Computer multitasking in the context of Semaphore (programming)

In computing, time-sharing is the concurrent sharing of a computing resource among many tasks or users by giving each task or user a small slice of processing time. This quick switch between tasks or users gives the illusion of simultaneous execution. It enables multi-tasking by a single user or enables multiple-user sessions.

Developed during the 1960s, its emergence as the prominent model of computing in the 1970s represented a major technological shift in the history of computing. By allowing many users to interact concurrently with a single computer, time-sharing dramatically lowered the cost of providing computing capability, made it possible for individuals and organizations to use a computer without owning one, and promoted the interactive use of computers and the development of new interactive applications.

The THE multiprogramming system (THE OS) was a computer operating system designed by a team led by Edsger W. Dijkstra, described in monographs in 1965-66 and published in 1968.Dijkstra never named the system; "THE" is simply the abbreviation of "Technische Hogeschool Eindhoven", then the name (in Dutch) of the Eindhoven University of Technology of the Netherlands. The THE system was primarily a batch system that supported multitasking; it was not designed as a multi-user operating system. It was much like the SDS 940, but "the set of processes in the THE system was static".

The THE system apparently introduced the first forms of software-based paged virtual memory (the Electrologica X8 did not support hardware-based memory management), freeing programs from being forced to use physical locations on the drum memory. It did this by using a modified ALGOL compiler (the only programming language supported by Dijkstra's system) to "automatically generate calls to system routines, which made sure the requested information was in memory, swapping if necessary". Paged virtual memory was also used for buffering input/output (I/O) device data, and for a significant portion of the operating system code, and nearly all the ALGOL 60 compiler. In this system, semaphores were used as a programming construct for the first time.

Memory management (also dynamic memory management, dynamic storage allocation, or dynamic memory allocation) is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when no longer needed. This is critical to any advanced computer system where more than a single process might be underway (multitasking) at any time.

Several methods have been devised that increase the effectiveness of memory management. Virtual memory systems separate the memory addresses used by a process from actual physical addresses, allowing separation of processes and increasing the size of the virtual address space beyond the available amount of RAM using paging or swapping to secondary storage. The quality of the virtual memory manager can have an extensive effect on overall system performance. The system allows a computer to appear as if it may have more memory available than physically present, thereby allowing multiple processes to share it.

Multi-user software is computer software that allows access by multiple users of a computer. Time-sharing systems are multi-user systems. Most batch processing systems for mainframe computers may also be considered "multi-user", to avoid leaving the CPU idle while it waits for I/O operations to complete. However, the term "multitasking" is more common in this context.

An example is a Unix or Unix-like system where multiple remote users have access (such as via a serial port or Secure Shell) to the Unix shell prompt at the same time. Another example uses multiple X Window sessions spread across multiple terminals powered by a single machine – this is an example of the use of thin client. Similar functions were also available in a variety of non-Unix-like operating systems, such as Multics, VM/CMS, OpenVMS, MP/M, Concurrent CP/M, Concurrent DOS, FlexOS, Multiuser DOS, REAL/32, OASIS, THEOS, PC-MOS, TSX-32 and VM/386.

Unix (/ˈjuːnɪks/ , YOO-niks; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Initially intended for use inside the Bell System, AT&T licensed Unix to outside parties in the late 1970s, leading to a variety of both academic and commercial Unix variants from vendors including University of California, Berkeley (BSD), Microsoft (Xenix), Sun Microsystems (SunOS/Solaris), HP/HPE (HP-UX), and IBM (AIX).

The early versions of Unix—which are retrospectively referred to as "Research Unix"—ran on computers such as the PDP-11 and VAX; Unix was commonly used on minicomputers and mainframes from the 1970s onwards. It distinguished itself from its predecessors as the first portable operating system: almost the entire operating system is written in the C programming language (in 1973), which allows Unix to operate on numerous platforms. Unix systems are characterized by a modular design that is sometimes called the "Unix philosophy". According to this philosophy, the operating system should provide a set of simple tools, each of which performs a limited, well-defined function. A unified and inode-based filesystem and an inter-process communication mechanism known as "pipes" serve as the main means of communication, and a shell scripting and command language (the Unix shell) is used to combine the tools to perform complex workflows.

NeXTSTEP is a discontinued object-oriented, multitasking operating system based on the Mach kernel and the UNIX-derived BSD. It was developed by NeXT, founded by Steve Jobs, in the late 1980s and early 1990s and was initially used for its range of proprietary workstation computers such as the NeXT Computer. It was later ported to several other computer architectures.

