IRQs And I/O Addresses, I/O Address Table

IRQs And I/O Addresses

Keep a record of the maintenance done on a computer, as well as the configuration information about that machine printed or on disk. In this notebook, dedicate a page to record the IRQ and I/O address assignments for the machine. When you decide to add a new device, the IRQs available for the device are at your fingertips if you keep the notebook up-to-date. Conflicts with interrupts arise when two peripherals have been assigned the same IRQ.

For example, because DOS does not use IRQ 7 to communicate with the printer, many older LAN cards are configured to use IRQ 7. However, Windows does use IRQ 7 to communicate with the printer. Therefore, if you are using Windows with a LAN card that uses IRQ 7, a conflict arises. You usually can solve these conflicts by changing the IRQ for the LAN card. Usually the card has a DIP switch or jumper that can change the IRQ.

In this situation, if you are using Windows 9x, the Plug-and-Play feature might not help you because the older LAN card is not Plug-and-Play. Even with Windows 9x, you might still need to assign the IRQ manually by setting the DIP switch on the board and informing Windows 9x of what you have done. Newer buses are designed to allow more than one device to share an IRQ.

I/O Address Table

The I/O address table, which is sometimes called the interrupt descriptor table, is an area at the beginning of RAM from 0000 to 03FFh (the table can take up more room if needed). Earlier PCs used a 4-byte table, called the interrupt vector table, which contained only the location in memory of the program that serviced the device issuing the

interrupt.

Beginning with 286 computers, the interrupt vector table was expanded to hold not only the memory address of the program that services the device, but also the addresses of data passed between the CPU and the device, as well as the commands (called tasks) communicated to the device. The I/O address table serves as the place where the CPU and the peripheral device communicate and it functions like a drop box or pick-up desk. Each device is assigned a range of I/O addresses that can be used by only a single device. If two devices try to share the same address, conflicts arise.

The ROM BIOS requests an I/O address during the boot process for a device. If two devices use the same I/O address, they cannot be installed on the same computer unless one of the devices can be assigned an alternative address by changing the device configuration.

The program that services the device is called the interrupt handler. This program can be a device driver stored in RAM, part of System BIOS, or part of ROM BIOS stored on the device controller. During booting, the I/O address range is assigned to a device, and the location of the interrupt handler is stored in a coded format in part of those I/O addresses.

The I/O addresses in the I/O address table can be accessed by the operating system to store data or tasks passed between the CPU and the device. An interrupt or IRQ can prompt the CPU to service a device, receive data, or perform other operations.