Operating system

• low level software that provides facilities for programs which are benefit to the user
• these facilities include:
  • task association
  • multitasking
  • error handling

Why an OS is needed

• the hardware is unusable without the OS; hides the complexity of hardware from the user
• acts as an interface between hardware and software

User-system interface

• allow the user to interact with the computer system
• consists of:
  • command-line interface (input)
  • graphical user interface or GUI (output)

Program-hardware interface

• the interaction between software programs and the computer hardware

Resource management

• provides resources during processes (the execution of programs)
• this manages:
  • scheduling of processes
  • solving conflicts when two processes require the same resource

Memory management

• ensures programs do not use the same memory locations as one another
• organises ways to improve efficiency in memory usage
• decides which processes are allocated to main memory at any one time

Secondary storage management

• allocates sectors for storage on a hard disk

Device management

• manages peripherals (input and output devices) and how they are used by processes
• aids in the installation of device driver software

File management

• manages:
  • file naming conventions
  • directory (folder) structures
  • access control mechanisms

Security management

• methods of action for data recovery
• prevention of intrusion
• ensuring data privacy

Error detection and recovery

• interrupts a program if an error has been found and runs diagnostics
• may restart the computer system

Disk analysis repair

• performs disk contents analysis
• undergoes attempts to recover data, otherwise deletes any that are unsalvageable
• marks damaged / corrupted sectors on the hard disk
  • these sectors will be avoided on use

File compression

• default file compressor provided by the OS
• used occasionally or when the user calls for

Virus checker

• scans files in the computer system in attempt to detect and take action against viruses
• requires updates to combat new viruses

Backup software

• creates a new backup file when there has been a change
• allows scheduled backups as a routine

Library programs

• programs stored in a 'program library' are called library programs
• they are subroutines that can be used by programmers in their code
  • e.g: Systems.IO is a library built into several programming languages

Why use library programs?

• rather than writing their own routines from scratch, programmers can use preexisting programs that have been tested and provided

Dynamic linker

• a part of the OS that loads and links to shared libraries during run-time execution
• the shared library is already loaded into memory, so when a DDL routine needs, the program is linked

Dynamic Link Library (DLL)

• a shared library implemented by Microsoft that can be dynamically linked

DDL file

• a shared library file
• available to several applications (at the same time)

Executable code

• code that is complete and can be directly executable

Object code

• partially formed machine code produced by translating assembly language through an assembler
• cannot be directly executed

Compiler

• translates high level language code to a lower level language (machine code)
• the most common reason for translating source code is to create an executable program

Advantages

• tends to produce faster code than interpreting source code
• produces an executable file, and therefore the program can be run without need of the source code
  • this is more secure

Disadvantages

• source code must be 100% correct for the executable file to be produced
• object code (code generated by compiler )needs to be produced before a final executable file, this can be a slow process

Interpreter

• instead of the reading the program all at once, it directly executes the program code line by line
• once the code is written, the program may be executed immediately without having to recompile

Advantages

• easy to debug as (syntax) errors would be quickly pinpointed
• easier to create multi-platform code, as each different platform would have an interpreter to run the same code
• useful for prototyping software

Disadvantages

• slower compared to compiler as it analyses each line
• source code is required for the interpreter program to be executed; this is insecure since this source code can be read

Machine / platform independent

• a source code that can run on many different operating systems
• Java is an example of a machine independent language
  • this is because it only requires the Java Virtual Machine (JVM) to compile Java code