1.2.2 - Applications Generation

• Applications Software

  • designed to be used by end-user to perform a specific task
  • requires systems software to run
• Applications Software - Examples

  • desktop publishing
  • word processing
  • spreadsheets
  • web browsers
• Systems Software

  • low-level software
  • responsible for running computer system smoothly, interacting with hardware and providing a platform for application software to run
  • user does not directly interact with systems software
  • ensures high performance for user
• Systems Software - Examples

  • library programs
  • utility programs
  • operating system
  • device drivers
• Utilities

  • key system software essential to ensuring consistent high performance of the OS
  • each program has a specific function linked to maintenance of the OS
• Utilities - Examples

  • compression
  • disk defragmentation
  • antivirus
  • automatic updating
  • backup
• Compression

  • enables files to be compressed and decompressed
  • used when compressing large files to be sent across the internet
  • commonly used to compress scanned files
• Disk Defragmentation

  • as hard disk fills, r/w times slow down because files become fragmented as they're stored in different parts of memory
  • this utility rearranges contents of the hard drive so they can be accessed faster
  • this improves performance
• Antivirus

  • responsible for detecting potential threats to computer
  • alerts user and removes threats
• Automatic Updating

  • ensures OS is kept up to date
  • any updates are automatically installed when computer is restarted
  • updates tackle bugs or security flaws
  • ensures system is less vulnerable to malware/hacking threats
• Backup

  • automatically creates routine copies of specific files chosen by user
  • how often the files are backed up is also chosen by the user
• Source Code

  • written by programmer
  • object code before it's been compiled
• Open Source

  • can be used by anyone without a license
  • is distributed with source code
• Open Source - Advantages

  • can be modified and improved by anyone
  • technical support from online community
  • can be modified and sold on
• Open Source - Disadvantages

  • support available online may be insufficient/incorrect
  • no user manuals
  • lower security as might not be developed in a controlled environment
• Closed Source

  • requires user to hold an appropriate license to use it
  • users cannot access source code as company owns copyright license
• Closed Source - Advantages

  • thorough, regular and well-tested updates
  • company owning software provides expert support and user manuals
  • high levels of security as developed professionally
• Closed Source - Disadvantages

  • license restricts how many people can use software at once
  • users cannot modify and improve software themselves
• Open vs Closed - What to Consider

  • suitability of software to the task it will be used for
  • costs - implementation, maintenance, training, license
  • functionality - features available, ease of use
• Translators

  • a programs that converts high-level source code into low-level object code
  • it's then ready to be executed by computer
• High-Level Code

  • written and understood by programmer but not the computer
• Low-Level Code

  • can be executed by computer but not directly understood
• Compiler

  • translates code all at once after carrying out checks and reporting back errors
  • initial process is longer than using an interpreter or assembler
  • if changes need to be made, whole program must be recompiled
  • once compiled, code can only be executed on certain devices as compiling code is specific to a certain processor type and OS
  • can be run without a translator present
• Interpreter

  • translates and executes code line-by-line
  • stops and produces error if line contains an error
  • initially appears faster than compilers as code is instantly executed but is slower as code must be translated each times it's executed with an interpreter
  • makes interpreters useful for testing code and pinpointing errors
  • requires interpreter in order to run on different devices
  • code can be executed on a range of platforms with right interpreter
  • makes code more portable
• Assembly Code

  • low-level language
  • the next step up from machine code
  • platform specific
  • instructions used are dependent on the instruction set of the processor
• Assembler

  • translate assembly code into machine code
  • each line of assembly is equal to almost one line of machine
  • translated on an almost 1 to 1 basis
• Stages of Compilation

  • lexical analysis
  • syntax analysis
  • code generation
  • optimisation
• Lexical Analysis

  • first stage of compilation
  • whitespace and comments are removed from code
  • remaining code is checked for keywords/names of variables and constants
  • these are replaced with tokens and info about each is stored in a symbol table
• Syntax Analysis

  • tokens are checked against grammar and rules of the programming language
  • tokens that break the rules are flagged as syntax errors and added to a list of errors
  • abstract syntax tree is produced
  • more detail about identifiers is added to symbol table
  • semantic analysis is carried out
• Syntax Errors - Examples

  • undeclared variable type
  • incomplete set of brackets
• Semantic Errors - Examples

  • multiple declaration
  • undeclared identifiers
• Code Generation

  • produced in syntax analysis stage
  • used to produce machine code
• Optimisation

  • searches through code for area that could be more efficient
  • aims to make code faster to execute but can add to overall time taken for compilation
  • redundant parts of code are detected and removed
  • repeated sections may be grouped and replaced with more efficient versions
  • excessive optimisation risks altering the way the program behaves
• Linkers

  • software responsible for linking external modules and libraries in the code
• Linkers - Types

  • static
  • dynamic
• Static

  • modules/libraries are added directly to main file
  • this increases file size
  • updates externally will not affect program
  • because of this, a specific version of the library can be used
• Dynamic

  • addresses of modules/libraries are included in file they're mentioned in
  • when program is run, loader retrieves program at the address so it can be executed
  • files remain small and they are affected by external updates
  • code does not need to be rewritten
• Loaders

  • programs provided by OS
  • when file is executed, loader retrieves the library/subroutine from given memory location
• Libraries

  • precompiled programs that can be added into other programs using static or dynamic linking
• Libraries - Advantages

  • ready-to-use
  • error free
  • save time developing and testing modules
  • can be reused in multiple programs
  • use already developed functions to save programmers having to rewrite them