There are 65,536 bytes available between 0000h and FFFFh. Table 13-3 does not list ev-
ery possible I/O address assignment, but because they are uniform in size or layout, there
are on occasion not enough I/O ports to go around. In addition to those listed in Table 13-3,
there are several other I/O address assignments that are commonly used for supplemental
space for some devices, such as IDE bus mastering, serial ports, parallel ports, and IDE con-
trollers, which further complicates their assignments.
I/O addresses are intended to be assigned to a single device. Multiple devices sharing
an I/O port would have the same disastrous results as multiple active devices sharing an
IRQ.TherewouldbenowayfortheCPUorthedevicestoknowthedeviceamessagewas
intended for or from which data was being sent. There are some legacy situations, such as
on parallel ports and ISA adapters, where it is necessary on occasion to double up on a
particular address, but they are disappearing. ISA adapter cards typically can be config-
ured to only one or two I/O addresses on the card with a jumper or DIP switch setting.
This limitation can create an I/O address collision with another legacy device.I/O Addresses in Windows
Like IRQs, I/O address assignments can be viewed on a Windows PC through the Device
Manager. Figure 13-12 shows the Computer Properties window with the I/O addresses
resources displayed. As you scroll down the list, you will see the PC’s devices assigned to
various I/O addresses. You may also see entries that are listed as In Use by an Unknown
Device or listed as Alias To entries for devices requiring additional space.
As with IRQs, you can access an individual device (see Figure 13-13) to view its spe-
cific I/O address assignment. You can also resolve any conflicts listed by assigning the(^300) PC Hardware: A Beginner’s Guide
Figure 13-12. The Computer Properties window displaying I/O address assignments