View source for Cylinder-head-sector

{{Short description|Historical method for giving addresses to physical data blocks on hard disk drives}}
[[File:Cylinder Head Sector.svg|right|thumb|Cylinder, head, and sector of a hard drive.]]
{{Use dmy dates|date=July 2020}}
'''Cylinder-head-sector''' ('''CHS''') is an early method for giving addresses to each physical block of data on a [[hard disk drive]].

It is a 3D-coordinate system made out of a vertical coordinate ''head'', a horizontal (or radial) coordinate ''cylinder'', and an angular coordinate ''sector''. Head selects a circular surface: a platter in the disk (and one of its two sides). Cylinder is a [[Cylinder|cylindrical]] intersection through the stack of platters in a disk, centered around the disk's spindle. Combined, cylinder and head intersect to a circular line, or more precisely: a circular strip of physical data blocks called ''track''. Sector finally selects which data block in this track is to be addressed, as the track is subdivided into several equally-sized portions, each of which is an arc of (360/n) degrees, where n is the number of sectors in the track.

CHS addresses were exposed, instead of simple linear addresses (going from ''0'' to the ''total block count on disk - 1''), because early hard drives didn't come with an embedded [[disk controller]], that would hide the physical layout. A separate ''generic'' controller card was used, so that the operating system had to know the exact physical "geometry" of the ''specific'' drive attached to the controller, to correctly address data blocks. The traditional limits were 512 bytes/sector × 63 sectors/track × 255 heads (tracks/cylinder) × 1024 cylinders, resulting in a limit of 8032.5 MiB for the total capacity of a disk.

As the geometry became more complicated (for example, with the introduction of [[zone bit recording]]) and drive sizes grew over time, the CHS addressing method became restrictive. Since the late 1980s, hard drives began shipping with an embedded disk controller<ref>{{Cite web|url=http://www.pcguide.com/ref/hdd/if/ide/over.htm|title=Overview and History of the IDE/ATA Interface|date=17 April 2001|website=The PC Guide|archive-url=https://web.archive.org/web/20190204014430/http://www.pcguide.com/ref/hdd/if/ide/over.htm|archive-date=2019-02-04}}</ref> that had good knowledge of the physical geometry; they would however report a false geometry to the computer, e.g., a larger number of heads than actually present, to gain more addressable space. These logical CHS values would be translated by the controller, thus CHS addressing no longer corresponded to any physical attributes of the drive.<ref name=":0">{{cite web
|url=http://jdebp.info/FGA/disc-partition-alignment.html
|title=The gen on disc partition alignment
|author=Jonathan de Boyne Pollard
|date=2011
|access-date=2022-11-21}}
</ref>

By the mid 1990s, hard drive interfaces replaced the CHS scheme with [[logical block addressing]] (LBA), but many tools for manipulating the [[master boot record]] (MBR) partition table still aligned partitions to cylinder boundaries; thus, artifacts of CHS addressing were still seen in partitioning software by the late 2000s.<ref name=":0" />

In the early 2010s, the disk size limitations imposed by MBR became problematic and the [[GUID Partition Table]] (GPT) was designed as a replacement; modern computers using [[UEFI]] firmware without MBR support no longer use any notions from CHS addressing.

==Definitions==
[[File:Hard drive geometry - English - 2019-05-30.svg|upright=2.5|thumb|schematic of the hard drive geometry]]

CHS addressing is the process of identifying individual [[#Sectors|sectors]] (aka. physical block of data) on a disk by their position in a [[#Tracks|track]], where the track is determined by the [[#Heads|head]] and [[#Cylinders|cylinder]] numbers. The terms are explained bottom up, for disk addressing the ''sector'' is the smallest unit. Disk controllers can introduce address translations to map logical to physical positions, e.g., [[zone bit recording]] stores fewer sectors in shorter (inner) tracks, physical disk formats are not necessarily cylindrical, and sector numbers in a track can be skewed.

