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The number of read/write heads the disk uses. For older drives, this is the number of physical heads the disk uses. For newer disks, it is the logical number of heads that the drive specifies for use in the BIOS setup, usually 16; see here for an explanation of physical and logical geometry.
For newer drives supported using BIOS geometry translation the BIOS increases the number of heads by the same number it uses to reduce the number of cylinders, so that the number of cylinders is less than the BIOS limit of 1,024.
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) the BIOS will record 767 cylinders, 128 heads and 63 sectors. Of course the drive doesn't really have 128 read/write heads (it doesn't even have 16, only 6), 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.