![Cylinder Head Sector](https://upload.wikimedia.org/wikipedia/commons/0/02/Cylinder_Head_Sector.svg)
Cylinder-head-sector (CHS) is an early method for giving addresses to each physical block of data on a
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
.
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
cylindrical
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infini ...
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
{{unreferenced, date=May 2010
The disk controller is the controller circuit which enables the CPU to communicate with a hard disk, floppy disk or other kind of disk drive. It also provides an interface between the disk drive and the bus conne ...
, 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 In computer storage, zone bit recording (ZBR) is a method used by disk drives to optimise the tracks for increased data capacity. It does this by placing more sectors per zone on outer tracks than on inner tracks. This contrasts with other approach ...
) 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 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.
[
]
By the mid 1990s, hard drive interfaces replaced the CHS scheme with
logical block addressing
Logical block addressing (LBA) is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disk drives. LBA is a particularly simple linear addressi ...
(LBA), but many tools for manipulating the
master boot record
A master boot record (MBR) is a special type of boot sector at the very beginning of partitioned computer mass storage devices like fixed disks or removable drives intended for use with IBM PC-compatible systems and beyond. The concept of MBR ...
(MBR) partition table still aligned partitions to cylinder boundaries; thus, artifacts of CHS addressing were still seen in partitioning software by the late 2000s.
In the early 2010s, the disk size limitations imposed by MBR became problematic and the
GUID Partition Table
The GUID Partition Table (GPT) is a standard for the layout of partition tables of a physical computer storage device, such as a hard disk drive or solid-state drive, using universally unique identifiers, which are also known as globally unique i ...
(GPT) was designed as a replacement; modern computers using
UEFI
UEFI (Unified Extensible Firmware Interface) is a set of specifications written by the UEFI Forum. They define the architecture of the platform firmware used for booting and its interface for interaction with the operating system. Examples of f ...
firmware without MBR support no longer use any notions from CHS addressing.
Definitions
![Hard drive geometry - English - 2019-05-30](https://upload.wikimedia.org/wikipedia/commons/4/41/Hard_drive_geometry_-_English_-_2019-05-30.svg)
CHS addressing is the process of identifying individual
sectors
Sector may refer to:
Places
* Sector, West Virginia, U.S.
Geometry
* Circular sector, the portion of a disc enclosed by two radii and a circular arc
* Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc
* Spherical sector, a p ...
(aka. physical block of data) on a disk by their position in a
track
Track or Tracks may refer to:
Routes or imprints
* Ancient trackway, any track or trail whose origin is lost in antiquity
* Animal track, imprints left on surfaces that an animal walks across
* Desire path, a line worn by people taking the shorte ...
, where the track is determined by the
head
A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may ...
and
cylinder
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infin ...
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 In computer storage, zone bit recording (ZBR) is a method used by disk drives to optimise the tracks for increased data capacity. It does this by placing more sectors per zone on outer tracks than on inner tracks. This contrasts with other approach ...
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 use physical sector sizes of 128, 256, 512 and 1024 bytes (e.g., PC/AX), whereby formats with 512 bytes per physical sector became dominant in the 1980s.
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 custom hard disks used sector sizes of 1024 bytes per sector.
Advanced Format
Advanced Format (AF) is any disk sector format used to store data on magnetic disks in hard disk drives (HDDs) that exceeds 512, 520, or 528 bytes per sector, such as the 4096, 4112, 4160, and 4224-byte (4 KB) sectors of an Advanced Format ...
hard disks use 4096 bytes per physical sector (
4Kn
Advanced Format (AF) is any disk sector format used to store data on magnetic disks in hard disk drives (HDDs) that exceeds 512, 520, or 528 bytes per sector, such as the 4096, 4112, 4160, and 4224-byte (4 KB) sectors of an Advanced Format ...
) since 2010, but will also be able to emulate 512 byte sectors (
512e
Advanced Format (AF) is any disk sector format used to store data on magnetic disks in hard disk drives (HDDs) that exceeds 512, 520, or 528 bytes per sector, such as the 4096, 4112, 4160, and 4224-byte (4 KB) sectors of an Advanced Format ...
) for a transitional period.
Magneto-optical drive
A magneto-optical drive is a kind of optical disc drive capable of writing and rewriting data upon a magneto-optical disc. Both 130 mm (5.25 in) and 90 mm (3.5 in) form factors exist. In 1983, just a year after the introduc ...
s use sector sizes of 512 and 1024 bytes on 5.25-inch drives and 512 and 2048 bytes on 3.5-inch drives.
