Introduction to Multiprocessors - Fundamentals of Computer Organization and Architecture - page 238

computer architectures:

1. Single-instruction single-data streams (SISD)
2. Single-instruction multiple-data streams (SIMD)
3. Multiple-instruction single-data streams (MISD)
4. Multiple-instruction multiple-data streams (MIMD)

Figure 11.2 shows the orthogonal organization of the streams according to Flynn's
classification.
Schematics for the four categories of architectures resulting from Flynn's classi-
fication are shown in Figure 11.3. Table 11.2 lists some of the commercial machines
belonging to each of the four categories.

Observations on Flynn's Classification

1. Flynn's classification is among the first of its kind to be introduced and as
such it must have inspired subsequent classifications.
2. The classification helped in categorizing architectures that were available
and those that have been introduced later. For example, the introduction of
the SIMD and MIMD machine models in the classification must have
inspired architects to introduce these new machine models.
3. The classification stresses the architectural relationship at the memory-
processor level. Other architectural levels are totally overlooked.
4. The classification stresses the external (morphological) features of architec-
tures. No information is included on the revolutionary relationship of archi-
tectures that belong to the same category.
5. Owing to its pure abstractness, no practically viable machine has exemplified
the MISD model introduced by the classification (at least so far). It should,
however, be noted that some architects have considered pipelined machines
(and perhaps systolic-array computers) as examples for MISD.
6. A very important aspect that is lacking in Flynn's classification is the issue of
machine performance. Although the classification gives the impression that
machines in the SIMD and the MIMD are superior to their SISD and MISD
counterparts, it gives no information on the relative performance of SIMD
and MIMD machines.

Data Stream

Single Multiple

Instruction Single SISD SIMD

Stream Multiple MISD MIMD

Figure 11.2 Flynn's classification