Computer Systems A Programmer's Perspective Second Edition
Running throughout the system is a collection of electrical conduits called
buses that carry bytes of information back and forth between the components. Buses
are typically designed to transfer fixed-sized chunks of bytes known as words. The
number of bytes in a word (the word size) is a fundamental system parameter that
varies across systems. Most machines today have word sizes of either 4 bytes (32
bits) or 8 bytes (64 bits). For the sake of our discussion here, we will assume a word
size of 4 bytes, and we will assume that buses transfer only one word at a time.
Every computer has a word size, indicating the nominal size of integer and pointer
data. Since a virtual address is encoded by such a word, the most important system
parameter determined by the word size is the maximum size of the virtual address
space. That is, for a machine with a w-bit word size, the virtual addresses can range
from 0 to 2w−1, giving the program access to at most 2w bytes.
Most personal computers today have a 32-bit word size. This limits the virtual
address space to 4 gigabytes (written 4 GB), that is, just over 4×109 bytes.
Although this is ample space for most applications, we have reached the point where
many large-scale scientific and database applications require larger amounts of
storage. Consequently, high-end machines with 64-bit word sizes are becoming in-
creasingly common as storage costs decrease. As hardware costs drop over time,
even desktop and laptop machines will switch to 64-bit word sizes, and so we will
consider the general case of aw-bit word size, as well as the special cases of w=32
and w=64.