PARALLEL COMPUTER ARCHITECTURES - Structured Computer Organization - page 591

simultaneously. Again, because objects are a strictly software concept, the imple-
mentation can be done by the run-time system without help from the operating sys-
tem or hardware. We will look at both Linda and Orca later in this chapter.

Taxonomy of Parallel Computers

Now let us get back to our main topic, the architecture of parallel computers.
Many kinds of parallel computers have been proposed and built over the years, so
it is natural to ask if there is some way of categorizing them into a taxonomy.
Many researchers have tried, with mixed results (Flynn, 1972, and Treleaven,
1985). Unfortunately, the Carolus Linnaeus† of parallel computing is yet to
emerge. The only scheme that is used much is Flynn's, and even his is, at best, a
very crude approximation. It is given in Fig. 8-22.

Instruction streams Data streams Name Examples

1 1 SISD Classical Von Neumann machine

1 Multiple SIMD Vector supercomputer, array processor

Multiple 1 MISD Arguably none

Multiple Multiple MIMD Multiprocessor, multicomputer

Figure 8-22. Flynn's taxonomy of parallel computers.

Flynn's classification is based on two concepts—instruction streams and data
streams. An instruction stream corresponds to a program counter. A system with
n CPUs has n program counters, hence n instruction streams.
A data stream consists of a set of operands. For example, in a weather-fore-
casting system, each of a large number of sensors might emit a stream of tempera-
tures at regular intervals.
The instruction and data streams are, to some extent, independent, so four
combinations exist, as listed in Fig. 8-22. SISD is just the classical, sequential von
Neumann computer. It has one instruction stream, one data stream, and does one
thing at a time. SIMD machines have a single control unit that issues one instruc-
tion at a time, but they have multiple ALUs to carry it out on multiple data sets si-
multaneously. The ILLIAC IV (Fig. 2-7) is the prototype of SIMD machines.
Mainstream SIMD machines are increasingly rare, but conventional computers
sometimes have some SIMD instructions for processing audiovisual material. The
Core i7 SSE instructions are SIMD. Nevertheless, there is one new area in which
some of the ideas from the SIMD world are playing a role: stream processors.

† Carolus Linnaeus (1707-1778) was the Swedish biologist who devised the system now used for
classifying all plants and animals into kingdom, phylum, class, order, family, genus, and species.