===Sectors===

Floppy disks and controllers had used physical sector sizes of 128, 256, 512 and 1024 bytes (e.g., PC/AX), but formats with 512 bytes per physical sector became dominant in the 1980s.<ref name="Ecma-107">{{cite web | url = https://www.ecma-international.org/publications-and-standards/standards/ecma-107/ | title = Volume and File Structure of Disk Cartridges for Information Interchange | work = Standard ECMA-107 (2nd ed., June 1995) | publisher = [[Ecma International|ECMA]] | year = 1995 | access-date = 2011-07-30 }}</ref><ref>{{cite web
|url=http://support.microsoft.com/kb/75131
|title=Standard Floppy Disk Formats Supported by MS-DOS
|work=KB75131
|date=2003-05-12
|publisher=[[Microsoft Knowledge Base]]
|archive-url=https://web.archive.org/web/20090131171358/http://support.microsoft.com/kb/75131
|access-date=2023-06-04|archive-date=31 January 2009
}}</ref>

The most common physical sector size for hard disks today is 512 bytes, but there have been hard disks with 520 bytes per sector as well for non-IBM compatible machines. In 2005 some [[Seagate Technology|Seagate]] custom hard disks used sector sizes of 1024 bytes per sector. [[Advanced Format]] hard disks use 4096 bytes per physical sector ([[4Kn]])<ref>{{cite web
|url=http://www.anandtech.com/show/2888
|title=Western Digital's Advanced Format: The 4K Sector Transition Begins
|publisher=[[AnandTech]]
|date=2009-12-18
|access-date=2011-07-29}}</ref> since 2010, but will also be able to emulate 512 byte sectors ([[512e]]) for a transitional period.<ref>{{cite web | url=http://www.hitachigst.com/tech/techlib.nsf/techdocs/3D2E8D174ACEA749882577AE006F3F05/$file/AFtechbrief.pdf | title=Advanced Format Technology Brief | publisher=[[Hitachi]]| year=2010 | page=1 | quote=512 byte emulation is sometimes referred to as 512e | access-date=2011-08-01 | url-status=dead | archive-url=https://web.archive.org/web/20110927024639/http://www.hitachigst.com/tech/techlib.nsf/techdocs/3D2E8D174ACEA749882577AE006F3F05/%24file/AFtechbrief.pdf | archive-date=27 September 2011}}</ref>

[[Magneto-optical drive]]s use sector sizes of 512 and 1024 bytes on 5.25-inch drives and 512 and 2048 bytes on 3.5-inch drives.<!-- TBD: Will have to recheck if these are actually emulated on this low level: Emulated physical sector sizes of 128, 256, 512 are also seen on some computers with bootable RAM or flash disks, e.g., the [[Atari Portfolio]]. -->

In CHS addressing the ''sector'' numbers always start at '''1''', there is no ''sector 0'',{{ref|a}} which can lead to confusion since logical sector addressing schemes typically start counting with 0, e.g., [[logical block addressing]] (LBA), or "relative sector addressing" used in DOS.

For physical disk geometries the maximal sector number is determined by the ''low level format'' of the disk. However, for disk access with the [[BIOS]] of IBM-PC compatible machines, the sector number was encoded in six bits, resulting in a maximal number of 111111 (63) sectors per track. This maximum is still in use for virtual CHS geometries.

===Tracks===
The [[track (disk drive)|track]]s are the thin [[concentric]] circular strips of sectors. At least one head is required to read a single track. With respect to disk geometries the terms ''track'' and ''cylinder'' are closely related. For a single or double sided [[floppy disk]] ''track'' is the common term; and for more than two heads ''cylinder'' is the common term.  Strictly speaking a ''track'' is a given <code><abbr title="Cylinder">C</abbr><abbr title="Head">H</abbr></code> combination consisting of<code><abbr title="Sectors Per Track">SPT</abbr></code> sectors, while a ''cylinder'' consists of<code>SPT×<abbr title="Heads">H</abbr></code> sectors.

===Cylinders===

A cylinder is a division of data in a [[Disk storage|disk drive]], as used in the CHS addressing mode of a [[fixed-block architecture]] (FBA) disk or the cylinder–head–record (CCHHR) addressing mode of a [[CKD disk]].

The concept is concentric, hollow, [[cylinder (geometry)|cylindrical]] slices through the physical disks ([[hard disk platter|platters]]), collecting the respective circular tracks aligned through the stack of platters. The number of cylinders of a disk drive exactly equals the number of tracks on a single surface in the drive. It comprises the same track number on each platter, spanning all such tracks across each platter surface that is able to store data (without regard to whether or not the track is "bad"). Cylinders are vertically formed by [[Track (disk drive)|tracks]]. In other words, track 12 on platter 0 plus track 12 on platter 1 etc. is cylinder 12.

Other forms of [[Direct access storage device|Direct Access Storage Device]] (DASD), such as [[drum memory]] devices or the [[IBM 2321 Data Cell]], might give blocks addresses that include a cylinder address, although the cylinder address doesn't select a (geometric) cylindrical slice of the device.