In CHS addressing the ''sector'' numbers always start at 1, there is no ''sector 0'', which can lead to confusion since logical sector addressing schemes typically start counting with 0, e.g.,
logical block addressing
Logical block addressing (LBA) is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disk drives. LBA is a particularly simple linear addressi ...
(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
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
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
Track or Tracks may refer to:
Routes or imprints
* Ancient trackway, any track or trail whose origin is lost in antiquity
* Animal track, imprints left on surfaces that an animal walks across
* Desire path, a line worn by people taking the shorte ...
s are the thin
concentric
In geometry, two or more objects are said to be concentric, coaxal, or coaxial when they share the same center or axis. Circles, regular polygons and regular polyhedra, and spheres may be concentric to one another (sharing the same center point ...
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
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
''track'' is the common term; and for more than two heads ''cylinder'' is the common term. Strictly speaking a ''track'' is a given
CH
combination consisting of
SPT
sectors, while a ''cylinder'' consists of
SPT×H
sectors.
Cylinders
A cylinder is a division of data in a
disk drive
Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...
, as used in the CHS addressing mode of a
Fixed Block Architecture
Fixed-block architecture (FBA) is an IBM term for the hard disk drive (HDD) layout in which each addressable block (more commonly, sector) on the disk has the same size, utilizing 4 byte block numbers and a new set of command codes. FBA as a ter ...
disk or the cylinder–head–record (CCHHR) addressing mode of a
CKD disk
Count key data (CKD) is a direct-access storage device (DASD) data recording format introduced in 1964, by IBM with its IBM System/360 and still being emulated on IBM mainframes. It is a self-defining format with each data record represented by a ...
.
The concept is concentric, hollow,
cylindrical
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infini ...
slices through the physical disks (
platters
The Platters was an American vocal group formed in 1952. They are one of the most successful vocal groups of the early rock and roll era. Their distinctive sound bridges the pre-rock Tin Pan Alley tradition and the new burgeoning genre. The ac ...
), 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
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
A direct-access storage device (DASD) (pronounced ) is a secondary storage device in which "each physical record has a discrete location and a unique address". The term was coined by IBM to describe devices that allowed random access to data, ...
(DASD), such as
drum memory
Drum memory was a magnetic data storage device invented by Gustav Tauschek in 1932 in Austria. Drums were widely used in the 1950s and into the 1960s as computer memory.
For many early computers, drum memory formed the main working memory ...
devices or the
IBM 2321 Data Cell
The IBM 2321 Data Cell announced in April 1964 (withdrawn January 1975) is a discontinued direct access storage device (DASD) for the IBM System/360. It holds up to 400 megabytes of data, with an access time of 95 milliseconds to 600 milliseconds ...
, 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
head
A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may ...
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
CHS
addressing supported in IBM-PC compatible
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
es code used eight bits for - theoretically up to 256 heads counted as head 0 up to 255 (
FFh
). However, a bug in all versions of
Microsoft DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...
/
IBM PC DOS
IBM PC DOS, an acronym for IBM Personal Computer Disk Operating System, is a discontinued disk operating system for IBM PC compatibles. It was manufactured and sold by IBM from the early 1980s into the 2000s. Developed by Microsoft, it was also ...
up to and including 7.10 will cause these operating systems to crash on boot when encountering volumes with 256 heads. Therefore, all compatible BIOSes will use mappings with up to 255 heads (
00h..FEh
) only, including in virtual
255×63
geometries.
This historical oddity can affect the maximum disk size in old BIOS
INT 13h
INT 13h is shorthand for BIOS interrupt call 13 hex, the 20th interrupt vector in an x86-based (IBM PC-descended) computer system. The BIOS typically sets up a real mode interrupt handler at this vector that provides sector-based hard disk and ...
code as well as old
PC DOS
PC or pc may refer to:
Arts and entertainment
* Player character or playable character, a fictional character controlled by a human player, usually in role-playing games or computer games
* ''Port Charles'', an American daytime TV soap opera
* ...
or similar operating systems:
(512 bytes/sector)×(63 sectors/track)×(255 heads (tracks/cylinder))×(1024 cylinders)=8032.5
MB, but actually
512×63×256×1024=8064
MB yields what is known as 8
GB limit.
In this context relevant definition of 8
GB = 8192
MB is another incorrect limit, because it would require CHS
512×64×256
with 64 sectors per track.
''Tracks'' and ''cylinders'' are counted from 0, i.e., track 0 is the first (outer-most) track on
floppy
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined wi ...
or other cylindrical disks. Old
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
code supported ten bits in CHS addressing with up to 1024 cylinders (
1024=210
). Adding six bits for
sectors
Sector may refer to:
Places
* Sector, West Virginia, U.S.
