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Classification of Computer Architectures 211

Processor

2

Processor

1

Processor

n

Shared memory

Data stream 1

Data stream 2

...... Data stream n

Master

control

unit

Instruction stream

Figure 16.2. SIMD computer

16.2.3 Multiple Instruction, Single Data Stream

The MISD architecture consists of multiple processors. Each processor executes

its own unique set of instructions (Fig. 16.3). However, all processors share a

single common data stream. Different processors execute different instructions

simultaneously to the same data stream. No practical example of a MISD has been

identified to date, and this architecture remains entirely theoretical.

16.2.4 Multiple Instruction, Multiple Data Streams

The MIMD architecture consists of a number of processors. They can share and

exchange data. Each processor has its own instruction and data stream, and all

processors execute independently. The processors used in MIMD computers are

usually complex contemporary microprocessors.

The MIMD architecture is becoming increasingly important as it is generally

recognized as the most flexible form of parallel computer (Kumar, 1994). A col￾lection of heterogeneous computers interconnected by a local network conforms

to the MIMD architecture.

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212 16. Computer Architecture

Processor

2

Processor

1

Processor

n

Shared memory

......

Control

unit

Instruction stream 2

Control

unit

Control

unit

Instruction stream 1 Instruction stream n

Data stream

Figure 16.3. MISD computer

MIMD computers are significantly more difficult to program than traditional

serial computers. Independent programs must be designed for each processor. The

programmer needs to take care of communication, synchronization and resource

allocation. MIMD architecture can be further divided into three categories accord￾ing to the method of connection between memory and processors.

16.2.4.1 Multicomputer (Distributed Memory Multiprocessor)

There is no global memory in Multicompter. Each processor has its own local

memory and works like a single-processor computer. A processor cannot read

data from other processors’ memory. However, it can read its own memory and

pass that data to another processor.

Synchronization of processes is achieved through message passing. They can be

scaled up to a large number of processors. Conceptually, there is little difference

between the operation of a distributed memory multiprocessor and that of a col￾lection of different computers operating over a local network or Internet/Intranet.

Thus, P2P network can be considered as multicomputer (Fig. 16.4).

16.2.4.2 Loosely Coupled Multiprocessor (Distributed

Shared Memory Multiprocessor)

A well-known example (Stone, 1980) of a loosely coupled multiprocessor is Cm*

of Carnegie-Mellon University. Each processor has its own local memory, local