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have a local memory distributed to each processor topther with a global memory accessible by
al1 processors. Shared memory MiMD computen are often called multiprocessor computen.
Message passing MIMD systems are often cailed multicomputers. In message passing
cornputers. each processor has its own local memory, but they do not have shared mernory, and
processors communicate only by sending and receiving messages over a communication network.
Each processor in message passing machines executes its own instruction streams and uses its
own local data, both stored in its local memory. Then. necessary data can be exchanged by
sending messages to each other over a network. Since the network determines the speed and
reliability of interpmcessor communication and the spatial distribution of the processoa. message
passins systems can be funher characterized into closely coupled systems and loosely coupled
systems (or distributed systems) by their communications networks.
Closely coupled systems. such as the architectures with mesh or hypercube networks.
have fast and reliable point-to-point links between processors which are physically close to one
another. Loosely coupled systems or distributed systems, such as workstations connected by local-
are3 network (LW) or wide-ares network (WAN), have relativeiy slow and unreliable
communication between processon that are physically dispersed. They have coarse-grained
parallelism. which distributes the data as large as possible with each processor spending most of
its time dotng computations and comrnunicating infrequently because of the expensive
communication cost.
The local-are? network 0 systems consist of several workstations connected by a
network. The locd area network allows communication beiween any two processors physically
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