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The number of cylinders on each side of each platter in the disk. For older drives, this is the number of physical cylinders the disk uses. For newer disks, it is the logical number of cylinders that the drive specifies for use in the BIOS setup; see here for an explanation of physical and logical geometry.
For newer drives using BIOS geometry translation, the BIOS reduces the number of logical cylinders by dividing it by an integer factor (2, 4, etc.) so that it is less than 1,024 (the BIOS limit for the number of cylinders for historical reasons) and then multiplies the number of heads by the same number.
For example, a Western Digital Caviar 33100 3.1 GB drive has nominal parameters (logical geometry) of 6,136 cylinders, 16 heads and 63 sectors. When you set up this drive (autodetect it) most BIOSes will record 767 cylinders, 128 heads and 63 sectors. Of course the drive doesn't really have only 767 cylinders, but this is the way that the BIOS gets around the infamous 504 MB restriction. See here for more details on BIOS translation.
Note: Some BIOSes will
continue to show the actual physical parameters even when set to a translating mode like
LBA. This can be confusing to see on the screen, but if the BIOS is set to "LBA"
or "Large", it should be using the translated parameters internally.
Note: IDE autodetection
will set this value automatically.