===Heads===
A device called a [[disk read-and-write head|head]] reads and writes data in a hard drive by manipulating the magnetic medium that composes the surface of an associated disk platter. Naturally, a platter has 2 sides and thus 2 surfaces on which data can be manipulated; usually there are 2 heads per platter, one per side. (Sometimes the term ''side'' is substituted for ''head,'' since platters might be separated from their head assemblies, as with the removable media of a ''floppy'' drive.)

The <code><abbr title="Cylinder">C</abbr><abbr title="Head">H</abbr><abbr title="Sector">S</abbr></code> addressing supported in IBM-PC compatible [[BIOS]]es code used eight bits for a maximum of 256 heads counted as head '''0''' up to '''255''' (<code>FFh</code>). However, a bug in all versions of [[MS-DOS|Microsoft DOS]]/[[IBM PC DOS]] up to and including 7.10 will cause these operating systems to crash on boot when encountering volumes with 256 heads{{ref|b}}. Therefore, all compatible BIOSes will use mappings with up to 255 heads (<code>00h..FEh</code>) only, including in virtual <code>255×63</code> geometries.

This historical oddity can affect the maximum disk size in old BIOS [[INT 13H|INT 13h]] code as well as old [[PC DOS]] or similar operating systems:

<code>(512 bytes/sector)×(63 sectors/track)×(255 heads (tracks/cylinder))×(1024 cylinders)=8032.5</code>&nbsp;[[Mebibyte|MiB]], but actually <code>512×63×256×1024=8064</code>&nbsp;[[Mebibyte|MiB]] yields what is known as '''8&nbsp;[[Gigabyte|GB]]''' limit.<ref name="eide" /> In this context relevant definition of 8&nbsp;[[Gibibyte|GiB]] = 8192&nbsp;[[Mebibyte|MiB]] is another incorrect limit, because it would require CHS <code>512×64×256</code> with 64 sectors per track.

''Tracks'' and ''cylinders'' are counted from 0, i.e., track 0 is the first (outer-most) track on [[floppy disk|floppy]] or other cylindrical disks. Old [[BIOS]] code supported ten bits in CHS addressing with up to 1024 cylinders (<code>1024=2<sup>10</sup></code>). Adding six bits for [[#Sectors|sectors]] and eight bits for [[#Heads|heads]] results in the 24 bits supported by [[INT 13H|BIOS interrupt 13h]]. Subtracting the disallowed sector number 0 in <code>1024×256</code> tracks corresponds to 128&nbsp;[[Mebibyte|MiB]] for a sector size of 512 bytes (<code>128&nbsp;[[Mebibyte|MiB]]=1024×256×(512 byte/sector)</code>); and <code>8192-128=8064</code> confirms the (roughly) '''8&nbsp;[[Gigabyte|GB]]''' limit.<ref>{{cite web
|url=http://support.microsoft.com/kb/q224526/en-us
|title=Windows NT 4.0 supports maximum of 7.8-GB system partition
|date=2007-02-23
|publisher=[[Microsoft]]
|access-date=2011-07-30}}</ref>

CHS addressing starts at <code>0/0/1</code> with a maximal value <code>1023/255/63</code> for <code>24=10+8+6</code> bits, or <code>1023/254/63</code> for 24 bits limited to 255 [[#Heads|heads]]. CHS values used to specify the geometry of a disk have to count cylinder 0 and head 0 resulting in
a maximum (<code>1024/256/63</code> or) <code>1024/255/63</code> for 24 bits with (256 or) 255 heads. In CHS tuples specifying a geometry S actually means sectors per track, and where the (virtual) geometry still matches the capacity the disk contains <code>C×H×S</code> sectors. As larger hard disks have come into use, a cylinder has become also a logical disk structure, standardised{{Citation needed|date=July 2011}} at 16&nbsp;065 sectors (<code>16065=255×63</code>).

