CompTIA A+ Certification All-in-One Exam Guide

Figure 3-18 Address bus
By turning the address bus wires on and off in different patterns, the CPU
tells the MCC which line of RAM it wants at any given moment. Every
different pattern of ones and zeros on these 20 wires points to one byte of
RAM. There are two big questions here. First, how many different patterns of
on-and-off wires can exist with 20 wires? And second, which pattern goes to
which row of RAM?
How Many Patterns?
Mathematics can answer the first question. Each wire in the address bus
exists in only one of two states: on or off. If the address bus consisted of only
one wire, that wire would at any given moment be either on or off.
Mathematically, that gives you (pull out your old pre-algebra books) 2 1 = 2
different combinations. If you have two address bus wires, the address bus
wires create 2 2 = 4 different combinations. If you have 20 wires, you would
have 2 20 (or 1,048,576) combinations. Because each pattern points to one line
of code and each line of RAM is one byte, if you know the number of wires
in the CPU’s address bus, you know the maximum amount of RAM that a
particular CPU can handle.