Loosely Coupled Systems

These systems don't share global memory since shared memory concept increases the problem of memory conflicts, which consecutively slows down the execution of instructions. Therefore, to ease this problem, every processor in loosely coupled systems is having a large local memory (LM), which isn't shared by other processor. So, such systems have multiple processors with their own local memory and a group of I/O devices. This set of memory, I/O devices and processor makes a computer system. Thus, these systems are as well known as multi-computer systems. These computer systems are connected mutually through message passing interconnection network by that processes communicate by passing messages to one another. As each computer system or node in multicomputer systems has a separate memory, they are termed as distributed multicomputer systems. These are also known as loosely coupled systems, means that nodes have little coupling between them as shown in Figure

Figure: Loosely coupled system organisation

Because local memories are available to the connected processor only, no processor can access remote memory. So these systems are also called as no remote memory access (NORMA) systems. A message passing interconnection network offers connection to each node as well as internodes communication with message depends on the type of interconnection network. E.g. interconnection network for non hierarchical system can share bus.