Operating system
Operating systems are found on many devices that contain a computer – from cellular phones and video game consoles to web servers and supercomputers.
As of September 2024[update], Android is the most popular operating system with a 46% market share, followed by Microsoft Windows at 26%, iOS and iPadOS at 18%, macOS at 5%, and Linux at 1%.
Some operating systems require installation or may come pre-installed with purchased computers (OEM-installation), whereas others may run directly from media (i.e. live CD) or flash memory (i.e. USB stick).
An operating system is difficult to define,[6] but has been called "the layer of software that manages a computer's resources for its users and their applications".
Multicomputers were developed because large multiprocessors are difficult to engineer and prohibitively expensive;[17] they are universal in cloud computing because of the size of the machine needed.
Hard real-time systems require exact timing and are common in manufacturing, avionics, military, and other similar uses.
[28] In order for hard real-time systems be sufficiently exact in their timing, often they are just a library with no protection between applications, such as eCos.
[14][29] Virtual machines can be paused, saved, and resumed, making them useful for operating systems research, development,[30] and debugging.
Note that this overhead is not negligible: to the direct cost of mode switching it's necessary to add the indirect pollution of important processor structures (like CPU caches, the instruction pipeline, and so on) which affects both user-mode and kernel-mode performance.
MINIX was a stripped-down version of UNIX, developed in 1987 for educational uses, that inspired the commercially available, free software Linux.
Windows later was rewritten as a stand-alone operating system, borrowing so many features from another (VAX VMS) that a large legal settlement was paid.
Later on, the open-source Android operating system (introduced 2008), with a Linux kernel and a C library (Bionic) partially based on BSD code, became most popular.
With the de facto obsoletion of DOS, all user software must interact with the operating system to access hardware.
An interrupt (also known as an abort, exception, fault, signal,[52] or trap)[53] provides an efficient way for most operating systems to react to the environment.
In both segmentation and paging, certain protected mode registers specify to the CPU what memory address it should allow a running program to access.
Attempts to access other addresses trigger an interrupt, which causes the CPU to re-enter supervisor mode, placing the kernel in charge.
In modern operating systems, memory which is accessed less frequently can be temporarily stored on a disk or other media to make that space available for use by other programs.
[91] Permanent storage devices used in twenty-first century computers, unlike volatile dynamic random-access memory (DRAM), are still accessible after a crash or power failure.
Permanent (non-volatile) storage is much cheaper per byte, but takes several orders of magnitude longer to access, read, and write.
[98][99] System calls (which are sometimes wrapped by libraries) enable applications to create, delete, open, and close files, as well as link, read, and write to them.
[100] The operating system's efforts to reduce latency include storing recently requested blocks of memory in a cache and prefetching data that the application has not asked for, but might need next.
[113] Other ways to increase security include simplicity to minimize the attack surface, locking access to resources by default, checking all requests for authorization, principle of least authority (granting the minimum privilege essential for performing a task), privilege separation, and reducing shared data.
[115] Unikernels are another approach that improves security by minimizing the kernel and separating out other operating systems functionality by application.
[125][126] Andrew S. Tanenbaum advises releasing the source code of all operating systems, arguing that it prevents developers from placing trust in secrecy and thus relying on the unreliable practice of security by obscurity.
This cost in supporting operating systems diversity can be avoided by instead writing applications against software platforms such as Java or Qt.
As of September 2024[update], Android is the most popular operating system with a 46% market share, followed by Microsoft Windows at 26%, iOS and iPadOS at 18%, macOS at 5%, and Linux at 1%.
[3] Linux is a free software distributed under the GNU General Public License (GPL), which means that all of its derivatives are legally required to release their source code.
[132] Linux was designed by programmers for their own use, thus emphasizing simplicity and consistency, with a small number of basic elements that can be combined in nearly unlimited ways, and avoiding redundancy.
[136] Linux has a graphical user interface (GUI) with a desktop, folder and file icons, as well as the option to access the operating system via a command line.
[139] Windows is a proprietary operating system that is widely used on desktop computers, laptops, tablets, phones, workstations, enterprise servers, and Xbox consoles.
