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Other authors prefer to refer to the operating system techniques as multiprogramming and reserve the term multiprocessing for the hardware aspect of having more than one processor. [2] [8] The remainder of this article discusses multiprocessing only in this hardware sense. In Flynn's taxonomy, multiprocessors as defined above are MIMD machines.
Multiprogramming is a computing technique that enables multiple programs to be concurrently loaded and executed into a computer's memory, allowing the CPU to switch between them swiftly. This optimizes CPU utilization by keeping it engaged with the execution of tasks, particularly useful when one program is waiting for I/O operations to complete.
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 a way that the process can restart later as efficiently as possible.
From the software standpoint, hardware support for multithreading is more visible to software, requiring more changes to both application programs and operating systems than multiprocessing. Hardware techniques used to support multithreading often parallel the software techniques used for computer multitasking. Thread scheduling is also a major ...
A process is said to own resources, of which an image of its program (in memory) is one such resource. However, in multiprocessing systems many processes may run off of, or share, the same reentrant program at the same location in memory, but each process is said to own its own image of the program.
"In a multiprogramming or multiprocessing environment, one or more sequences of instructions treated by a control program as an element of work to be accomplished by a computer." The term task in OS/360 through z/OS is roughly equivalent to light-weight process; the tasks in a job step share an address space.
Multiprocessor system with a shared memory closely connected to the processors. A symmetric multiprocessing system is a system with centralized shared memory called main memory (MM) operating under a single operating system with two or more homogeneous processors. There are two types of systems: Uniform memory-access (UMA) system; NUMA system
In shared memory model the processors are all connected to a "globally available" memory, via either software or hardware means. The operating system usually maintains its memory coherence. [4] From a programmer's point of view, this memory model is better understood than the distributed memory model.