In the figure, the two areas of the address space available for memory are not contiguous.
Instead, a hole is located between them. Furthermore, a hole is located
between the second memory area and the device area.
When a computer system is constructed, the owner must follow the address map.
For example, the sixteen-bit bus in Figure 14.14 only allows two blocks of memory that
total 32,768 bytes. The owner can choose to install less than a full complement of
memory, but not more.
The device space in a bus address map is especially interesting because the space
reserved for devices is often much larger than necessary. In particular, most address
maps reserve a large piece of the address space for devices, making it possible for the
bus to accommodate extreme cases with thousands of devices. However, a typical computer
has fewer than a dozen devices, and a typical device uses a few addresses. The
consequence is:
In a typical computer, the part of the address space available to devices
is sparsely populated — only a small percentage of the addresses
are used.