CHS addressing with 28 bits ([[Parallel ATA#EIDE and ATA-2|EIDE and ATA-2]]) permits eight bits for sectors still starting at 1, i.e., sectors 1...255, four bits for heads 0...15, and sixteen bits for cylinders 0...65535.<ref>{{cite web
|url=http://www.hitachigst.com/tech/techlib.nsf/techdocs/1F70FDFFB4DE1EBB86257538007E4CFE/$file/TS5K500.B_OEMSpec_r12a.pdf
|title=5K500.B SATA OEM Specification Revision 1.2
|publisher=[[Hitachi]]
|date=2009-03-17
|page=51
|url-status=dead
|archive-url=https://web.archive.org/web/20110927024620/http://www.hitachigst.com/tech/techlib.nsf/techdocs/1F70FDFFB4DE1EBB86257538007E4CFE/%24file/TS5K500.B_OEMSpec_r12a.pdf
|archive-date=27 September 2011}}</ref> This results in a roughly '''128&nbsp;[[Gigabyte|GB]]''' limit; actually <code>65536×16×255=267386880</code> sectors corresponding to 130560&nbsp;[[Mebibyte|MiB]] for a sector size of 512 bytes.<ref name="eide">{{cite web
|url=http://tldp.org/HOWTO/Large-Disk-HOWTO-4.html
|title=History of BIOS and IDE limits
|work=Large Disk HOWTO v2.5
|author=Andries Brouwer
|author-link=Andries Brouwer
|date=2004-11-01
|access-date=2011-07-30}}</ref> The <code>28=16+4+8</code> bits in the [[Parallel ATA#EIDE and ATA-2|ATA-2]] specification are also covered by [[Ralf Brown's Interrupt List]], and an old working draft of this now expired standard was published.<ref>{{cite web
|url=http://www.t13.org/Documents/UploadedDocuments/project/d0948r4c-ATA-2.pdf
|title=ATA-2
|publisher=[[INCITS]] Technical Committee T13 AT Attachment
|date=1996-03-18
|work=X3T10/0948D
|url-status=dead
|archive-url=https://web.archive.org/web/20110728081254/http://www.t13.org/Documents/UploadedDocuments/project/d0948r4c-ATA-2.pdf
|archive-date=28 July 2011}}</ref>

With an old [[BIOS]] limit of 1024 cylinders and the [[Parallel ATA|ATA]] limit of 16 heads<ref>{{cite web
|url=http://www.t13.org/Documents/UploadedDocuments/project/d0791r4c-ATA-1.pdf
|title=ATA-1
|publisher=[[INCITS]] Technical Committee T10 SCSI Storage Interfaces
|year=1994
|work=X3T10/791D
|url-status=dead
|archive-url=https://web.archive.org/web/20120321035657/http://www.t13.org/Documents/UploadedDocuments/project/d0791r4c-ATA-1.pdf
|archive-date=21 March 2012}}</ref> the combined effect was <code>1024×16×63=1032192</code> sectors, i.e., a '''504&nbsp;[[Mebibyte|MiB]]''' limit for sector size 512. [[BIOS]] translation schemes known as <abbr title="Extended CHS">ECHS</abbr> and ''revised ECHS'' mitigated this limitation by using 128 or 240 instead of 16 heads, simultaneously reducing the numbers of cylinders and sectors to fit into <code>1024/128/63</code> (ECHS limit: 4032&nbsp;[[Mebibyte|MiB]]) or <code>1024/240/63</code> (revised ECHS limit: 7560&nbsp;[[Mebibyte|MiB]]) for the given total number of sectors on a disk.<ref name="eide" />

===Blocks and clusters===
The [[Unix]] communities employ the term ''block'' to refer to a sector or group of sectors. For example, the Linux [[fdisk]] utility, before version 2.25,<ref>{{cite web|title=Util-linux 2.25 Release Notes|url=https://www.kernel.org/pub/linux/utils/util-linux/v2.25/v2.25-ReleaseNotes|website=The Linux Kernel Archives|access-date=24 March 2016}}</ref> displayed partition sizes using 1024-byte ''blocks''.

''Clusters'' are allocation units for data on various file systems ([[File Allocation Table|FAT]], [[NTFS]], etc.), where ''data'' mainly consists of files. ''Clusters'' are not directly affected by the physical or virtual geometry of the disk, i.e., a cluster can begin at a sector near the end of a given <code><abbr title="Cylinder">C</abbr><abbr title="Head">H</abbr></code> track, and end in a sector on the physically or logically next <code><abbr title="Cylinder">C</abbr><abbr title="Head">H</abbr></code> [[#Tracks|track]].

