Paper 1 - Unit 5 - Systems Software

Why an operating system is needed

The hardware is unusable without an OS // hides complexity of hardware from user

Acts as an interface / controls communications between user and hardware / hardware and software

Provides software platform / environment on which other programs can be run

Operating system management tasks

• Main memory management

• File / secondary storage management

• Peripheral / hardware / device management

• Provision of a user interface

• Process / task / resource management - Interrupt handling

• Security management

• Input / output management

• Error detection / recovery

• Provision of a platform for software

Memory management tasks

Allocates / deallocates RAM to programs/processes

Memory protection. Prevents a process from accessing memory that is not allocated to it

Keeps track of allocated and free memory locations

Swaps data to and from the hard drive

Handles virtual memory

Paging // segmentation

Secondary Storage/File management

• OS maintains a file directory structure and FAT

• Provides file naming conventions

• Storage space divided into file allocation units

• Space allocated to particular files

Controls access // implements access rights // implements password protection // Makes file sharing possible

Specifies tasks that can be performed on a file (e.g. open, close, delete, copy, create, move)

Peripheral Management

• Manages installation of appropriate driver software

Manages communication between devices / hardware and software:

• Controls access to data being sent to/from hardware/peripherals

• Controls access to hardware/peripherals

Printer management example

Installs printer driver

Sends data to the printer / buffer // sends documents to the print queue

Sends commands to printer

Receives and handles (error) messages/interrupts from the printer

Provision of a User interface

• Allows user interaction with the computer system // Facilitates human-computer communication

• Hides the complexity of the hardware from the user

e.g. GUI, command line interface, Menu-driven

Process Management

Manages the scheduling of processes

Allows multi-tasking

Ensures fair access of CPU

Handles interrupts

Manages / allocates which resources the processes require

Facilitates the sharing and exchange of data between processes

Prevents interference between processes

Interrupt

A signal from a device/program that it requires attention from the processor

Interrupt Handling

Check for interrupt at a point during the fetch-execute cycle.

If an interrupt flag/bit is set in the interrupt register:

• Halt the execution of the current process

• Store the values of the current process on a stack/register

• Loads and executes the appropriate ISR code

• Use of priorities for handling simultaneous interrupts

• Saves data on power outage

Security Management

• Makes provision for recovery when data is lost

• Automatic backup. Enables system restore / roll back (to previous stable state)

• Provides and checks usernames and passwords (Authentication) / encryption / user accounts

• Implements access rights to prevent unauthorized access.

Utility program

Provide additional functionality that make it easier to maintain and manage the system

They can be packaged as part of the operating system or bought as stand-alone software programs

Utility program examples

• System clean up

• Automatic update

• Disk contents analysis / Disk checking / Disk repair

• File compression

• Disk formatter

• Firewall

• Disk Defragmenter

Disk repair software

Checks for and resolves any errors, inconsistencies, or bad sectors on the disk:

• Retrieves files / data from a damaged disk // re-constructs directory // recovers disc when data corrupt

• Marks bad sectors on the disk as unusable

Disc contents analysis

Visually demonstrates the disc usage by showing sizes of files/folders

Disk formatter

• Configures the disc for use / initialise tracks and sectors

• Initialises a file system (e.g. FAT, NTFS) / create a file directory

• Install a boot sector (if creating a bootable disk)

• Checks all sectors and mark bad sectors

• Makes existing data inaccessible

• Partitions the disk into logical drives

Disk Defragmenter

Re-organises the disk contents

Moves split files so they are contiguous

Creates a larger area of (contiguous) free space

File compression software

Compresses/decompresses the contents of the disc, thus increasing number of files that can be stored.

Anti-virus software

Scans files for viruses // detects/identifies a virus

Can constantly run in background

Can run scheduled scans

Can automatically update virus definitions

Can quarantine or delete viruses

Completes heuristic checking

Notifies user of possible viruses

Backup software

Makes copy of files on another medium in case of corruption / loss of data

Firewall

Prevents unauthorised access to computer systems from external sources

Sits between the computer or LAN and the Internet/WAN and permits or blocks traffic to/from the network

Can be software and/or hardware

Can make precise decisions about what to allow or block as it can detect attempts by specific software to connect to Internet

Can help to block hacking and prevent viruses from reaching a computer

Authentication

• The process of determining whether somebody/something is who/what they claim to be

• frequently done through log-on passwords/biometrics, although digital certification is also used as passwords can be cracked/stolen

• helps to prevent unauthorised access to data

Assembly language

Low level programming language.

Uses mnemonic codes.

Specific to the computer hardware.

Often used to create drivers for hardware.

Machine code

A computer programming language consisting of binary or hexadecimal instructions which a computer can respond to directly.

