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CHAPTER 2
INTRODUCTION TO THE INTEL ARCHITECTURE
A strong case can be made that the exponential growth of both the power and breadth of usage
of the computer has made it the most important force that is reshaping human technology, busi-
ness, and society in the second half of the twentieth century. Further, the computer promises to
continue to dominate technological growth well into the twenty-first century, in part since other
powerful technological forces that are just emerging are strongly dependent on the growth of
computing power for their own existence and growth (such as the Internet, and genetics devel-
opments like recombinant DNA research and development). The Intel Architecture (IA) is
clearly todays preferred computer architecture, as measured by number of computers in use and
total computing power available in the world. Thus it is hard to overestimate the importance of
the IA.
2.1.BRIEF HISTORY OF THE INTEL ARCHITECTURE
The developments leading to the IA can be traced back through the 8085 and 8080 microproces-
sors to the 4004 microprocessor (the first microprocessor, designed by Intel in 1969). However,
the first actual processor in the IA family is the 8086, quickly followed by a more cost effective
version for smaller systems, the 8088. The object code programs created for these processors
starting in 1978 will still execute on the latest members of the IA family.
The 8086 has 16-bit registers and a 16-bit external data bus, with 20-bit addressing giving a 1-
MByte address space. The 8088 is identical except for a smaller external data bus of 8 bits.
These processors introduced IA segmentation, but only in Real Mode; 16-bit registers can act
as pointers to address into segments of up to 64 KBytes in size. The four segment registers hold
the (effectively) 20-bit base addresses of the currently active segments; up to 256 KBytes can
be addressed without switching between segments, and a total address range of 1 MByte is avail-
able.
The Intel 80286 processor introduced the Protected Mode into the IA. This new mode uses the
segment register contents as selectors or pointers into descriptor tables. The descriptors provide
24-bit base addresses, allowing a maximum physical memory size of up to 16 MBytes, support
for virtual memory management on a segment swapping basis, and various protection mecha-
nisms. These include segment limit checking, read-only and execute-only segment options, and
up to four privilege levels to protect operating system code (in several subdivisions, if desired)
from application or user programs. Furthermore, hardware task switching and the local
descriptor tables allow the operating system to protect application or user programs from each
other.
The Intel386 processor introduced 32-bit registers into the architecture, for use both as oper-
ands for calculations and for addressing. The lower half of each 32-bit register retained the prop-
erties of one of the 16-bit registers of the earlier two generations, to provide complete upward
compatibility. A new virtual-8086 mode was provided to yield greater efficiency when
