Cylinder Head Sector

Hard drive geometry, schematic representation of the basic principle

Cylinder, head, sector and track in relation to the data disks on a hard disk

The term cylinder / head / sector (per track) (CHS) or cylinder / head / sector (per track) (ZKS) describes an addressing method for hard disks . This enables the computer to recognize the hard drive geometry of the data carrier and its division into partitions using a partition table .

With older computers, the number of cylinders, heads and sectors often had to be entered manually in the BIOS so that the computer or the operating system could correctly control the hard disk. Incorrect information could easily lead to data loss. The reference to the hardware was not very direct: Sometimes hard disks also worked if incorrect information was entered, as long as the specified size did not exceed the actual size of the hard disk. The BIOS of modern computers can normally determine the CHS information from hard drives independently.

With CHS formatting, the number of cylinders, heads and sectors of the hard disk are entered in the register set of the hard disk controller. Since the register space was limited for reasons of space, the BIOS could only control 1024 cylinders, 16 heads and 63 sectors. With a sector size of 512 bytes, this resulted in a maximum of 504 MiB . Later the maximum number of heads was increased to 256, which increased the capacity limit to 8064 MiB.

In the case of RAID , this information may This can play an important role, because with some forms of data mirroring you have to make sure that both hard drives have identical properties - not just the same size.

The sectors are counted from one, while the cylinders and heads (as usual in computer science) start counting from zero.

Conversion from CHS to LBA

With the LBA method ( Logical Block Addressing ) the blocks are counted continuously starting with zero. The conversion of the CHS data into the LBA block takes place according to the formula

{\ displaystyle LBA = (c \ times H + h) \ times S + s-1}

${\ displaystyle LBA}$ : Address of the block according to the LBA method
${\ displaystyle c}$ : Cylinder number
${\ displaystyle H}$ : Number of read heads
${\ displaystyle h}$ : Read head number
${\ displaystyle S}$ : Number of sectors (= number of blocks per cylinder head lane or track)
${\ displaystyle s}$ : Sector number