An operating system is system software that manages computer hardware and software resources and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may include accounting software for cost allocation of processor time, mass storage 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, although the application code is executed directly by the hardware and makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computer – from cellular phones and video game consoles to web servers and supercomputers; the dominant desktop operating system is Microsoft Windows with a market share of around 82.74%. MacOS by Apple Inc. is in second place, the varieties of Linux are collectively in third place. In the mobile sector, use in 2017 is up to 70% of Google's Android and according to third quarter 2016 data, Android on smartphones is dominant with 87.5 percent and a growth rate 10.3 percent per year, followed by Apple's iOS with 12.1 percent and a per year decrease in market share of 5.2 percent, while other operating systems amount to just 0.3 percent.
Linux distributions are dominant in supercomputing sectors. Other specialized classes of operating systems, such as embedded and real-time systems, exist for many applications. A single-tasking system can only run one program at a time, while a multi-tasking operating system allows more than one program to be running in concurrency; this is achieved by time-sharing, where the available processor time is divided between multiple processes. These processes are each interrupted in time slices by a task-scheduling subsystem of the operating system. Multi-tasking may be characterized in co-operative types. In preemptive multitasking, the operating system slices the CPU time and dedicates a slot to each of the programs. Unix-like operating systems, such as Solaris and Linux—as well as non-Unix-like, such as AmigaOS—support preemptive multitasking. Cooperative multitasking is achieved by relying on each process to provide 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 preemptive multi-tasking. Single-user operating systems have no facilities to distinguish users, but may allow multiple programs to run in tandem. A multi-user operating system extends the basic concept of multi-tasking with facilities that identify processes and resources, such as disk space, belonging to multiple users, the system permits multiple users to interact with the system at the same time. Time-sharing operating systems schedule tasks for efficient use of the system and may include accounting software for cost allocation of processor time, mass storage and other resources to multiple users. A distributed operating system manages a group of distinct 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 form a distributed system.
In an OS, distributed and cloud computing context, templating refers to creating a single virtual machine image as a guest operating system saving it as a tool for multiple running virtual machines. The technique is used both in virtualization and cloud computing management, is common in large server warehouses. 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 compact and efficient by design. Windows CE and Minix 3 are some examples of embedded operating systems. A real-time operating system is an operating system that guarantees to process events or data by a specific moment in time. A real-time operating system may be single- or multi-tasking, but when multitasking, it uses specialized scheduling algorithms so that a deterministic nature of behavior is achieved. 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.
A library operating system is one in which the services that a typical operating system provides, such as networking, are provided in the form of libraries and composed with the application and configuration code to construct a unikernel: a specialized, single address space, machine image that can be deployed to cloud or embedded environments. Early computers were built to perform a series of single tasks, like a calculator. Basic operating system features were developed in the 1950s, such as resident monitor functions that could automatically run different programs in succession to speed up processing. Operating systems did not exist in their more complex forms until the early 1960s. Hardware features were added, that enabled use of runtime libraries and parallel processing; when personal computers became popular in the 1980s, operating systems were made for them similar in concept to those used on larger computers. In the 1940s, the earliest electronic digital systems had no operating systems.
Electronic systems of this time were programmed on rows of mechanical switches or by jumper wires on plug boards. These were special-purpose systems that, for example, generated ballistics tables for the military or controlled the pri
The symbol # is most known as the number sign, hash, or pound sign. The symbol has been used for a wide range of purposes, including the designation of an ordinal number and as a ligatured abbreviation for pounds avoirdupois. Since 2007, widespread usage of the symbol to introduce metadata tags on social media platforms has led to such tags being known as "hashtags" and from that, the symbol itself is sometimes incorrectly called a "hashtag"; the symbol is defined in ASCII as U +0023 # NUMBER SIGN and & num. It is graphically similar to several other symbols, including the sharp from musical notation and the equal-and-parallel symbol from mathematics, but is distinguished by its combination of level horizontal strokes and right-tilting vertical strokes, it is believed that the symbol traces its origins to the symbol ℔, an abbreviation of the Roman term libra pondo, which translates as "pound weight". This abbreviation was printed with a dedicated ligature type, with a horizontal line across, so that the lowercase letter "l" would not be mistaken for the numeral "1".
The symbol was reduced for clarity as an overlay of two horizontal strokes "=" across two slash-like strokes "//". Examples of it being used to indicate pounds exist at least as far back as 1850; the symbol is described as the "number" character in an 1853 treatise on bookkeeping. And its double meaning is described in a bookkeeping text from 1880; the instruction manual of the Blickensderfer model 5 typewriter appears to refer to the symbol as the "number mark". Some early-20th-century U. S. sources refer to it as the "number sign", although this could refer to the numero sign. A 1917 manual distinguishes between two uses of the sign: "number"; the use of the phrase "pound sign" to refer to this symbol is found from 1932 in U. S. usage. Before this time, still outside the United States, the term "pound sign" was used to refer to the pound currency symbol or the pound weight symbol. An alternative theory is that the name "pound sign" arose from the fact that character encodings used the same code for both the number sign and the British pound sign "£".
Claims have included ISO 646-GB as well as the Baudot code in the late 19th century. The apparent use of the sign to mean pounds weight in 1850 appears to rule out both of these code sets as the origin, although that same reference admits that the earliest reference in print was a decade after Baudot code."Hash sign" is found in South African writings from the late 1960s, from other non-North-American sources in the 1970s. The symbol appears to be used in handwritten material, while in the printing business, the numero symbol and barred-lb are used for "number" and "pounds" respectively. For mechanical devices, it appeared on the keyboard of the Remington Standard typewriter, but was not used on the keyboards used for typesetting, it appeared in many of the early teleprinter codes and from there was copied to ASCII which made it available on computers and thus caused many more uses to be found for the character. The symbol was introduced on the bottom right button of touch-tone keypads in 1968, but that button was not extensively used until the advent of large scale voicemail in the early 1980s.
Mainstream use in the United States is as follows: when it prefixes a number, it is read as "number", as in "a #2 pencil". The one exception is with the # key on a phone, always referred to as the "pound key" or "pound", thus instructions to dial an extension such as #77 are always read as "pound seven seven". When the symbol follows a number, the symbol indicates weight in pounds; this traditional usage still finds handwritten use, may be seen on some signs in markets and groceries. It is commonly known as the "pound sign". In Canada the symbol is called both the "number sign" and the "pound sign"; the American company Avaya has an option in their programming to denote Canadian English, which in turn instructs the system to say "number sign" to callers instead of "pound sign". In the United Kingdom and Ireland, it is called a hash, it is not used as weight or currency. It is not called the "pound sign"; the use of "#" as an abbreviation for "number" may be understood in Britain and Ireland, where there has been business or educational contact with American usage, but use in print is rare and British typewriters had "£" in place of the American "#".
Where Americans might write "Symphony #5", the British and Irish are more to write "Symphony No. 5", or use the numero sign—"Symphony № 5". To add to the confusion between "£" and "#", in BS 4730, 0x23 represents "£", whereas in ASCII, it represents "#", thus it was common for the same character code to display "#" on US equipment and "£" on British equipment; the symbol has many other names in English: Comment sign Taken from its use in many shell scripts and some programming languages to start comments. Hash, or hash mark Hashtag The word "hashtag" is used when reading social media messages aloud, indicating the start of a hashtag. For instance the text "#foo" is read out loud as "hashtag, foo" (as opposed to "h