Everything in a computer, whether hardware or software, requires a unique name and address for the CPU to be able to identify what is going on. Bus-mastering devices might seem to get around this requirement, but they have their own controllers that track local traffic and "talk" to the CPU as needed.
I/O addresses are patterns of 1s and 0s transmitted across the address bus by the CPU. The CPU must identify the device before any data is placed on the bus. The CPU uses two bus wires-the IOR (Input/Output Read) wire and the IOW (Input/Output Write) wire-to notify the devices that the address bus is not being used to specify an address in memory, but rather to read to or write from a particular device. The address bus has at least 20 wires. However, when the IOW or IOR wire has voltage, only the first 16 wires are monitored.
To allow communication directly between the CPU and a device, each device responds to unique patterns or code built into it. If the CPU needs to check the error status of a hard disk drive controller, for instance, it activates the IOW wire and puts the correct pattern of 1s and 0s onto the address bus. The controller then sends back a message describing its error status.
All I/O addresses define the range of patterns assigned to each device's command set. The device ignores all commands outside its range. All devices must have an I/O address, and no two devices can have overlapping ranges. Basic devices on the address list have preset I/O addresses that cannot be changed. Other devices must be assigned to the open addresses, and they must be configured at installation. The following table lists standard PC I/O port address assignments.
|PC/XT Port||Used By||PC/XT Port||Used By|
|000h-00Fh||DMA chip 8237A||2F0h-2F7h||Reserved|
|040h-043h||PIT 8253||300h-31Fh||Prototype adapter|
|060h-063h||PPI 8255||320h-32Fh||Hard disk controller|
|080h-083h||DMA page register||378h-37Fh||Parallel interface|
|0A0h-0AFh||NMI mask register||380h-38Fh||SDLC adapter|
|0E0h-0EFh||Reserved||3B0h-3BFh||Monochrome adapter/ parallel interface|
|220h-24Fh||Reserved||3F0h-3F7h||Floppy disk controller|
|AT Port||Used By||AT Port||Used By|
|000h-00Fh||First DMA chip 8237A||278h-27Fh||Second Parallel interface|
|020h-021h||First PIC 8259A||2B0h-2DFh||EGA|
|060h-063h||Keyboard controller 8042||300h-31Fh||Prototype adapter|
|080h-083h||DMA page register||378h-37Fh||First parallel interface|
|0A0h-0AFh||Second PIC 8259A||380h-38Fh||SDLC adapter|
|0C0h-0CFh||Second DMA chip 8237A||3A0h-3AFh||Reserved|
|0E0h-0EFh||Reserved||3B0h-3BFh||Monochrome adapter/parallel interface|
|0F0h-0FFh||Reserved for coprocessor 80287||3c0h-3CFh||EGA|
|210h-217h||Reserved||3F0h-3F7h||Floppy disk controller|
I/O addresses have several important characteristics to remember:
- I/O addresses have 16 bits; they are displayed with a hexadecimal number.
- By convention, the lead 0 is dropped (because all I/O addresses have it).
- Hexadecimal I/O addresses must use capital letters; they are case sensitive.
Setting I/0 Addresses
As mentioned, each device in a computer must have an I/O address. If a device qualifies as a basic device, it will have a standard, preset I/O address. The default setting for the I/O address will work and no changes are required.
If a device is not a basic device, and does not conform to the PCI Plug and Play specification on a Plug and Play-compatible system, read the manual that came with it. The manual will explain how to set the I/O address and define the limits for that device. I/O addresses are set by changing jumpers, switches, or through use of software.
On Plug and Play systems, PCI cards are self-configuring, and usually no intervention is needed to set I/O addresses for those cards. It is possible for Plug and Play cards to conflict with older ISA cards, which don't recognize the Plug and Play devices. If you are confronted with this problem, refer to the cards and the motherboard manual for possible resolution.
Managing I/0 Addresses
Devices assigned overlapping I/O addresses usually do not respond to commands and stop functioning. In such a scenario, a modem will dial but not connect; a sound card will start to play but will stop; a mouse pointer will appear but the mouse will not move. I/O overlaps can sometimes cause the machine to lock up intermittently.
I/O overlaps never happen independently. They usually appear immediately after a new device is installed. The best way to prevent I/O address overlaps is to document all I/O addresses. There are many commercially available programs that will check the I/O addresses for every device on your computer. You can also use Microsoft Diagnostics (MSD), a program provided with MS-DOS.
If you are running Windows 95 or Windows 98 or the Windows NT operating system, you can use the Device Manager or System Information to locate and resolve IRQ and address conflicts. (See Tutorial 16, "Windows 95 and Beyond," for more information on the Device Manager.)