[ The PC Guide | Systems and Components Reference Guide | Motherboard and System Devices | System
Resources | Input / Output (I/O) Addresses ]
I/O Address Details By Number
Here I describe some of the more interesting I/O addresses in use in the typical PC. Of
particular interest are those where conflicts are likely to occur, due to a large number
of devices using the address or offering it as an option. A complete list of I/O addresses
is provided in the summary in the next section:
- 060h and 064h: These two addresses are used by the keyboard controller, which
operates both the keyboard and the PS/2 style mouse (on devices that use it).
- 130-14Fh and 140-15Fh: These addresses are sometimes offered as options for SCSI
host adapters. Note that these options partially overlap (from 140-14Fh).
- 220-22Fh: This is the default address for many sound cards. It is also an option
for some SCSI host adapters (first 16 bytes).
- 240-24Fh: This is an optional address for sound cards and network cards (first 16
bytes for NE2000 cards).
- 260-26Fh and 270-27Fh: This is an optional address for sound cards and network
cards. NE2000-compatible network cards take 32 bytes; if set to use this I/O address, they
will conflict with several system devices as well as the I/O address for either LPT2 or
LPT3 in the 270-27Fh area.
- 280-28Fh: This is an optional address for sound cards and network cards (first 16
bytes for NE2000 cards).
- 300-30Fh: This is the default for many network cards (NE2000 cards extend to
31Fh). 300-301h is also an option for the MIDI port on many sound cards.
- 320-32Fh and 330-33Fh: This is a busy area in the I/O memory map. First, 330-331h
is the default for the MIDI port on many sound cards. 320-33Fh is an option for some
NE2000-compatible network cards and will conflict with the MIDI port at this setting. Some
SCSI host adapters also offer 330-34Fh as an option. Finally, the old PC/XT hard disk
controller also uses 320-323h.
- 340-34Fh: Optional areas for several device types overlap here, including two
options for SCSI host adapters (330-34Fh and 340-35Fh) as well as network cards.
- 360-36Fh and 370-37Fh: This is another "high traffic" area. 378-37Fh is
used on most systems for the first parallel port, and 376-377h is used for the secondary
IDE controller's slave drive. These can conflict with an NE2000-compatible network card
placed at location 360h. Tape accelerator cards often default to 370h, which will also
conflict with a network card placed at 360h).
- 3B0-3BBh and 3C0-3DFh: These are used by VGA video adapters. They take all of the
areas originally assigned for monochrome cards (3B0-3BBh), CGA adapters (3D0-3DFh) and EGA
- 3E8-3EFh: There is a potential conflict here in locations 3EE-3EFh if you are
using a third serial port (COM3) and a tertiary IDE controller.
- 3F0-3F7h: There is actually a "standard" resource conflict here: the
floppy disk controller and the slave drive on the primary IDE controller "share"
locations 3F6-3F7h. These devices are actually both present in many systems. Fortunately,
this conflict (which exists for historical reasons) is fairly well known and compensated
for, so it will not result in problems in a typical system. Note that some tape
accelerator cards also offer the use of 3F0h as an option, which will conflict with the
floppy disk controller.
Next:I/O Address Summary Map
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