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NwekeFavour (talk | contribs) m a typo error, changed :became" to "become" and also added a synonym of de-allocated just for clarity of what information the line was trying to pass. |
m →Multiprogramming: fixed two minor grammar issues. If I did something wrong & this needs to be reverted, PLEASE contact me, thanks! |
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== Multiprogramming ==
In any modern operating system, there can be more than one instance of a [[computer program|program]] loaded in memory at the same time. For example, more than one user could be executing the same program, each user having separate copies of the program loaded into memory. With some programs, it is possible to have one copy loaded into memory, while several users have shared access to it so that they can each execute the same program-code. Such a program is called [[Reentrant (subroutine)|re-entrant]].{{Relevance inline|date=November 2023}} The [[central processing unit|processor]] at any instant can only be executing one instruction from one program but several processes can be sustained over a period of time by assigning each process to the processor at intervals while the remainder become temporarily inactive. The execution of multiple processes over a period of time, rather than simultaneously, is called concurrent execution.
A [[multiprogramming]] or [[Computer multitasking|multitasking]] OS is a system that can execute many processes concurrently. Multiprogramming requires that the processor be allocated to each process for a period of time and de-allocated or issued at an appropriate moment. If the processor is de-allocated during the execution of a process, it must be done in such a way that it can be restarted later as easily as possible.
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== The Kernel system concept ==
{{Repetition section|date=November 2023}}<!-- This appears to be a second section in a row describing CPU modes. -->
The parts of the [[operating system|OS]] that are critical to its correct operation execute in [[kernel mode]], while other [[software]] (such as generic system software) and all application programs execute in [[user mode]]. This fundamental distinction is usually the irrefutable distinction between the operating system and other [[system software]]. The part of the system executing in the kernel supervisor state is called the [[kernel (computer science)|kernel]], or nucleus, of the [[operating system]]. The kernel operates as trusted software, meaning that when it was designed and implemented, it was intended to implement protection mechanisms that could not be covertly changed through the actions of untrusted software executing in user space. Extensions to the OS execute in [[user mode]], so the OS does not rely on the correctness of those parts of the system software for the correct operation of the OS. Hence, a fundamental design decision for any function to be incorporated into the OS is whether it needs to be implemented in the kernel. If it is implemented in the kernel, it will execute in kernel (supervisor) space, and have access to other parts of the kernel. It will also be trusted software by the other parts of the kernel. If the function is implemented to execute in [[user mode]], it will have no access to kernel data structures. However, the advantage is that it will normally require very limited effort to invoke the function. While kernel-implemented functions may be easy to implement, the trap mechanism and authentication at the time of the call are usually relatively expensive. The kernel code runs fast, but there is a large performance overhead in the actual call. This is a subtle, but important point.
== Requesting system services ==
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