What does all of this mean? For Linux users only one thing:
that they must make sure that LILO and
How does LILO know about the geometry?
It asks the kernel, using the
How does the kernel know what to answer?
Well, first of all, the user may have specified an explicit geometry
with a `
Note that values guessed by the kernel are very unreliable. The kernel does not have a good way of finding out what values the BIOS uses, or indeed whether the disk is known to the BIOS at all.
It is possible (since Linux 2.1.79) to change the kernel's ideas
about the geometry by using the
This is especially useful if you need so many boot parameters that you overflow LILO's (very limited) command line length. (It also helps if you want to influence a utility that gets its idea of the geometry from the kernel via the HDIO_GETGEO ioctl.)
Since Linux 2.6.5 the kernel will (when compiled with CONFIG_EDD)
ask the BIOS for legacy_cylinders, legacy_heads, legacy_sectors
using INT 13/AH=08h. The values obtained are made available in
Thus, we see here a disk with 255 heads and 120064896 sectors in all. Careful comparison shows that this is
How does the BIOS know about the geometry? The user may have specified it in the CMOS setup. Or the geometry is read from the disk, and possibly translated as specified in the setup. In the case of SCSI disks, where no geometry exists, the geometry that the BIOS has to invent can often be specified by jumpers or setup options. (For example, Adaptec controllers have the possibility to choose between the usual H=64, S=32 and the `extended translation' H=255, S=63.) Sometimes the BIOS reads the partition table to see with what geometry the disk was last partitioned - it will assume that a valid partition table is present when the 55aa signature is present. This is good, in that it allows moving disks to a different machine. But having the BIOS behaviour depend on the disk contents also causes strange problems. (For example, it has been reported that a 2.5 GB disk was seen as having 528 MB because the BIOS read the partition table and concluded that it should use untranslated CHS. Another effect is found in the report that unpartitioned disks were slower than partitioned ones, because the BIOS tested 32-bit mode by reading the MBR and seeing whether it correctly got the 55aa signature.)
How does the disk know about the geometry? Well, the manufacturer invents a geometry that multiplies out to approximately the right capacity. Many disks have jumpers that change the reported geometry, in order to avoid BIOS bugs. For example, all IBM disks allow the user to choose between 15 and 16 heads, and many manufacturers add jumpers to make the disk seem smaller than 2.1 GB or 33.8 GB. See also below. Sometimes there are utilities that change the disk firmware.
Sometimes it is useful to force a certain geometry
by adding `
gives two ways of finding the total number of sectors 71346240. Recent kernels also give the precise size in the boot messages:
Older kernels only give MB and CHS. In general the CHS value is rounded down, so that the above output tells us that there are at least 70780*16*63=71346240 sectors. In this example that happens to be the precise value. The MB value may be rounded instead of truncated, and in old kernels may be `binary' (MiB) instead of decimal. Note the agreement between the kernel size in MB and the Maxtor model number. Also in the case of SCSI disks the precise number of sectors is given in the kernel boot messages:
Next Previous Contents