==CHS to LBA mapping==
{{main|Logical Block Addressing#CHS conversion}}

In 2002 the [[Parallel ATA|ATA-6]] specification introduced an optional 48 bits [[Logical Block Addressing]] and declared CHS addressing as obsolete, but still allowed to implement the ATA-5 translations.<ref>{{cite web
|url=http://www.t10.org/t13/project/d1410r3a-ATA-ATAPI-6.pdf
|title=ATA-6
|publisher=[[INCITS]] Technical Committee T13 ATA Storage Interface|quote=In standards ATA/ATAPI-5 and earlier, a CHS translation was defined. This translation is obsolete but may be implemented as defined in ATA/ATAPI-5.
|page=22
|year=2002
|work=T13/1410D
|access-date=2011-07-30
|url-status=dead
|archive-url=https://web.archive.org/web/20110728081452/http://www.t10.org/t13/project/d1410r3a-ATA-ATAPI-6.pdf
|archive-date=28 July 2011}}</ref> Unsurprisingly the CHS to LBA translation formula given below also matches the last ATA-5 CHS translation. In the ATA-5 specification CHS support was mandatory for up to 16&nbsp;514&nbsp;064 sectors and optional for larger disks. The ATA-5 limit corresponds to CHS <code>16383 16 63</code> or equivalent disk capacities {{nowrap|1=(16514064 = 16383 × 16 × 63}} {{nowrap|1== 1032 × 254 × 63)}}, and requires {{nowrap|1=24 = 14 + 4 + 6 bits}} ({{nowrap|1=16383 + 1 = 2<sup>14</sup>}}).<ref>{{cite web
|url=http://www.t10.org/t13/project/d1321r3-ATA-ATAPI-5.pdf
|title=ATA-5
|publisher=[[INCITS]] Technical Committee T13 ATA Storage Interface|quote=If the device's capacity is greater than or equal to one sector and less than or equal to 16,514,064 sectors, then the device shall support CHS translation.
|page=19
|year=2000
|work=T13/1321D
|access-date=2011-07-30
|url-status=dead
|archive-url=https://web.archive.org/web/20110728081430/http://www.t10.org/t13/project/d1321r3-ATA-ATAPI-5.pdf
|archive-date=28 July 2011}}</ref>

CHS [[tuple]]s can be mapped onto LBA addresses using the following formula:

<math display=block>A = (c \times N_\mathrm{heads} + h) \times N_\mathrm{sectors} + (s-1),</math>

where {{mvar|A}} is the LBA address, {{math|''N''<sub>heads</sub>}} is the number of heads on the disk, {{math|''N''<sub>sectors</sub>}} is the maximum number of sectors per track, and {{math|(''c'', ''h'', ''s'')}} is the CHS address.

A ''Logical Sector Number'' formula in the [[Ecma International|ECMA]]-107<ref name="Ecma-107"/> and [[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]]&nbsp;9293:1994<ref name="ISO_9293_1994">{{cite web | url = http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=21273 | title = Information technology -- Volume and file structure of disk cartridges for information interchange | work = ISO/IEC 9293:1994 | publisher = [[International Organization for Standardization|ISO]] catalogue | year = 1994 | access-date = 2012-01-06 }}</ref> (superseding ISO&nbsp;9293:1987<ref name="ISO_9293_1987">{{cite web | url = http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=16948 | title = Information processing -- Volume and file structure of flexible disk cartridges for information interchange | work = ISO 9293:1987 | publisher = [[International Organization for Standardization|ISO]] catalogue | year = 1987 | access-date = 2012-01-06 }}</ref>) standards for [[File Allocation Table|FAT]] file systems matches exactly the LBA formula given above: ''Logical Block Address'' and ''Logical Sector Number'' (LSN) are synonyms.<ref name="Ecma-107"/><ref name="ISO_9293_1994"/><ref name="ISO_9293_1987"/> The formula does not use the number of cylinders, but requires the number of heads and the number of sectors per track in the disk geometry, because the same CHS tuple addresses different logical sector numbers depending on the geometry. 

'''Examples''':
:For geometry <code>1020 16 63</code> of a disk with 1028160 sectors, CHS <code>3 2 1</code> is LBA {{nowrap|1=3150 = ((3 × 16) + 2) × 63 + (1 – 1)}};
:For geometry <code>1008 4 255</code> of a disk with 1028160 sectors, CHS <code>3 2 1</code> is LBA {{nowrap|1=3570 = ((3 × 4) + 2) × 255 + (1 – 1)}}
:For geometry <code>&nbsp;64 255 63</code> of a disk with 1028160 sectors, CHS <code>3 2 1</code> is LBA {{nowrap|1=48321=((3 × 255) + 2) × 63 + (1 – 1)}}
:For geometry <code>2142 15 32</code> of a disk with 1028160 sectors, CHS <code>3 2 1</code> is LBA {{nowrap|1=1504 = ((3 × 15) + 2) × 32 + (1 – 1)}}

To help visualize the sequencing of sectors into a linear LBA model, note that: 
:The first LBA sector is sector # zero, the same sector in a CHS model is called sector # one.
:All the sectors of each head/track get counted before incrementing to the next head/track.
:All the heads/tracks of the same cylinder get counted before incrementing to the next cylinder.
:The outside half of a whole hard drive would be the first half of the drive.

