Applications Generation

Applications Generation

1. Nature of Applications

   • General Purpose
     • Word processor
     • Spreadsheet
     • Used for different purposes
   • Special Purpose
     • Single specific task or set of tasks
     • Hotel booking systems
     • Fingerprint scanning systems
     • Off-The-Shelf
       • Less expensive
       • May contain unwanted features
       • Installed immediately
       • Well documented
         • Well-tested
           • Error-free
     • Bespoke
       • More costly
       • Features customized
       • May take long time to develop
       • May contain errors

2. Utilities

   • Optimize performance
   • Perform tasks
     • Disk Defragmentation
       • Files split into blocks recombined
     • Automatic Backup
       • Where to store backup
       • What to backup
       • How to run backup
       • When to run backup
     • Automatic Updating
       • Any software installed on computer up-to-date
     • Virus Checker
       • Checks hard drive, incoming emails and internet downloads for viruses and removes them
     • Compression Software
       • Supplied as part of operating system

3. Open vs Closed Source

   • Open
     • Distributed with source code
     • Anyone can use
     • Developers can sell software created
     • New software created from open source must be open
       • Distributed so others can read and edit
     • No helpline or regular updates
   • Closed/Proprietary
     • Sold as license to use
     • Restrictions on use
     • Writer holds copyright
       • No access to source code
       • Users cannot modify or sell package
         • Infringe Copyright, Designs and Patents Act
     • Support available from company
       • Regular updates
       • Support lines

4. Translators

   • Assembler
     • Assembly code to machine code
     • Input source code
       • Output object code
   • Compiler
     • High-level language to machine code
     • input source code
       • Compile
         • Output object code
     • Different hardware platforms need different compilers
       • Object code hardware-specific
     • Advantages
       • Object code saved on disk and run without recompiling
       • Object code executes faster
       • Object code distributed without compiler present
       • Object code more secure
   • Interpreter
     • Program written and saved
       • Computer instructed to run program
         • Interpreter analyses each line of code
           • Translates to machine code if no syntax errors
             • Runs program
     • Advantages
       • Platform independence
       • Useful for program development

5. Stages of Compilation

   • Lexical Analysis
     • Removes spaces
     • Comments identified and removed
     • Simple error-checking performed
       • Illegal identifier flagged as error
       • Detect attempt to assign illegal value to constant
     • Replace keywords, constants and identifiers with tokens
       • Unique symbols
   • Syntax Analysis
     • Determines whether code forms valid sentence
     • Parsing
       • Apply set of rules to each statement
     • Semantics checked
       • Defines meaning rather than grammar of language
   • Code Generation
     • High-level language to machine code statements
   • Optimization
     • Reduce execution time
     • Disadvantages
       • Increase compilation time
       • May produce unexpected results

6. Linkers, Loaders and Libraries

   • Linker
     • Put appropriate machine addresses on external call and return instructions
       • Modules linked together correctly
   • Relocating Loader
     • Load object code anywhere in memory
   • Use of Libraries
     • Ready compiled programs
       • Run when needed
     • Tested and error-free
     • Save programmer time