Intel 80x86 Base Architecture
16.1 Why Study the 80x86?
Any introduction to processor architecture should be followed by an
investigation of the architecture of a specific processor. The choice then
becomes which processor to examine. There are so many. Some
approaches use a virtual processor, i.e., one that exists only on paper or
as a simulator. This method simplifies the learning process by
concealing the complexities and idiosyncrasies of a real processor.
At the other extreme, we could examine a modern processor such as
the Intel® Pentium® 4 Processor Extreme Edition with its Hyper-
Threading Technology™, Hyper-Pipelined Technology™, enhanced
branch prediction, three levels of 8-way cache including a split L1
cache, and multiple ALUs. Or we could look at the Apple® PowerPC®
G5 with its 64-bit architecture, two double-precision floating point
units, and twelve functional units. If you are a student who has just
been introduced to processor architecture, this can be like trying to
swallow an elephant. Too many new concepts must be explained before
even a minimal understanding of the processor can be had.
A third method is to examine the simplest processor from a family
of existing processors. This particular processor should provide the
closest match to the processor architecture discussed in Chapter 15
while providing a link to the most modern processor of the family. It
eliminates the need for a discussion of advanced computer architecture
concepts while giving the student a real processor that they can
The processor we present here is the original 16-bit Intel processor,
the 80186, the root of the Intel processor family that is commonly
referred to as the 80x86 family. The 'x' in 80x86 represents the
generation of the processor, 1, 2, 3, and so on. Table 16-1 presents a
summary of the bus characteristics of some of the 80x86 processors.
The 80186 has 16 data lines allowing it to perform operations on
unsigned integers from 0 to 216 – 1 = 65,535