Logically, a bridge performs a one-to-one mapping from the address space of one
bus to the address space of another. That is, the bridge makes a set of addresses on one
bus appear in the address space of the other. To understand why bridging is popular,
consider a common case where an architect needs to add a new device to a computer
that already has a bus. If the interface on the new device does not match the
computer’s main bus, the architect can design adapter hardware or use a bridge to add
an auxiliary bus to the system. Using a bridge has two advantages: a computer owner
can add other devices to the auxiliary bus without changing the hardware, and an architect
has less work because bridges are available for most buses. Figure 14.16 illustrates
the concept of address mapping.