The Intel 80x86 Family
Modern Intel processors can trace their heritage all the way back to the earliest Intel chips. As
customers became more sophisticated and demanding in their use of chip sets, Intel was always ready
with compatible follow-on processors. Compatibility made it easy for customers to move to newer
chips, but it severely restricted the amount of innovation that was possible. The modern Pentium is a
direct descendant of Intel's 8086 from 15 years before, and it contains architectural irregularities to
provide backwards compatibility with it (meaning that programs compiled for an 8086 will run on a
Pentium). Referring to the innovation/compatibility trade-off, some people have unkindly commented
that "Intel puts the 'backward' in 'backward compatible'..." (see Figure 7-1).
Figure 7-1. The Intel 80x86 Family: Putting the "Backward" in "Backward Compatible"The Intel 4004 was a 4-bit microcontroller built in 1970 to satisfy the specific needs of a single
customer, Busicom—a Japanese calculator company. The Intel design engineer conceived the idea of
producing a general-purpose programmable chip, instead of the custom logic for each different
customer that was the rule at the time. Intel thought they'd sell a few hundred, but a general-purpose
design turned out to have vastly wider applicability. Four bits was too limiting, so in April 1972 an 8-
bit version, the 8008, was launched. Two years later, that in turn spawned the 8080, which was the
first chip powerful enough to be called a microcomputer. It included the entire 8008 instruction set
and added 30 more instructions of its own, initiating a trend that continues to this day. If the 4004 was