The 8086 PC Family

One of the keys to understanding the PC family is understanding the
microprocessor that acts as the working "brains" of the computer. Unlike
the mainframe computer tradition, microcomputers like our PC aren't
designed and built in an independent way. Instead, nearly all microcomput­
ers incorporate many standard components that are designed independently
of the computers in which they are used.
If a computer maker, such as IBM, designs a computer from scratch,
then they can determine what the features and capabilities of the computer
will be. This includes the instruction set, or internal language, that the
computer will have. However, most microcomputers, including our PC,
aren't made that way. Instead, they get their thinking power (and instruc­
tion set) from one of several standard microprocessors offered by com­
puter chip makers. IBM could have chosen from several possible
microprocessors to serve as the brains of the PC. The one they chose
would both define the current instruction set, or language, for the PC, and
also define a great many things about the direction the computer could
take in the future.
IBM chose the Intel 8088 microprocessor as the brains or engine inside
the Pc. The 8088 is just one member of a whole family of microprocessors,
called the 8086 family, that was designed by the pioneering silicon chip
maker, Intel. By choosing the 8088 for the original PC, IBM committed the
PC family to live within the range of possibilities that are defined by its
microprocessor fanlily. To understand what the PC family is (and can
become), we need to understand the main points of its microprocessor
family, the 8086 family.

Before I cause any confusion, I need to make clear that while each
member of the PC family of computers uses a member of the 8086 micro­
processor family for its brain, there isn't any direct correspondence between
the PC family and the 8086 family. There isn't a separate member of the
PC family for each member of the 8086 family. Since each PC includes
something from the 8086 family, knowing about this family can help us
understand the directions that the PC family can take.
The founding member of the Intel 8086 family is the 8086 chip itself,
the chip that the whole family is named after. The 8086 was designed to
introduce the concept of 16-bit computing, which means that the computer
can deal with 16 bits of information at a time (we'll get a clearer idea of
what that means when we discuss bits and our computer's data in Chapter
3). The previous generation of Intel microprocessors, the 8080, were 8-bit
The 8086, as a 16-bit microprocessor, had a much greater range of
capabilities than its predecessors. The power of a microprocessor is only
very loosely implied by describing it as "8-bit" or "16-bit" or "32-bit";
the features of each new generation of computer chips go far beyond what
the bit rating suggests. But this bit rating does at least tell us how much data
the computer can sling around at a time, and the 8086 could sling twice as
much as the 8080 that went before it.
There was an inherent practical problem, though, in using the 8086 as
the base of a computer design. While the 8086 had 16-bit capabilities
internally-which is very good-it also had to work exclusively with other
computer components that handle 16 bits at a time as well. When the PC
was being designed, 8-bit parts were plentiful and cheap; 16-bit parts were
more expensive and in shorter supply. This presented an obstacle to anyone
designing a computer around the 8086.
Intel found a simple, practical solution to this problem with the 8088
chip. The 8088 internally has all the 16-bit skills of the 8086, but when it
communicates with other circuitry around it, it talks only 8 bits at a time;
this slightly reduces the speed of the 8088, but it makes it possible for the
8088 to work with other components that are cheap and plentiful.
For practical reasons, IBM designed the original model of PC around
the 8088-a microprocessor with 16-bit power, but 8-bit economy. The
8088 formed the heart of the first four models of PC from IBM-the PC,
the XT, the Portable PC, and the PCjr-as well as the first two Compaq
contributions to the family-the Compaq and the Compaq Plus. Most other
"PC clones" also used the 8088. However, when Compaq wanted to add
more computing power to their third model, the DeskPro, they used the
8086 for its greater speed.