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Unit 2: (Assembly Code (Libraries (Pre-written functions, Examples could…

- - - - Complier translates a whole program written in a high level language into executable machine code, going through several stages
      - The resulting machine code is called object code
      - The object code produced is also hardware specific
      - Interpreter
        
        An interpreter also translates code written in a high level language into machine code
        
        An interpreter does this translation line by line rather than translating the whole program before any of it can be executed
      - Stages to convert source code to object code
        
        Lexical analysis
        
        All unnecessary spaces and all comments are removed
        
        Symbol Table
        
        The lexer will build up a symbol table for every keyword and identifier in the program
        
        The symbol table helps to keep track of the run-time memory address for each identifier
        
        Syntax rules
        
        The rules of the language need to be defined
        
        Semantic analysis
        
        It is possible to create a sequence of tokens which is valid syntax but is not a valid program
        
        Semantic analysis checks for this kind of error
        
        Code Generation
        
        Once the program has been checked, the compiler generates the machine code
        
        It may do this in several ‘passes’ over the code because code optimisation will also take place
  - - - Program can be run many times without the need to recompile
      - Faster to execute
      - Executable code does not require the interpreter to run
      - Compiled code cannot be easily read and copied by others
    - - Source code can be run on any machine with the interpreter
      - If a small error is found, no need to recompile the entire program
  - - - Job of loader is to copy the program and any subroutines into main memory to run
      - When the executable code was created it may assume the program will load in memory address 0
      - However, memory addresses in the program will need to be relocated by the loader because some memory will already be in use
- - - - Programmer has no control over the task distribution as this is entrley handled by the OS
  - - - Must respond quickly to inputs
      - May need to cope with many inputs simultaneously
- - - - Operating system provides a platform for the user to interact with the computer
      - Types of User interfaces
        
        GUI
        
        Menu
        
        Command Line
    - - The operating system must mange the allocation of RAM to he different programs to the different programs. This allocation happens when there is not sufficient RAM for all desired processes
    - - Sent to the CPU by the software
      - When an interrupt is detected , processor stops fetching instructions and instead pushes the current contents of it's registers onto a stack
      - The CPU uses an interrupt service routine to process the interrupt
      - Interrupt Priority
        
        Interrupts have different priorities , will be processed in order of priority
        
        Interrupts can themselves be interrupted if the new interrupt is of a higher priority
    - - Process instructions for one application at a time
      - The Operating system must schedule when each application can use the CPU
      - Application gives the illusion of multi-tasking - Multiple applications appear to be running simultaneously
      - Aim of scheduling
        
        Provide acceptable response time to all users
        
        Maximise the time given to the CPU
        
        To ensure fairness on a multi-user system
- - - - Utility Programs
        
        Utility programs maximise the performance of the computer and perform useful background tasks
        
        Disk Defragmenter
        
        Large files stored on a magnetic hard drive may be split up across several physicaldisk locations
        
        Disk defragmenter reorganises the hard drive so that files are in sequential blocks where possible
        
        The result s that files can be read more quickly
        
        Automatic backup
        
        Data in permanent storage needs to be regularly backed up
        
        Process can be automated, specifying:
        Where-Portable hard drive , cloud , local server
        When-Usually when the computer is not in use
        What-Which data should be backed up
        How-Should backups be compressed
        
        Automatic updating
        
        Runs in background detecting software update releases and automatically updates them
        
        Some updates add new software features
        
        Some updates fix security issues which could be exploited by hackers
        
        Compression Software
        
        Compression software is used to reduce the size of files
      - VIrus Checker
        
        Scans permanent storage for viruses by comparing files to known virus definitions
        
        May scan files on access as they are opened
        
        Must be kept up to date with new virus definitions to be effective
        
        Antivirus software use heuristics - it knows what types of behaviour are likely to be used by a malicious program
    - - Allow programs to be pre-written routines and to be translated into executable code
    - - Open Source
        
        Allows anyone to access it's source code
        
        The software is open source licensed but free to use
        
        Anyone can modify software and sell it, on the condition that the software produced is also open source
        
        Different from freeware - software that is for anyone to use but the source code is not available
      - Proprietary Source software
        
        No access to the source code
        
        User may pay the person or company who owns the copyright for a licence to use the software
      - Off the Shelf
        
        Ready made software available to anyone to purchase is known as off the shelf software
      - Bespoke
        
        Software that is custom created for a specific user is known as bespoke software
        
        Bespoke software is mostly used by businesses rather than individuals
      - Selecting an application: Functionality - Does it do what i want Hardware - Will it run on my hardware
        Availability - Already exists or specially written
        Cost - How much is it
        Reliability - Are there bugs, has it been tested