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Process management is an integral part of any modern-day operating system (OS). The OS must allocate resources to processes, enable processes to share and exchange information, protect the resources of each process from other processes and enable synchronisation among processes. To meet these requirements, the OS must maintain a data structure for each process, which describes the state and resource ownership of that process, and which enables the OS to exert control over each process. == Multiprogramming == In many modern operating systems, there can be more than one instance of a 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 each can execute the same program-code. Such a program is said to be re-entrant. The 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. A number of processes being executed over a period of time instead of at the same time is called concurrent execution. A multiprogramming or multitasking OS is a system executing many processes concurrently. Multiprogramming requires that the processor be allocated to each process for a period of time and de-allocated 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. There are two possible ways for an OS to regain control of the processor during a program’s execution in order for the OS to perform de-allocation or allocation: # The process issues a system call (sometimes called a ''software interrupt''); for example, an I/O request occurs requesting to access a file on hard disk. # A hardware interrupt occurs; for example, a key was pressed on the keyboard, or a timer runs out (used in pre-emptive multitasking). The stopping of one process and starting (or restarting) of another process is called a context switch or context change. In many modern operating systems, processes can consist of many sub-processes. This introduces the concept of a ''thread''. A thread may be viewed as a ''sub-process''; that is, a separate, independent sequence of execution within the code of one process. Threads are becoming increasingly important in the design of distributed and client–server systems and in software run on multi-processor systems. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Process management (computing)」の詳細全文を読む スポンサード リンク
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