A basic understanding of the BIOS is a fundamental skill for all serious computer users. While many modern computers allow you to boot directly from the media of your choice, such as an RHEL 5 installation CD or a rescue USB key, that may not be possible during your troubleshooting. Therefore, you need to know how to modify the BIOS menu to boot from
the media of your choice.
Basics of the BIOS
When you power up a computer successfully, the first thing that starts is the BIOS. Based on settings stored in stable, read-only memory, BIOS performs a series of diagnostics to detect and connect the CPU and key controllers. This is known as the Power On Self Test (POST). If you hear beeps during this process, you may have a hardware problem such as an improperly connected hard drive controller. The BIOS then looks for attached devices such as the graphics card. After the graphics hardware is detected, you may see a screen. Once complete, the BIOS passes control to the MBR of the boot device, normally the first hard drive. At this point, you should see a boot loader screen.
The default boot loader is GRUB, and the first part of it is installed in the MBR of the default drive. Normally, the BIOS should automatically start the boot loader, with a message similar to: Booting Red Hat Enterprise Linux Server (2.6.18-8.el5) in 5 seconds…
If you’re working with an older PC, the BIOS can’t find your boot loader unless it’s located within the first 1024 cylinders of the hard disk, which is why the /boot partition is normally a primary partition.
BIOSs overcome this problem with logical block addressing, which is also known as LBA mode. LBA mode reads “logical” values for the cylinder, head, and sector, which allows the BIOS to “see” a larger disk drive. If you have multiple hard drives, there is one more caveat. If your drives are IDE (PATA) hard drives, the /boot directory must be on a hard drive attached to the primary IDE controller. If your drives are all SCSI hard drives, the /boot directory must be located on a hard drive with SCSI ID 0 or ID 1. If you have a mix of hard drives, the /boot directory must be located on either the first IDE drive or a SCSI drive with ID 0. However, I believe most computers used for the Red Hat exams have only one hard drive. This saves costs for the exam site and means you probably won’t have to worry about which hard drive contains the /boot