In computing, multitasking is a process by which multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for that task. Multitasking solves the problem by scheduling which task may be the one running at any given time, and when another waiting task gets a turn. The act of reassigning a CPU from one task to another one is called a context switch. When context switches occur frequently enough the illusion of parallelism is achieved. Even on computers with more than one CPU (called multiprocessor machines), multitasking allows many more tasks to be run than there are CPUs.

Operating systems may adopt one of many different scheduling approaches, which typically fall into the following categories:

In multiprogramming systems, the running task keeps running until it performs an operation that requires waiting for an external event (e.g. reading from a tape) or until the computer's scheduler forcibly swaps the running task out of the CPU. Multiprogramming systems are designed to maximize CPU usage.

In time-sharing systems, the running task is required to relinquish the CPU, either voluntarily or by an external event such as a hardware interrupt. Time sharing systems are designed to allow several programs to execute apparently simultaneously.

In real-time systems, some waiting tasks are guaranteed to be given the CPU when an external event occurs. Real time systems are designed to control mechanical devices such as industrial robots, which require timely processing.

The term time-sharing is no longer frequently used, having been replaced by simply multitasking.

Multiprogramming

In the early days of computing, CPU time was expensive, and peripherals were very slow. When the computer ran a program that needed access to a peripheral, the CPU would have to stop executing program instructions while the peripheral processed the data. This was deemed very inefficient.

The first efforts to create multiprogramming systems took place in the 1960s. Several different programs in batch were loaded in the computer memory, and the first one began to run. When the first program reached an instruction waiting for a peripheral, the context of this program was stored away, and the second program in memory was given a chance to run. The process continued until all programs finished running.

Multiprogramming doesn't give any guarantee that a program will run in a timely manner. Indeed, the very first program may very well run for hours without needing access to a peripheral. As there were no users waiting at an interactive terminal, this was no problem: users handed a deck of punched cards to an operator, and came back a few hours later for printed results. Multiprogramming greatly reduced the waiting.

As noted in the OS/360 entry its original O/S followed the above model but was replaced the very next year, 1967, by MFT which set limits on the amount of CPU time any single process could consume before being switched out. The hours-long wait referred to above typically was related to the fact that computers simply weren't very fast as compared to today.

Cooperative multitasking/time-sharing

When computer usage evolved from batch mode to interactive mode, multiprogramming was no longer a suitable approach. Each user wanted to see his program running as if it was the only program in the computer. The use of time sharing made this possible, with the concession that the computer would not seem as fast to any one user as it really would be if it were running only that user's program.

Early multitasking systems consisted of suites of related applications that voluntarily ceded time to each other. This approach, which was eventually supported by many computer operating systems, is today known as cooperative multitasking. Although it is rarely used in larger systems, Microsoft Windows prior to Windows 95 (32-bit applications only) and Windows NT, and Mac OS prior to Mac OS X both used cooperative multitasking to enable the running of multiple applications simultaneously.

Because a cooperatively multitasked system relies on each process to regularly give time to other processes on the system, one poorly designed program can cause the whole system to hang.

Preemptive multitasking/time-sharing

Main article: Preemption (computing)

Preemptive multitasking allows the computer system to more reliably guarantee each process a regular "slice" of operating time. It also allows the system to rapidly deal with important external events like incoming data, which might require the immediate attention of one or another process.

At any specific time, processes can be grouped into two categories: those that are waiting for input or output (called "I/O bound"), and those that are fully utilizing the CPU ("CPU bound"). In early systems, processes would often "poll", or "busywait" while waiting for requested input (such as disk, keyboard or network input). During this time, the process was not performing useful work, but still maintained complete control of the CPU. With the advent of interrupts and preemptive multitasking, these I/O bound processes could be "blocked", or put on hold, pending the arrival of the necessary data, allowing other processes to utilize the CPU. As the arrival of the requested data would generate an interrupt, blocked processes could be guaranteed a timely return to execution.

The earliest pre-emptive multitasking OS available to home users was the AmigaOS released in 1985. Pre-emptive multitasking was later adopted by MacOS 9.x, X for Apple Macintoshes, and Windows NT, XP, and Vista for IBM PC-compatibles.

Real time

A further reason for multitasking was in the design of real-time computing systems, where a number of possibly unrelated external activities needed to be controlled by a single processor system. In such systems a hierarchical interrupt system was coupled with process prioritization to ensure that key activities were given a greater share of available process time.

Multithreading

As multitasking greatly improved the throughput of computers, programmers started to implement applications as sets of cooperating processes (e.g. one process gathering input data, one process processing input data, one process writing out results on disk.) This, however, required some tools to allow processes to efficiently exchange data.

Threads were born from the idea that the most efficient way for cooperating processes to exchange data would be to share their entire memory space.Therefore, threads are basically processes that run in the same memory context. Threads are described as lightweight because switching between threads does not involve changing the memory context.

While threads are scheduled preemptively, some operating systems provide a variant to threads, named fibers, that are scheduled cooperatively. On operating systems that do not provide fibers, an application may implement its own fibers using repeated calls to worker functions. Fibers are even more lightweight than threads, and somewhat easier to program with, although they tend to lose some or all of the benefits of threads on machines with multiple processors.

Some systems directly support multithreading in hardware.

Memory protection

When multiple programs are present in memory, an ill-behaved program may (inadvertently or deliberately) overwrite memory belonging to another program, or even to the operating system itself.

The operating system therefore restricts the memory accessible to the running program. A program trying to access memory outside its allowed range is instantly stopped before it can change memory belonging to another process.

A further key innovation was the idea of privilege levels. Low privilege tasks are not allowed some kinds of memory access and are not allowed to perform certain instructions. When a task tries to perform a privileged operation a trap occurs and a supervisory program running at a higher level is allowed to decide how to respond. This created the possibility of virtualizing the entire system, including virtual peripheral devices. Such a simulation is called a virtual machine operating system. Early virtual machine systems did not have virtual memory, but both are common today.

Memory swapping

Use of a swap file or swap partition is a way for the operating system to give more memory than is physically available by keeping portions of the primary memory in secondary storage. While multitasking and memory swapping are two completely unrelated techniques, they are very often used together, as swapping memory allows more tasks to be loaded at the same time. Typically, a multitasking system allows another process to run when the running process hits a point where it has to wait for some portion of memory to be reloaded from secondary storage.

Programming in a multitasking environment

Processes that are completely independent are not much trouble to program. Most of the complexity in multitasking systems comes from the need to share computer resources between tasks and to synchronize the operation of co-operating tasks. Various concurrent computing techniques are used to avoid potential problems caused by multiple tasks attempting to access the same resource.

Bigger computer systems were sometimes built with a central processor(s) and some number of I/O processors, a kind of asymmetric multi-processing. One use for interrupts is to allow a simpler processor to simulate the dedicated I/O processors that it does not have.

Over the years, multitasking systems have been refined. Modern operating systems typically include detailed mechanisms for prioritizing processes, while symmetric multiprocessing has introduced new complexities and capabilities.