When we use fdisk to check a hard disk, it would show something like:

Disk /dev/sda/: 21.5 GB, 21474836480 bytes
255 heads, 63 sectors/track, 2610 cylinders, total 41943040 sectors

But when we multiply 255 x 63 x 2610, the number is 41929650, slightly less than the total no. of sectors shown above. Why?


1 Answer 1


The head, cylinder, sector numbers are displayed for historical purposes only, and for use by some really old software. Logical Block Addressing is used to address disks today. The geometry of modern disks typically have a variable number of sectors, outer tracks have more sectors than inner tracks.

Various interfaces have maximum values of 255 heads, 63 sectors/track, so to be able to express the CHS values for a large disk, these are set to their respective maximum values and the number of cylinders is rounded down to the nearest integer value giving an approximate value of the number of total sectors on the disk. The real number of sectors is typically different, as you have seen. The actual geometry of the disk is not even close to the CHS values; probably no disk ever manufactured has 255 heads, for example.

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    "Logical Block Addressing is used to address disks today" – Just to give some perspective on the timeframe: 22-bit LBA was introduced in 1986, 28-bit in 1994, 48-bit in 2003. As of 1996, all drives support LBA. Without LBA, the maximum addressable drive size is ~7.8 GiByte compared to 128 PiByte with LBA48. Jul 14, 2019 at 8:52

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