Computer Systems A Programmer‘s Perspective Second Edition
As we will discuss, the extension of IA32 to 64 bits, termed x86-64, was origi-
nally developed by Advanced Micro Devices (AMD), Intel’s biggest competitor.
Whereas a 32-bit machine can only make use of around 4 gigabytes (2
32 bytes) of random-access memory, current 64-bit machines can use up to 256 terabytes (2
48 bytes). The computer industry is currently in the midst of a transition from 32-
bit to 64-bit machines. Most of the microprocessors in recent server and desktop
machines, as well as in many laptops, support either 32-bit or 64-bit operation.
However, most of the operating systems running on these machines support only
32-bit applications, and so the capabilities of the hardware are not fully utilized.
As memory prices drop, and the desire to perform computations involving very
large data sets increases, 64-bit machines and applications will become common-
place. It is therefore appropriate to take a close look at x86-64. We will see that in
making the transition from 32 to 64 bits, the engineers at AMD also incorporated
features that make the machines better targets for optimizing compilers and that
improve system performance.
256 terabytes random-access memory
原文：http://www.cnblogs.com/yuanjiangw/p/6133537.html
