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An operating system (OS) is a computer program that manages the hardware and software resources of a computer. more...

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At the foundation of all system software, the OS performs basic tasks such as controlling and allocating memory, prioritizing system requests, controlling input and output devices, facilitating networking, and managing files. It also may provide a graphical user interface for higher level functions.

Services

Process management

Every program running on a computer, be it background services or applications, is a process. As long as a von Neumann architecture is used to build computers, only one process per CPU can be run at a time. Older microcomputer OS such as MS-DOS did not attempt to bypass this limit, with the exception of interrupt processing, and only one process could be run under them (although DOS itself featured TSR as a very partial and not too easy to use solution). Mainframe operating systems have had multitasking capabilities since the early 1960's. Modern operating systems enable concurrent execution of many processes at once via multitasking even with one CPU. Process management is an operating system's way of dealing with running multiple processes. Since most computers contain one processor with one core, multitasking is done by simply switching processes quickly. Depending on the operating system, as more processes run, either each time slice will become smaller or there will be a longer delay before each process is given a chance to run. Process management involves computing and distributing CPU time as well as other resources. Most operating sytems allow a process to be assigned a priority which affects its allocation of CPU time. Interactive operating systems also employ some level of feedback in which the task with which the user is working receives higher priority. Interrupt driven processes will normally run at a very high priority. In many systems there is a background process, such as the System Idle Process in Windows, which will run when no other process is waiting for the CPU.

Memory management

According to Parkinson's law "Data expands to fill the space available for storage". Programmers prefer a memory of infinite size and infinite speed. Nowadays most of the computer's memory is arranged in a hierarchical manner, starting from the fastest registers, CPU cache, RAM and disk storage. The memory manager in an OS coordinates the memories by tracking which one is available, which is to be allocated or deallocated and how to swap between the main memory and secondary memories. This activity, which is usually referred to as virtual memory management, greatly increases the amount of memory available for each process. There is a speed penalty associated with swapping RAM to disk or other slower storage. If running processes require significantly more RAM than is available, either because one process requires a large amount of RAM or two (or more) processes each require less RAM than is available in the system but multitasking results in constant swapping of each process to slower storage, leading to thrashing.

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