Geometry
* Circular sector, the portion of a disc enclosed by two radii and a circular arc
* Hyperbolic sector, a region enclosed by two radii and a hyperbolic arc
* Spherical sector, a p ...
and eight bits for
heads
A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may no ...
results in the 24 bits supported by
BIOS interrupt 13h. Subtracting the disallowed sector number 0 in
1024×256
tracks corresponds to 128
MB for a sector size of 512 bytes (
128 MB=1024×256×(512 byte/sector)
); and
8192-128=8064
confirms the (roughly) 8
GB limit.
CHS addressing starts at
0/0/1
with a maximal value
1023/255/63
for
24=10+8+6
bits, or
1023/254/63
for 24 bits limited to 255
heads
A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may no ...
. CHS values used to specify the geometry of a disk have to count cylinder 0 and head 0 resulting in
a maximum (
1024/256/63
or)
1024/255/63
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
C×H×S
sectors. As larger hard disks have come into use, a cylinder has become also a logical disk structure, standardised at 16 065 sectors (
16065=255×63
).
CHS addressing with 28 bits (
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. This results in a roughly 128
GB limit; actually
65536×16×255=267386880
sectors corresponding to 130560
MB for a sector size of 512 bytes.
The
28=16+4+8
bits in the
ATA-2 specification are also covered by
Ralf Brown's Interrupt List
Ralf Brown's Interrupt List (aka RBIL, x86 Interrupt List, MS-DOS Interrupt List or INTER) is a comprehensive list of interrupts, calls, hooks, interfaces, data structures, CMOS settings, memory and port addresses, as well as processor opcodes a ...
, and an old working draft of this now expired standard was published.
With an old
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
limit of 1024 cylinders and the
ATA limit of 16 heads the combined effect was
1024×16×63=1032192
sectors, i.e., a 504
MB limit for sector size 512.
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
translation schemes known as
ECHS 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
1024/128/63
(ECHS limit: 4032
MB) or
1024/240/63
(revised ECHS limit: 7560
MB) for the given total number of sectors on a disk.
Blocks and clusters
The
Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
communities employ the term ''block'' to refer to a sector or group of sectors. For example, the Linux
fdisk
In computing, the fdisk command-line utility provides disk-partitioning functions, preparatory to defining file systems. fdisk features in the DOS, DR FlexOS, IBM OS/2, and Microsoft Windows operating systems, and in certain ports of FreeBS ...
utility, before version 2.25, displayed partition sizes using 1024-byte ''blocks''.
''Clusters'' are allocation units for data on various file systems (
FAT
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers spec ...
,
NTFS
New Technology File System (NTFS) is a proprietary journaling file system developed by Microsoft. Starting with Windows NT 3.1, it is the default file system of the Windows NT family. It superseded File Allocation Table (FAT) as the preferred fil ...
, 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
CH
track, and end in a sector on the physically or logically next
CH
track
Track or Tracks may refer to:
Routes or imprints
* Ancient trackway, any track or trail whose origin is lost in antiquity
* Animal track, imprints left on surfaces that an animal walks across
* Desire path, a line worn by people taking the shorte ...
.
CHS to LBA mapping
In 2002 the
ATA-6 specification introduced an optional 48 bits
Logical Block Addressing
Logical block addressing (LBA) is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disk drives. LBA is a particularly simple linear addressi ...
and declared CHS addressing as obsolete, but still allowed to implement the ATA-5 translations. 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 16 514 064 sectors and optional for larger disks. The ATA-5 limit corresponds to CHS
16383 16 63
or equivalent disk capacities (16514064 = 16383×16×63 = 1032×254×63), and requires 24 = 14+4+6 bits (16383 + 1 = 2
14).
CHS
tuple
In mathematics, a tuple is a finite ordered list (sequence) of elements. An -tuple is a sequence (or ordered list) of elements, where is a non-negative integer. There is only one 0-tuple, referred to as ''the empty tuple''. An -tuple is defi ...
s can be mapped onto LBA addresses using the following formula:
:''A'' = (''c'' ⋅ ''N''
heads + ''h'') ⋅ ''N''
sectors + (''s'' − 1),
where ''A'' is the LBA address, ''N''
heads is the number of heads on the disk, ''N''
sectors is the maximum number of sectors per track, and (''c'', ''h'', ''s'') is the CHS address.
A ''Logical Sector Number'' formula in the
ECMA-107
and
ISO
ISO is the most common abbreviation for the International Organization for Standardization.
