An operating system (OS) is a collection of software that manages computer hardware resources and provides common services for computer programs. The operating system is an essential component of the system software in a computer system. Application programs usually require an operating system to function.
Time-sharing operating systems schedule tasks for efficient use of the system and may also include accounting software for cost allocation of processor time, mass storage, printing, and other resources.
For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware,[1][2] although the application code is usually executed directly by the hardware and will frequently make a system call to an OS function or be interrupted by it. Operating systems can be found on almost any device that contains a computer—from cellular phones and video game consoles to supercomputers and web servers.
Examples of popular modern operating systems include Android, BSD, iOS, Linux, OS X, QNX, Microsoft Windows,[3] Windows Phone, and IBM z/OS. All these, except Windows, Windows Phone and z/OS, share roots in UNIX.
Types of operating systems
Real-time
A real-time operating system is a multitasking operating system that aims at executing real-time applications. Real-time operating systems often use specialized scheduling algorithms so that they can achieve a deterministic nature of behavior. The main objective of real-time operating systems is their quick and predictable response to events. They have an event-driven or time-sharing design and often aspects of both. An event-driven system switches between tasks based on their priorities or external events while time-sharing operating systems switch tasks based on clock interrupts.
Multi-user
A multi-user operating system allows multiple users to access a computer system at the same time. Time-sharing systems and Internet servers can be classified as multi-user systems as they enable multiple-user access to a computer through the sharing of time. Single-user operating systems have only one user but may allow multiple programs to run at the same time.
Multi-tasking vs. single-tasking
A multi-tasking operating system allows more than one program to be running at the same time, from the point of view of human time scales. A single-tasking system has only one running program. Multi-tasking can be of two types: pre-emptive and co-operative. In pre-emptive multitasking, the operating system slices the CPU time and dedicates one slot to each of the programs. Unix-like operating systems such as Solaris and Linux support pre-emptive multitasking, as does AmigaOS. Cooperative multitasking is achieved by relying on each process to give time to the other processes in a defined manner. 16-bit versions of Microsoft Windows used cooperative multi-tasking. 32-bit versions of both Windows NT and Win9x, used pre-emptive multi-tasking. Mac OS prior to OS X used to support cooperative
multitasking.
Distributed system
A distributed operating system manages a group of independent computers and makes them appear to be a single computer. The development of networked computers that could be linked and communicate with each other gave rise to distributed computing. Distributed computations are carried out on more than one machine. When computers in a group work in cooperation, they make a distributed system.
Embedded
Embedded operating systems are designed to be used in embedded computer systems. They are designed to operate on small machines like PDAs with less autonomy. They are able to operate with a limited number of resources. They are very compact and extremely efficient by design. Windows CE and Minix 3 are some examples of embedded operating systems.
Lists of Operating Systems
Microsoft Windows
Microsoft Windows is a series of graphical interface operating systems developed, marketed, and sold by Microsoft. Microsoft introduced an operating environment named Windows on November 20, 1985 as a graphical operating system shell for MS-DOS in response to the growing interest in graphical user interfaces (GUI).[4] Microsoft Windows came to dominate the world’s personal computer market with over 90% market share, overtaking Mac OS, which had been introduced in 1984.
As of October 2013, the most recent versions of Windows for personal computers, smartphones, server computers and embedded devices are respectively Windows 8.1, Windows Phone 8, Windows Server 2012 R2 and Windows Embedded 8.
Mac OS
Mac OS is a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems. The original version was the integral and unnamed system software first introduced in 1984 with the original Macintosh, and referred to simply as the “System” software. The System was renamed to Mac OS in 1996 with version 7.6. The System is credited with popularizing the graphical user interface concept.
Macintosh operating systems have been released in two major series. Up to major revision 9, from 1984 to 2000, it is historically known as Classic Mac OS. Major revision 10 (revisioned minorly, such as 10.0 through 10.9), from 2001 to present, has had the brand name of Mac OS X, later changing to only OS X. Both series share a general interface design and some shared application frameworks for compatibility, but also have deeply different architectures.
Linux
Linux began its life as a Unix-clone, but has over time become the dominant Unix-type platform in use today (at the time I write this). Created by Linus Torvalds in 1991, it is commonly found on servers, but can also be found in embedded devices, desktop computers, and so on. The core of the system, the Linux kernel, is licensed under the GNU General Public License. As distributed, Linux is packaged differently by a variety of vendors, with each vendor adding its own software mix and external visual appearance to the overall package. These packages are called “distributions” in the Linux world, and you can find a list of the most popular distributions from the above link. Instead of installing a distribution, you can also download a Linux Live-CD which allows you to boot from the CD/DVD into a complete Linux system without installing anything.
Chromium OS (Google Chrome OS)
Chromium OS is the open source operating system from Google that will form the basis of Google’s Chrome OS. It uses a modified Linux kernel and is primarily intended for devices like netbooks. As such, the main user interface is probably the web browser, with most (if not all) applications that a user runs being web applications. Although the system is still under development, the source code is already available for you to download and play with.
