2.4 I/O Addresses (principles relevant to other resources too)
Devices were originally located in I/O address space but today they may use space in main memory. An I/0 address is sometimes just called "I/O", "IO", "i/o" or "io". The terms "I/O port" or "I/O range" are also used. Don't confuse these IO ports with "IO memory" located in main memory. There are two main steps to allocate the I/O addresses (or some other bus-resources such as interrupts on the ISA bus):
- Set the I/O address, etc. in the hardware (in one of its registers)
- Let its device driver know what this I/O address, etc. is
Often, the device driver does both of these (sort of). The device driver doesn't actually need to set an I/O address if it finds out that the address has been previously set (perhaps by the BIOS) and is willing to accept that address. Once the driver has either found out what address has been previously set or sets the address itself, then it obviously knows what the address is so there is no need to let the driver know the address --it already knows it.
The two step process above (1. Set the address in the hardware. 2. Let the driver know it.) is something like the two part problem of finding someone's house number on a street. Someone must install a number on the front of the house so that it may be found and then people who might want to go to this address must obtain (and write down) this house number so that they can find the house. For computers, the device hardware must first get its address put into a special register in its hardware (put up the house number) and then the device driver must obtain this address (write the house number in its address book). Both of these must be done, either automatically by software or by entering the data manually into configuration files. Problems may occur when only one of them gets done right.
For manual PnP configuration some people make the mistake of doing only one of these two steps and then wonder why the computer can't find the device. For example, they may use "setserial" to assign an address to a serial port without realizing that this only tells the driver an address. It doesn't set the address in the serial port hardware itself. If you told the driver wrong then you're in trouble. Another way to tell the driver is to give the address as an option to a kernel module (device driver). If what you tell it is wrong, there could be problems. A smart driver may detect how the hardware is actually set and reject the incorrect information supplied by the option (or at least issue an error message).
An obvious requirement is that before the device driver can use an address it must be first set in the physical device (such as a card). Since device drivers often start up soon after you start the computer, they sometimes try to access a card (to see if it's there, etc.) before the address has been set in the card by a PnP configuration program. Then you see an error message that they can't find the card even though it's there (but doesn't yet have an address yet).
What was said in the last few paragraphs regarding I/O addresses applies with equal force to most other bus-resources: Memory Ranges, IRQs --Overview and DMA Channels. What these are will be explained in the next 3 sections. The exception is that interrupts on the PCI bus are not set by card registers but are instead routed (mapped) to IRQs by a chip on the motherboard. Then the IRQ a PCI card is routed to is written into the card's register for information purposes only.
To see what IO addresses are used on your PC, look at the /proc/ioports file.
* License
