Operating Systems

Operating systems

Most important program that runs on a computer Every general-purpose (such as desktop) computer must

have OS to run other programs Manages the hardware and software resources of the

system Provides stable and consistent way for applications to

deal with the hardware without having to know all the details

Most common Windows, UNIX, Macintosh

Many other OS designed for special-purpose applications mainframes, robotics, manufacturing, real-time control

systems

                                       

                                   

Managing hardware and software

Various programs and input methods compete for the attention of the central processing unit (CPU) programs and methods demand memory, storage

and input/output (I/O) bandwidth for their own purposes

OS makes sure each application gets the necessary resources while cooperating with the other applications economizes capacity of the system to greatest

benefit of users and applications.

Software OS Hardware

Providing consistent application interface

Important if more than one type of computer is using OS or if computer hardware is open to change

A consistent application program interface (API) allows software developer to write an application on one computer and have a high level of confidence that it will run on another computer of the same type even if the amount of memory or the quantity of

storage is different on the two machines OS can ensure that applications continue to run when

hardware upgrades and updates occur OS and not the application is charged with

managing the hardware and the distribution of its resources

Tasks of the OS

A task refers to the combination of a program being executed and bookkeeping information used by the operating system

Tasks of the OS: Processor management Memory management Device management Storage management Application interface User interface

Processor management

Ensuring that each process and application receives enough of the processor's time to function properly

Using as many processor cycles for real work as is possible

Processes (thread) schedule work done by processor CPU can only do one thing at a time OS has to switch between different processes

thousands of times a second OS sets a certain number of CPU execution cycles

to one program, after those cycles switches to another program, etc.

Memory and storage management

Each process must have enough memory in which to execute, and it can neither run into the memory space of another process nor be run into by another process

The different types of memory in the system must be used properly so that each process can run most effectively

Memory types High-speed cache - fast, small amounts of memory

available to the CPU, where cache controllers predict which data CPU will need next and pull it from main memory

Main memory - RAM Secondary memory - disk

Paging

OS must balance the needs of the various processes with the availability of the different types of memory

Not enough memory to run all processes simultaneously Processes not running take up space in memory Paging: allocating physical memory to processes in

fixed-size blocks called pages Separate programs into pages and keep only a few

pages of each running program in memory at one time active pages kept in memory inactive pages kept on disk, brought in as needed

Moving pages in and out of memory is called swapping

Thrashing

If user runs too many processes at the same time, OS will use majority of its available CPU cycles to swap between processes rather than run processes

Paging or swapping systems that are overloaded waste most of their time moving data rather than performing useful computation

Thrashing CPU utilization, system throughput and system

response time decrease resulting poor performance of a system

Risk of thrashing increases with degree of multiprogramming

Device management

Device driver path between the operating system and (most)

hardware not on computer's motherboard translator between signals of the hardware

subsystems and high-level programming languages of the OS and application programs

takes file data from OS and translates into bit streams put in specific locations on storage devices

Drivers are run when device is required, function basically like any other process

OS assigns high-priority blocks to drivers so hardware resources can be released and readied for further use as quickly as possible

Application and user interfaces

Application Program Interfaces (APIs) allow application programmers to use functions of

computer and OS without worrying about details of CPU operation

provides a consistent way for applications to use the resources of the computer system

User Interface (UI) brings structure to the interaction between a user

and the computer program or set of programs that sits as a layer

above the OS usually a graphical user interface (GUI)

Classification

Multi-user allows many different users to take advantage of the

computer's resources simultaneously balance requirements of users, programs have sufficient and

separate resources Single-user, multi-tasking

most desktop and laptop computers today single user operates many programs at same time

Single-user, single task one user can effectively do one thing at a time Palm OS

Real time (RTOS) particular operation executes in precisely the same amount

of time every time it occurs machinery, scientific instruments and industrial systems

Types of operating systems

Open Source

Description of software distributed as source code under licenses that guarantee everyone the right to freely use, change, or redistribute the source code

Initiated by Bruce Perens primary author of The Open Source Definition, the

formative document of Open Source movement

The basic idea behind open source is very simple: Whenprogrammers can read, redistribute, and modify the sourcecode for a piece of software, the software evolves. Peopleimprove it, people adapt it, people fix bugs. And this canhappen at a speed that, if one is used to the slow pace ofconventional software development, seems astonishing.

www.opensource.org

Open Source definition

Distribution terms of open-source software must comply with the following criteria:1. free redistribution2. source code3. derived works4. integrity of the author's source code5. no discrimination against persons or groups6. no discrimination against fields of endeavor7. distribution of license8. license must not be specific to a product9. the license must not restrict other software10. the license must be technology-neutral

www.opensource.org

The GNU Project

Founded by Richard Stallman Launched in 1984 to develop a complete free

Unix- like operating system: GNU GNU Manifesto

written by Stallman at start of GNU Project to ask for participation and support

GNU is free software everyone is free to copy

and redistribute it as well as make changes either large or small

www.gnu.org

Linux

Linux is a Unix-like kernel, created by Linus Torvalds in 1991

Linux together with the GNU operating system made a complete free system: a Linux-based version of the GNU system GNU/Linux system

GNU/Linux systemsare in widespread use estimated 20 million users

Complete, free OS

References

www.webopedia.com www.howstuffworks.com www.linux.org www.microsoft.com www.apple.com www.opensource.org www.stallman.org www.gnu.org www.perens.com FOLDOC