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9. Consequences

What does all of this mean? For Linux users only one thing: that they must make sure that LILO and fdisk use the right geometry where `right' is defined for fdisk as the geometry used by the other operating systems on the same disk, and for LILO as the geometry that will enable successful interaction with the BIOS at boot time. (Usually these two coincide.)

How does fdisk know about the geometry? It asks the kernel, using the HDIO_GETGEO ioctl. But the user can override the geometry interactively or on the command line.

How does LILO know about the geometry? It asks the kernel, using the HDIO_GETGEO ioctl. But the user can override the geometry using the `disk=' option in /etc/lilo.conf (see lilo.conf(5)). One may also give the linear option to LILO, and it will store LBA addresses instead of CHS addresses in its map file, and find out of the geometry to use at boot time (by using INT 13 Function 8 to ask for the drive geometry).

How does the kernel know what to answer? Well, first of all, the user may have specified an explicit geometry with a `hda=cyls,heads,secs' kernel command line option (see bootparam(7)). And otherwise the kernel will guess, possibly using values obtained from the BIOS or the hardware.


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