LILO is usually able to obtain information about the hard disks and floppy disks on the system by reading the kernel. On some systems(especially some SCSI adapters and adapters that do not behave as IDE or SCSI normal devices), though, this isn’t possible. When LILO can’t obtain the disk parameter information, it generates an error message about “bad geometry.”

The disk parameters can be physically read by LILO from the file /etc/lilo/disktab, which exists only with some versions of Linux. When the disktab file exists, it takes precedence over any auto-detected values. The disktab file contains the device number (hexadecimal), its BIOS code, and the disk geometry. A sample disktab file could have the following entries:

# /etc/lilo/disktab - LILO disk paramter table

#

# Dev. num BIOS code Secs/track Heads/cyl Cyls Part. Offset

#

0×800 0×80 32 64 1714 0

0×801 0×80 32 64 1714 1001

This example shows a SCSI disk with two partitions. The first partition /dev/sda1 has a device number 800, and the second partition /dev/sda2 has the device number 801. Both partitions have the BIOS code 80. Both the device number and BIOS code have to be given in hex format, which accounts for the leading 0x. The disk has 32 sectors per track, 64 heads per cylinder, and 1714 cylinders. Because both partitions are on the same disk, these parameters will be the same.

The partition offset is an optional field. The first partition starts at offset 0, and the second starts at offset 1001 (the number of sectors from the start of the disk). The partition offsets need only be explicitly given when the kernel cannot obtain that information. Most hard disks (including removable and optical disks) don’t need the partition offsets, but CD-ROMs sometimes do.

When filling in the /etc/lilo/disktab file you don’t have to have the details exactly right. Most systems will remap the drive parameters to 32 sectors per track and 64 heads, whether those numbers are correct or not. (This is a BIOS action.) The number of cylinders must be at least equal to or higher than the number of actual cylinders to avoid truncation of the disk space the operating system recognizes.

Some BIOS versions will not allow disks exceeding certain values. This problem usually occurs with IDE and SCSI controller cards that are designed for DOS systems with relatively small (<1G) disk drives and older BIOSs. Device drivers allow some high-capacity drives to be used, although some systems will have a problem accessing files beyond the 1G limit.