One of the most commonly encountered problems is hardware that is not recognized properly. This problem can happen to a CD-ROM, a network card, and even a hard disk. Most of the time, a conflict in the IRQ (interrupt), DMA (Direct Memory Address), or I/O address settings causes this problem. When two devices have the same settings on any one of these three characteristics, Linux and the BIOS may not be able to communicate with the device properly.

A symptom of this problem may be Linux hanging when it tries to find a specific device, as explained in the boot messages. When Linux boots up, it generates messages that explain what it is doing. If you see a message that it is trying to connect to the network card, for example, and it never gets past that point, chances are that the network card has a conflict with another device. (Totally failed cards are very rare and don’t usually stop the boot process, as Linux ignores devices it can’t access. The problem with a working card with conflicting settings is that Linux is getting messages from two devices that don’t act the same.)

To check for conflicts, run a diagnostic utility under DOS, such as MSD or Norton Info. These utilities can show you the current IRQ, DMA, and I/O addresses and pinpoint any conflicts. you can also use them to find available settings. Alternatively, you can check the settings of every device in your system for conflicts. Usually, network cards conflict with sound boards, non-SCSI tape driver cards, video cards, and similar add-on cards. Most cards use DIPs or jumpers to set these parameters, so check them against the documentation. To help isolate the problem, remove cards that are not necessary, such as a sound card, and see whether the boot process moves past the device that caused the hangup.

Another problem that can occur is with SCSI devices (and a few others, although much rarer) that must have specific settings in the kernel image. Some kernels, especially special-purpose kernels that have been developed for non-mainstream adapters, were compiled with settings that are default values for adapters or disk drives, and if the settings have been changed, the kernel hangs up. To check for this type of problem, investigate any documentation that came with the kernel image.

The most common devices in a PC (COM ports, parallel ports, and floppy disks) and their IRQ, DMA, and I/O addresses are shown in in the following list. These are the default values for a PC, but they may be changed by users. Because only two COM ports (serial ports) are usually supported by DOS, they share IRQ values. The I/O addresses are different, though. Both floppy disks share the same I/O addresses, IRQ, and DMA.
Device IRQ DMA I/O Address (Hex)
COM 1 (/dev/ttyS0) 4 N/A 3F8
COM 2 (/dev/ttyS1) 3 N/A 2F8
COM 3 (/dev/ttyS2) 4 N/A 3E8
COM 4 (/dev/ttys3) 3 N/A 2E8
LPT 1 (/dev/lp0) 7 N/A 378-37F
LPT 2 (/dev/lp1) 5 N/A 278-27F
Floppy A (/dev/fd0) 6 2 3F0-3F7
Floppy B (/dev/fd1) 6 2 3F0-3F7

You may have noticed that the serial ports are called /dev/ttyS0, /dev/ttyS1, and so on in the list of devices. Yet they were called /dev/cua0, /dev/cua1, and so on when you configured the modem. The ports are the same (/dev/ttyS0 is the same as /dev/cua0); Linux just handles the devices differently. Don’t get too confused about these device driver names yet. Just remember that /dev/cua refers to a modem port.

Network cards, SCSI adapters, sound boards, video cards, and other peripherals all must have unique IRQ, DMA, and I/O addresses, which can be difficult to arrange with a fully loaded system. For more information on available values, check your device or card installation manual for recommended values and potential conflicts.