Sometimes used interchangeably with object code

Object code

Code generated by a compiler or other translator, consisting of machine code, byte code, or possibly both

Combined with additional metadata that will enable a linker or loader...

...to assemble it with other object code modules into executable machine code

Translator

Converts a (higher-level) programming language to a different form

e.g. compilers, interpreters and assemblers

Compiler

Translates a high-level language program into machine code for the processor to execute.

Reads the source code and reports all errors at the end.

An executable file is produced.

Different hardware platforms need different compilers to translate the source code into the machine code.

Interpreter

Translates a high-level language program into machine code for the processor to execute.

Reads each statement and checks it before running it.

Halts when it encounters a syntax error.

Assembler

Translates low-level statements into machine code for the processor to execute.

Compiler benefits

• Once translated, the compiler software is not needed to run the program.

• Compiled code should execute faster.

• The executable file produced by a compiler can be distributed without users having sight of the source code, which is kept secure. Users are unable to make changes to the program.

• Cross-compilation is possible.

Interpreter benefits

Easier de-bugging:

• The interpreter stops when error encountered, so errors can be corrected in real time

• The interpreter translates a statement then executes it immediately

• Parts of the program can be tested, without all the program code being available

Interpreter and Compiler used together

Interpreter:

• Easier debugging because errors are reported as they are found and can be corrected immediately.

• No need to wait until the end of the process for the error report.

• Used during development.

Compiler:

• Produces an executable file (.exe)

• After compilation the compiler does not need to be present for the program to run

• The program can be given to others without giving access to source code

• Final program does not need to be re-compiled each time it is run

• Cross-compilation, the program can be compiled to run on different platforms

• Used when development complete // ready for distribution

Integrated Development Environment (IDE)

An application that includes an editor, debugger and compiler/interpreter among other useful tools in order to program.

Features:

• Colour coding // pretty printing

• Auto-complete

• Auto-correct

• Context-sensitive prompts

• Expand and collapse code blocks

• Dynamic syntax checking

• Debugging tools including single stepping, breakpoints, variables/expressions report window

Java

An object-oriented programming language designed specifically for programs (particularly multimedia) to be used over the Internet.

Java allows programmers to create small programs or applications to enhance websites.

Machine independent

The program code can be translated to run on any processor / platform

Source code is translated into machine independent intermediate code, not machine dependent code

How a Java source code program is translated

Java code is partially interpreted - partially compiled. Java uses a two-step translation process:

• Code is translated first into intermediate code/bytecode using the Java compiler

• bytecode file is portable, because each computer that can run Java has a Java Virtual Machine

• The bytecode is then interpreted by the Java Virtual Machine

Library routine

Pre-existing / pre-compiled / pre-written modules / code

Can be linked/imported into the program

They are called to perform common / complex tasks

Typically statically-linked

Library routine/program benefits

• Saves time: less code needs to be written

• Simplifies the program, since only the name of the function is included in the source code

• Reliability: pre-tested/used by many people. Reduces time spent testing. Can be fairly sure that it will function as it should.

• Convenience: can be complex algorithms (e.g. mathematical/graphics functions). Programmer won’t need to figure out how to implement it from scratch.

• Can be written in a different programming language, making use of special features of that language

• Domain specialists can contribute their knowledge to the library

Library routine/program drawbacks

• Compatibility issues: may not work with the other code/may require changing program for it to work

• Not guaranteed thorough testing: there may be unknown or unexpected bugs / virus

• May not meet exact needs or need editing, and then it may give unexpected results/errors after modification.

DLL file

Dynamic Link Library

A shared library file format

Code is saved separately from the main .exe files

Code is only loaded into main memory when required at run-time

Can be made available to several applications (at the same time)

DLL file benefits

• The executable file is smaller as it does not need to contain all the library routines.

• DLL files are only loaded into memory when required.

• Changes to the DLL file code are done independently of the programs that use it, so there is no need to recompile the other programs. Improvements to the DLL file will benefit all programs that use it.

• A single DLL file can be made available to several application programs, saving memory space.

• Routines are pre-written, saving the developers time and money

• Routines are pre-tested so should be reliable

DLL file drawbacks

• The executable code is not self-contained. The DLL files need to be included at run time.

• Appropriate linking software must be available at run-time to link / import the DLL files.

• The DLL file must be present, otherwise you get (unable to find X.dll) errors

• Unexpected changes to the DLL file / corrupted DLL file could mean the program stops working as expected

• Malicious changes to the DLL file could install a virus on the user's computer / related files could be corrupted

• “DLL hell” - DLL file version conflicts. Different applications need different versions of the same DLL file.