: loads from MEM into ACC
  - BRA: branch always
  - BRZ: branch if zero
  - BRP: branch if positive
  - INP: input
  - OUT: output
  - HLT: end the program
  - DAT: data location

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Description: Mind Map on AS/A2 Computer Science OCR: Unit 1.2, created by Sam Gue on 19/11/2016.
Created by: Sam Gue
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