Section 2 ½ The Computer Space 35

in biological evolution, where it is possible to draw a strict genealogical tree. Nevertheless, the term is useful in stressing that the history of computer systems is not just a story of particular scientists discovering or building particular things, but of a somewhat more impersonal and widespread series of advances that have changed computer systems radically.

The generations are best defined solely in terms of logic technology (see Table 1): the first generation is that of vacuum tubes (1945-1958); the second generation is that of discrete transistors (1958-1966); the third, small- and medium-scale integrated circuits (1966-1972); the fourth, large-scale integration with 100 ~ 10,000 gates per chip (1972-1978); and the fifth, very-large-scale integration (1978-). Chip complexities in the fourth generation were large enough to allow the integration of a processor on a single chip.

It is a measure of American industry's generally ahistorical view of things that the title of "first" generation has been allowed to be attached to a collection of machines that were some generations removed from the beginnings by any reasonable accounting. Mechanical and electromechanical computers existed prior to electronic ones. Furthermore, they were the functional equivalents of electronic computers and were realized to be such. They were also separated by a wide gap in performance and structure, both from each other and from vacuum-tube machines. Thus, by reasonable reckoning, we are currently in the seventh generation of computers, not the fifth. But usage is now too well established to change. The concept of precomputer generation handles this anomaly.

Actually, it was not always viewed thus. Figure 1 reproduces a genealogical tree of the early computers prepared by the National