ISO or Iso may also refer to: Business and finance
* Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007
* Iso ...
/
IEC
The International Electrotechnical Commission (IEC; in French: ''Commission électrotechnique internationale'') is an international standards organization that prepares and publishes international standards for all electrical, electronic and r ...
9293:1994
(superseding ISO 9293:1987
) standards for
FAT
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers spec ...
file systems matches exactly the LBA formula given above: ''Logical Block Address'' and ''Logical Sector Number'' (LSN) are synonyms.
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
1020 16 63
of a disk with 1028160 sectors, CHS
3 2 1
is LBA
3150=((3× 16)+2)× 63 + (1-1)
:For geometry
1008 4 255
of a disk with 1028160 sectors, CHS
3 2 1
is LBA
3570=((3× 4)+2)×255 + (1-1)
:For geometry
64 255 63
of a disk with 1028160 sectors, CHS
3 2 1
is LBA
48321=((3×255)+2)× 63 + (1-1)
:For geometry
2142 15 32
of a disk with 1028160 sectors, CHS
3 2 1
is LBA
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
Count key data (CKD) is a direct-access storage device (DASD) data recording format introduced in 1964, by IBM with its IBM System/360 and still being emulated on IBM mainframes. It is a self-defining format with each data record represented by a ...
(CKD) hard disks on
IBM mainframes
IBM mainframes are large computer systems produced by IBM since 1952. During the 1960s and 1970s, IBM dominated the large computer market. Current mainframe computers in IBM's line of business computers are developments of the basic design of th ...
since at least the 1960s. This is largely comparable to the Cylinder Head Sector format used by PCs, save 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.
Earlier hard drives used in the PC, such as
MFM and
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
500 4 32
would have 500 tracks per side on each platter, two platters (4 heads), and 32 sectors per track, with a total of 32 768 000
bytes
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable unit ...
(31.25
MiB).
ATA/IDE drives were much more efficient at storing data and have replaced the now ''archaic'' MFM and RLL drives. They use
zone bit recording In computer storage, zone bit recording (ZBR) is a method used by disk drives to optimise the tracks for increased data capacity. It does this by placing more sectors per zone on outer tracks than on inner tracks. This contrasts with other approach ...
(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.
An ATA/IDE drive can be set in the system
BIOS
In computing, BIOS (, ; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the ...
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.
For operating systems such as
Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
DOS
DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems.
DOS may also refer to:
Computing
* Data over signalling (DoS), multiplexing data onto a signalling channel
* Denial-of-service attack (DoS), an attack on a communicat ...
or older version of
Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
, each partition must start and end at a cylinder boundary. Only some of the most modern
operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
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
Multi-booting is the act of installing multiple operating systems on a single computer, and being able to choose which one to boot. The term dual-booting refers to the common configuration of specifically two operating systems. Multi-booting m ...
ing on the same drive. Microsoft does not follow this rule with internal disk partition tools since Windows Vista.
See also
*
CD-ROM format
*
Block (data storage)
In computing (specifically data transmission and data storage), a block, sometimes called a physical record, is a sequence of bytes or bits, usually containing some whole number of records, having a maximum length; a ''block size''. Data thus ...
*
Disk storage
Disk storage (also sometimes called drive storage) is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is ...
*
Disk formatting
Disk formatting is the process of preparing a data storage device such as a hard disk drive, solid-state drive, floppy disk, memory card or USB flash drive for initial use. In some cases, the formatting operation may also create one or more new fil ...
*
File Allocation Table
File Allocation Table (FAT) is a file system developed for personal computers. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. It is often supported for compatibility reasons by ...
*
Disk partitioning
Disk partitioning or disk slicing is the creation of one or more regions on secondary storage, so that each region can be managed separately. These regions are called partitions. It is typically the first step of preparing a newly installed disk ...
References
Notes
:1.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
KB format used by BBC Master 512 with DOS Plus 2.1), where the first sector in a track is named "0" not "1".
:2.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
overflow a head count of 255 (
0xFF
) into 0 (
0x100 & 0xFF = 0x00
) instead of the 256 that would be expected. This was fixed with DOS 8, but by then, it had become a
de facto
''De facto'' ( ; , "in fact") describes practices that exist in reality, whether or not they are officially recognized by laws or other formal norms. It is commonly used to refer to what happens in practice, in contrast with ''de jure'' ("by la ...
standard to not use a head value of 255.
{{DEFAULTSORT:Cylinder-Head-Sector
AT Attachment
BIOS
Computer file systems
Hard disk computer storage
Rotating disc computer storage media
Computer storage devices
IBM storage devices