Vol. 1 2-1

CHAPTER 2

INTEL

®

 64 AND IA-32 ARCHITECTURES

The exponential growth of computing power and ownership has made the computer one of the most important 
forces shaping business and society. Intel 64 and IA-32 architectures have been at the forefront of the computer 
revolution and is today the preferred computer architecture, as measured by computers in use and the total 
computing power available in the world.

2.1 

BRIEF HISTORY OF INTEL

®

 64 AND IA-32 ARCHITECTURE

The following sections provide a summary of the major technical evolutions from IA-32 to Intel 64 architecture: 
starting from the Intel 8086 processor to the latest Intel® Core® 2 Duo, Core 2 Quad and Intel Xeon processor 
5300 and 7300 series. Object code created for processors released as early as 1978 still executes on the latest 
processors in the Intel 64 and IA-32 architecture families.

2.1.1 

16-bit Processors and Segmentation (1978)

The IA-32 architecture family was preceded by 16-bit processors, the 8086 and 8088. The 8086 has 16-bit regis-
ters and a 16-bit external data bus, with 20-bit addressing giving a 1-MByte address space. The 8088 is similar to 
the 8086 except it has an 8-bit external data bus. 
The 8086/8088 introduced segmentation to the IA-32 architecture. With segmentation, a 16-bit segment register 
contains a pointer to a memory segment of up to 64 KBytes. Using four segment registers at a time, 8086/8088 
processors are able to address up to 256 KBytes without switching between segments. The 20-bit addresses that 
can be formed using a segment register and an additional 16-bit pointer provide a total address range of 1 MByte.

2.1.2 The 

Intel

®

 

286 Processor (1982)

The Intel 286 processor introduced protected mode operation into the IA-32 architecture. Protected mode uses the 
segment register content as selectors or pointers into descriptor tables. Descriptors provide 24-bit base addresses 
with a physical memory size of up to 16 MBytes, support for virtual memory management on a segment swapping 
basis, and a number of protection mechanisms. These mechanisms include: 

•

Segment limit checking

•

Read-only and execute-only segment options

•

Four privilege levels 

2.1.3 The 

Intel386

™

 Processor (1985)

The Intel386 processor was the first 32-bit processor in the IA-32 architecture family. It introduced 32-bit registers 
for use both to hold operands and for addressing. The lower half of each 32-bit Intel386 register retains the prop-
erties of the 16-bit registers of earlier generations, permitting backward compatibility. The processor also provides 
a virtual-8086 mode that allows for even greater efficiency when executing programs created for 8086/8088 
processors. 
In addition, the Intel386 processor has support for:

•

A 32-bit address bus that supports up to 4-GBytes of physical memory

•

A segmented-memory model and a flat memory model

•

Paging, with a fixed 4-KByte page size providing a method for virtual memory management

•

Support for parallel stages