Although relatively unsuccessful at the time, it attracted interest from computer scientists and researchers. It hosted the original development of the Electronic AppWrapper, the first commercial electronic software distribution catalog to collectively manage encryption and provide digital rights for application software and digital media, a forerunner of the modern "app store" concept. It is the platform on which Tim Berners-Lee created the first web browser, and on which id Software developed the video games Doom and Quake.

In computer operating systems, a light-weight process (LWP) is a means of achieving multitasking. In the traditional meaning of the term, as used in Unix System V and Solaris, a LWP runs in user space on top of a single kernel thread and shares its address space and system resources with other LWPs within the same process. Multiple user-level threads, managed by a thread library, can be placed on top of one or many LWPs - allowing multitasking to be done at the user level, which can have some performance benefits.

In some operating systems, there is no separate LWP layer between kernel threads and user threads. This means that user threads are implemented directly on top of kernel threads. In those contexts, the term "light-weight process" typically refers to kernel threads and the term "threads" can refer to user threads. On Linux, user threads are implemented by allowing certain processes to share resources, which sometimes leads to these processes to be called "light weight processes". Similarly, in SunOS version 4 onwards (prior to Solaris) "light weight process" referred to user threads.

An embedded operating system (EOS) is an operating system designed specifically for embedded computer systems. These systems aim to enhance functionality and reliability to perform dedicated tasks. When the multitasking method employed allows for timely task execution, such an OS may qualify as a real-time operating system (RTOS).

webOS, also known as LG webOS, is a Linux kernel-based multitasking operating system for smart devices, such as smart TVs, that has also been used as a mobile operating system. Initially developed by Palm, Inc. (which was acquired by Hewlett-Packard), HP made the platform open source, at which point it became Open webOS.

The operating system was later sold to LG Electronics, and was made primarily a smart TV operating system for LG televisions as a successor to NetCast. In January 2014, Qualcomm announced that it had acquired technology patents from HP, which included all the webOS and Palm patents; LG licenses them to use in their devices.

BeOS is a discontinued operating system for personal computers that was developed by Be Inc. It was conceived for the company's BeBox personal computer which was released in 1995. BeOS was designed for multitasking, multithreading, and a graphical user interface. The OS was later sold to OEMs, retail, and directly to users; its last version was released as freeware.

Early BeOS releases are for PowerPC. It was ported to Macintosh, then x86. Be was ultimately unable to achieve a significant market share and ended development with dwindling finances, so Palm acquired the BeOS assets in 2001. Enthusiasts have since created derivate operating systems including Haiku, which will retain BeOS 5 compatibility as of Release R1.

The Nintendo eShop is a digital distribution service for the Nintendo Switch and the Nintendo Switch 2, and formerly available via the Nintendo Network for the Wii U and Nintendo 3DS. Launched in June 2011 on the Nintendo 3DS, the Nintendo eShop served as the successor to both the Wii Shop Channel and DSi Shop. It is also a multitasking application, which means it is easily accessible even when a game is already running in the background through the system software. The Nintendo eShop features downloadable games, demos, applications, streaming videos, consumer rating feedback, and other information on upcoming game releases. The service was discontinued globally for the Wii U and Nintendo 3DS in March 2023, continuing only on the Switch and Switch 2.

In computing, a desk accessory (DA) or desklet is a small transient or auxiliary application that can be run concurrently in a desktop environment with any other application on the system. Early examples, such as Sidekick and Macintosh desk accessories, used special programming models to provide a small degree of multitasking on systems that initially did not have any other multitasking ability.

In digital computers, an interrupt is a request for the processor to interrupt currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, the processor will suspend its current activities, save its state, and execute a function called an interrupt handler (or an interrupt service routine, ISR) to deal with the event. This interruption is often temporary, allowing the software to resume normal activities after the interrupt handler finishes, although the interrupt could instead indicate a fatal error.

Interrupts are commonly used by hardware devices to indicate electronic or physical state changes that require time-sensitive attention. Interrupts are also commonly used to implement computer multitasking and system calls, especially in real-time computing. Systems that use interrupts in these ways are said to be interrupt-driven.

Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20-bit segmented memory address space (giving 1 MB of addressable memory) and unlimited direct software access to all addressable memory, I/O addresses and peripheral hardware. Real mode provides no support for memory protection, multitasking, or code privilege levels.

Before the introduction of protected mode with the release of the 80286, real mode was the only available mode for x86 CPUs; and for backward compatibility, all x86 CPUs start in real mode when reset, though it is possible to emulate real mode on other systems when starting in other modes.