DexOS New
This is an x86 32-bit operating system written completely in assembly language, that is optimised (“optimized” in US English) for speed. Only a single process can execute at a time, although that process can have multiple threads. Since the process runs at ring 0, it has complete access to all hardware. The system (by design) does not support virtual memory (paging) nor indeed even provide any memory allocation API (other than functions to get the conventional and extended memory size and the top of usable memory); your program manages memory itself. It does however have a TCP/IP stack (for Internet support), support for some Ethernet cards, comes with an integrated development environment (IDE), and a basic user interface. The entire OS is very small, and (at the time I write this) can apparently fit into less than 64KB of memory. (The entire download file is also very small: about 8.8 MB at the time I checked it.) You don’t have to install to your hard disk if you only want to try it: the OS can be booted from a USB flash drive, a floppy disk or a CD.
Qubes OS
Qubes OS is an operating system designed with security foremost in mind. It does this by using “security by isolation” where not only are programs isolated from each other, but even some system components (like the networking and storage subsystems) are also sandboxed. This is done with the aid of virtualisation (“virtualization”). This open source system is based on Xen, Linux and the X Window system. At the time this entry was written, the software is still under development (that is, it’s not ready for production use).
Singularity Project
This is a research project by Microsoft intended to use the advances in software engineering in the design and implementation of an operating system. The operating system runs every program, device driver and system extension in its own isolated space, which they call Software Isolated Process (SIP). Programs cannot share memory or even modify their own code. The aim is to make the system more reliable and robust. The research development kit (RDK) for Singularity contains the source code, build tools, test suites and documentation for the project. It is licensed for academic non-commercial use only (read their licence for the details).
Plan 9 from Bell Labs
Plan 9 is a research system originally designed by Ken Thompson, Rob Pike, Dave Presotto and Phil Winterbottom. Unlike many other operating systems, all resources in Plan 9 are accessed like files in a hierarchical file system. While Unix-type systems already provided such an encapsulation, with many devices mapped to the file system, Plan 9 goes even further, so that even things like the network and user interface window are also accessed in this way, through a standard protocol called “9P”. The operating system can also (optionally) work as a distributed system, with its components distributed among different hardware platforms. The system is released under the GNU General Public License.
MINIX 3
MINIX is a POSIX compliant operating system for the PC (although ports to Xscale and PowerPC are supposed to be underway) with a microkernel architecture (where the operating system core, or kernel, does the bare mininum and everything else is implemented separately [in user space]). It has TCP/IP networking, supports the X Window system, runs the GNU compiler tools and many other Unix utilities, has device drivers that run as user processes, etc. The system is released as open source.
Android
This is a complete software stack for mobile devices comprising the base operating system as well as applications that run on it. The operating system itself is a fork of the Linux kernel, while the software in the stack are Java applications. The system runs on ARM processors, although there is apparently also an x86 version. Android is currently used on many devices including smartphones and netbooks.
BSD-based Systems: FreeBSD, OpenBSD, NetBSD, Darwin
BSD started out as a derivative of the original AT&T Unix system, although today, the modern open source BSD-based systems no longer has any AT&T code. It is released under a permissive BSD licence. See the page linked-to above for more details as well as links to the various free BSD systems.
BeOS and Clones
BeOS is an OS from the 1990s designed for its own hardware that has since been ported to the PC. It was optimized for work with digital media and has many modern operating system features. Although the original BeOS is no longer being maintained, an open source project called Haiku continues its development. See the page linked to above for details.
MS-DOS and Clones: FreeDOS, DR-DOS, IBM PC-DOS
MSDOS, PC-DOS, and its clones, DR-DOS and FreeDOS, were operating systems widely used on PCs in the 1980s and early 1990s. The last I checked, the open source equivalent, FreeDOS, is still under development and runs fine on modern hardware.
Windows Clones and Emulation Layers
Windows clones and emulation layers let you run Windows programs without having to buy a copy of Windows. See the page linked-to for more details.
Symbian (no longer available)
Symbian, from Nokia, is an operating system for smartphones and other mobile devices. It uses a microkernel architecture, has real-time features, supports pre-emptive multitasking and memory protection, and is designed with the philosophy that resources are scarce. It is a popular mobile operating system in current (at the time this was written) use. Update (28 November 2010): The Symbian website will apparently be closed on 17 December 2010. If you want to download the source code, you’ll have to do it before that date. Update (2011): Well, I just checked, and it’s no longer available.
OpenSolaris (no longer available)
OpenSolaris is a Unix derivative, based on Solaris, which is in turn based on Unix System V Release 4. It runs on the x86 family of processors as well as SPARC (UltraSPARC, SPARC64). The majority of the code is released as open source under the Common Development and Distribution License (CDDL), although there are apparently also some binary-only components released under the OpenSolaris Binary License. Update (August 2010): with the purchase of Sun by Oracle, and consequently Solaris, it looks like OpenSolaris will no longer be updated or maintained. Update (25 Mar 2013): Another one bites the dust. It looks like OpenSolaris is no longer available.
