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A loosely coupled multiprocessor system is a type of multiprocessing where the individual processors are configured with their own memory and are capable of executing user and operating system instructions independent of each other. This type of architecture paves the way for parallel processing. Loosely coupled multiprocessor systems are connected via high-speed communication networks.
Loosely coupled multiprocessor systems are also known as distributed memory, as the processors do not share physical memory and have their own IO channels.
A multiprocessor system makes use of more than one CPU along with memory and IO channels. They are capable of processing multiple instruction, multiple data (MIMD) programming. Thus, they support concurrent operations. The configuration of processors in a multiprocessor system can be loosely coupled or tightly coupled. The major distinction between these two types of multiprocessors is the way memory is organized.
Tightly coupled systems share a single memory space and share information through the shared common memory. Loosely coupled multiprocessors consist of distributed memory where each processor has its own memory and IO channels. The processors communicate with each other via message passing or interconnection switching. Each processor may also run a different operating system and have its own bus control logic.
Loosely coupled systems are less costly than tightly coupled systems, but are physically bigger and have a low performance compared to tightly coupled systems. The individual nodes in a loosely coupled system can be easily replaced and are usually inexpensive. Loosely coupled systems also draw more power than tightly coupled systems. Loosely coupled systems are more robust and can resist failures. A single node failure does not break down the entire system and it is also easy to add more nodes to an existing system. But the need for extra hardware required to provide communication between the individual processors makes them complex and less portable.
Some of the characteristics of loosely coupled multiprocessors are: