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 todaydesktoplaptop  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: When programmers can read, redistribute, and modify the source code for a piece of software, the software evolves. People improve it, people adapt it, people fix bugs. And this can happen at a speed that, if one is used to the slow pace of conventional software development, seems astonishing.

Open Source definition  Distribution terms of open-source software must comply with the following criteria: 1. free redistribution 2. source code 3. derived works 4. integrity of the author's source code 5. no discrimination against persons or groups 6. no discrimination against fields of endeavor 7. distribution of license 8. license must not be specific to a product 9. the license must not restrict other software 10. the license must be technology-neutral

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 

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 systems are in widespread use  estimated 20 million users  Complete, free OS

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