==History==
Cylinder Head Record format has been used by [[Count Key Data]] (CKD) hard disks on [[IBM mainframes]] since at least the 1960s. This is largely comparable to the Cylinder Head Sector format used by PCs, with the exception that the sector size was not fixed but could vary from track to track based on the needs of each application. In contemporary use, the disk geometry presented to the mainframe is emulated by the storage firmware, and no longer has any relation to physical disk geometry.{{citation needed|date=August 2021}}

Earlier hard drives used in the PC, such as [[Modified Frequency Modulation|MFM]] and [[Run Length Limited|RLL]] drives, divided each cylinder into an equal number of sectors, so the CHS values matched the physical properties of the drive. A drive with a CHS tuple of <code>500 4 32</code> would have 500 tracks per side on each platter, two platters (4 heads), and 32 sectors per track, with a total of 32&nbsp;768&nbsp;000 [[bytes]] (31.25&nbsp;[[Mebibyte|MiB]]).{{citation needed|date=August 2021}}

[[Advanced Technology Attachment|ATA/IDE]] drives were much more efficient at storing data and have replaced the now-obsolete MFM and RLL drives. They use [[zone bit recording]] (ZBR), where the number of sectors dividing each track varies with the location of groups of tracks on the surface of the platter. Tracks nearer to the edge of the platter contain more blocks of data than tracks close to the spindle, because there is more physical space within a given track near the edge of the platter. Thus, the CHS addressing scheme cannot correspond directly with the physical geometry of such drives, due to the varying number of sectors per track for different regions on a platter. Because of this, many drives still have a surplus of sectors (less than 1 cylinder in size) at the end of the drive, since the total number of sectors rarely, if ever, ends on a cylinder boundary.{{citation needed|date=August 2021}}

An ATA/IDE drive can be set in the system [[BIOS]] with any configuration of cylinders, heads and sectors that do not exceed the capacity of the drive (or the BIOS), since the drive will convert any given CHS value into an actual address for its specific hardware configuration. This however can cause compatibility problems.{{citation needed|date=August 2021}}

For operating systems such as [[Microsoft]] [[DOS]] or older version of [[Microsoft Windows|Windows]], each partition must start and end at a cylinder boundary.{{Citation needed|date=April 2017}} Only some of the relatively modern [[operating system]]s (Windows XP included) may disregard this rule, but doing so can still cause some compatibility issues, especially if the user wants to perform [[dual boot]]ing on the same drive. Microsoft does not follow this rule with internal disk partition tools since Windows Vista.<ref>{{cite web
|url=http://support.microsoft.com/kb/931760/en-us
|title=KB931760
|work=Microsoft Windows XP Support
|date=2009-07-23
|publisher=[[Microsoft Knowledge Base]]
|access-date=2011-07-30}}</ref>

==See also==
* [[CD-ROM#CD-ROM format|CD-ROM format]]
* [[Block (data storage)]]
* [[Disk storage]]
* [[Disk formatting]]
* [[File Allocation Table]]
* [[Disk partitioning]]

==References==
{{Reflist|30em}}

==Notes==
:1.{{note|a}}This rule is true at least for all formats where the physical sectors are named 1 upwards. However, there are a few odd floppy formats (e.g., the 640&nbsp;[[Kilobyte|KB]] format used by BBC Master 512 with DOS Plus 2.1), where the first sector in a track is named "0" not "1".<!-- So, is this rule above really universal or does it only apply to the former type of formats starting with sector "1"? -->
:2.{{note|b}}While computers begin counting at 0, DOS would begin counting at 1. In order to do this, DOS would add a 1 to the head count before displaying it on the screen. However, instead of converting the 8-bit unsigned integer to a larger size (such as a 16-bit integer) first, DOS just added the 1. This would [[integer overflow|overflow]] a head count of 255 (<code>0xFF</code>) into 0 (<code>0x100 & 0xFF = 0x00</code>) instead of the 256 that would be expected. This was fixed with DOS 8<!-- or 7.11 if there was one -->, but by then, it had become a [[de facto]] standard to not use a head value of 255.

{{DEFAULTSORT:Cylinder-Head-Sector}}
[[Category:AT Attachment]]
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