Encyclopedia
Logical block addressing (LBA) is a common scheme used for specifying the location of blocks of data stored on computer storage
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....

devices, generally secondary storage systems such as hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

s.

LBA is a particularly simple linear addressing scheme; blocks are located by an integer index, with the first block being LBA 0, the second LBA 1, and so on.

IDE standard included 22-bit LBA as an option, which was further extended to 28-bit with the release of ATA-1 (1994) and to 48-bit with the release of ATA-6 (2003). Most hard drives released after 1996 implement Logical block addressing.

## Overview

In logical block addressing, only one number is used to address data, and each linear base address describes a single block.

The LBA scheme replaces earlier schemes which exposed the physical details of the storage device to the software of the operating system. Chief among these was the cylinder-head-sector
Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.Though CHS...

(CHS) scheme, where blocks were addressed by means of a tuple
Tuple
In mathematics and computer science, a tuple is an ordered list of elements. In set theory, an n-tuple is a sequence of n elements, where n is a positive integer. There is also one 0-tuple, an empty sequence. An n-tuple is defined inductively using the construction of an ordered pair...

which defined the cylinder, head, and sector at which they appeared on the hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

. CHS didn't map well to devices other than hard disks (such as tapes and networked storage), and was generally not used for them. CHS was used in early MFM
Modified Frequency Modulation
Modified Frequency Modulation, commonly MFM, is a line coding scheme used to encode the actual data-bits on most floppy disk formats, hardware examples include Amiga, most CP/M machines as well as IBM PC compatibles. Early hard disk drives also used this coding.MFM is a modification to the original...

and RLL
Run Length Limited
Run length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. This is used in both telecommunication and storage systems which move a medium past a fixed head. Specifically, RLL bounds the length of stretches ...

drives, and both it and its successor Extended Cylinder-Head-Sector (ECHS) were used in the first ATA drives. However, current disk drives use zone bit recording
Zone bit recording
Zone Bit Recording is used by disk drives to store more sectors per track on outer tracks than on inner tracks. It is also called Zone Constant Angular Velocity ....

, where the number of sectors per track depends on the track number. Even though the disk drive will report some CHS values as sectors per track (SPT) and heads per cylinder (HPC), they have little to do with the disk drive's true geometry.

LBA was first introduced in SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...

as an abstraction. While the drive controller still addresses data blocks by their CHS address, this information is generally not used by the SCSI device driver, the OS, filesystem code, or any applications (such as databases) that access the "raw" disk. System calls requiring block-level I/O pass LBA definitions to the storage device driver; for simple cases (where one volume maps to one physical drive), this LBA is then passed directly to the drive controller.

In RAID devices and SAN
Storage area network
A storage area network is a dedicated network that provides access to consolidated, block level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the devices appear like locally attached devices...

s and where logical drives (LUNs) are composed via LUN virtualization and aggregation), LBA addressing of individual disk should be translated by a software layer to provide uniform LBA addressing for the entire storage device.

## Enhanced BIOS

The earlier IDE standard from Western Digital introduced 22 bit LBA addressing; in 1994, the ATA-1 standard
AT Attachment
Parallel ATA , originally ATA, is an interface standard for the connection of storage devices such as hard disks, solid-state drives, floppy drives, and optical disc drives in computers. The standard is maintained by X3/INCITS committee...

allowed for 28 bit addresses in both LBA and CHS modes. The CHS scheme used 16 bits for cylinder, 4 bits for head and 8 bits for sector, counting sectors from 1 to 255. This means the reported number of heads never exceeds 16 (0-15), the number of sectors can be 255 (1-255; though 63 is often the largest used) and the number of cylinders can be as large as 65,536 (0-65535), limiting disk size to 128 GiB (≈137.4 GB), assuming 512 byte sectors. These values can be accessed by issuing the ATA command "Identify Device" (`EC`h) to the drive.

However IBM BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

implementation defined in the INT 13H disk access routines used quite a different 24-bit scheme for CHS addressing, with 10 bits for cylinder, 8 bits for head, and 6 bits for sector, or 1024 cylinders, 256 heads, and 63 sectors. This INT 13H implementation had pre-dated the ATA standard, as it was introduced when the IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...

Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...

storage, and when hard disk drives were introduced on the IBM PC/XT, INT 13H interface could not be practically redesigned due to backward compatibility
Backward compatibility
In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...

issues. Overlapping ATA CHS mapping with BIOS CHS mapping produced the lowest common denominator of 10:4:6 bits, or 1024 cylinders, 16 heads, and 63 sectors, which gave the practical limit of 1024×16×63 sectors and 528 Mbytes (504 MiB
MIB
MIB may refer to any of several concepts:* Master of International Business, a postgraduate business degree* Melayu Islam Beraja, the adopted national philosophy of Brunei* Motion induced blindness, a visual illusion in peripheral vision...

