The bits that comprise physical memory are divided into blocks of N bits per
block, where N is the memory transfer size. A block of N bits is sometimes called a
word, and the transfer size is called the word size or the width of a word.
Each word of physical memory is assigned a unique number known as a physical
memory address; the approach is known as word addressing. As Figure 10.6 illustrates,
we can think of physical memory as an array of words, with addresses starting at zero.