), assuming 512 byte sectors.

In order for BIOS to overcome this limit and successfully work with large hard drives, a CHS translation scheme had to be implemented in BIOS disk I/O routines which would convert between 24-bit CHS used by INT 13H and 28-bit CHS numbering used by ATA. The translation scheme was called Large or Bit Shift Translation. This method would remap 16:4:8 bit ATA cylinders and heads to 10:8:6 bit scheme used by INT 13H, generating much more "virtual" drive heads than the physical disk reported. This increased the practical limit to 1024×256×63 sectors, or 8.4 Gbytes (7.8 GiB
Gibibyte
The gibibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The gibibyte unit symbol is GiB....

).

To further overcome this limit, INT 13H Extensions
INT 13
INT 13H or INT 19 is shorthand for BIOS interrupt call 13hex, the 20th interrupt vector in an x86-based computer system. The BIOS typically sets up a real mode interrupt handler at this vector that provides sector-based hard disk and floppy disk read and write services using cylinder-head-sector ...

were introduced with BIOS Enhanced Disk Drive Services specification, which removed practical limits on disk size for operating systems which are aware of this new interface, such as DOS 7.0 component in Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...

. This Enhanced BIOS subsystem supports LBA addressing with LBA or LBA-Assist method, which uses native 28-bit LBA for addressing ATA disks and performs CHS conversion as needed.

The Normal or None method reverts to the earlier 10:4:6 bit CHS mode which does not support addressing more than 528 Mbytes.

Until the release of ATA-2 standard in 1996, there were a handful of large hard drives which did not support LBA addressing, so only Large or Normal methods could be used. However using the Large method also introduced portability problems, as different BIOSes often used different and incompatible translation methods, and hard drives partitioned on a computer with BIOS from a particular vendor often could not be read on a computer with a different make of BIOS. The solution was to use conversion software such as OnTrack Disk Manager
Disk Manager
Disk Manager was a popular 1980s and 1990s software and documentation package for MS-DOS and PC DOS computers, useful for self-installing PC hard disks...

, EZ-Drive, etc., which installed to the disk's OS loader
Master boot record
A master boot record is a type of boot sector popularized by the IBM Personal Computer. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk...

and replaced INT 13H routines at boot time with custom code. This software could also enable LBA and INT 13H Extensions support for older computers with non LBA-compliant BIOSes.

The current 48-bit LBA scheme, introduced in 2003 with ATA-6 standard, allows addressing up to 128 PiB
Pebibyte
The pebibyte is a standards-based binary multiple of the byte, a unit of digital information storage. The pebibyte unit symbol is PiB....

. Current PC-Compatible computers support INT 13H Extensions, which use 64-bit structures for LBA addressing and should encompass any future extension of LBA addressing, though modern operating systems implement direct disk access and do not use the BIOS
BIOS
In IBM PC compatible computers, the basic input/output system , also known as the System BIOS or ROM BIOS , is a de facto standard defining a firmware interface....

subsystems, except at boot load time. However, the common DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...

style Master boot record
Master boot record
A master boot record is a type of boot sector popularized by the IBM Personal Computer. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk...

partition table only supports disk partitions up to 2 TiB in size. For large partitions this needs to be replaced by another scheme for instance the GUID Partition Table
GUID Partition Table
In computer hardware, GUID Partition Table is a standard for the layout of the partition table on a physical hard disk. Although it forms a part of the Extensible Firmware Interface standard , it is also used on some BIOS systems because of the limitations of MBR partition tables, which restrict...

which has the same 64-bit limit as the current INT 13H Extensions.

## CHS conversion

LBA and CHS equivalence with 16 heads per cylinder
LBA Value CHS Tuple
0    0, 0, 1
1    0, 0, 2
2    0, 0, 3
62    0, 0, 63
945    0, 15, 1
1007    0, 15, 63
1008    1, 0, 1
1070    1, 0, 63
1071    1, 1, 1
1133    1, 1, 63
1134    1, 2, 1
2015    1, 15, 63
2016    2, 0, 1
16,127    15, 15, 63
16,128    16, 0, 1
32,255    31, 15, 63
32,256    32, 0, 1
16,450,559    16319, 15, 63
16,514,063    16382, 15, 63

CHS (cylinder/head/sector) tuples can be mapped to LBA address with the following formula:

where,
• C, H and S are the cylinder number, the head number, and the sector number
• LBA is the logical block address
• HPC is the maximum number of heads per cylinder (reported by disk drive, typically 16 for 28-bit LBA)
• SPT is the maximum number of sectors per track (reported by disk drive, typically 63 for 28-bit LBA)

LBA addresses can be mapped to CHS tuples with the following formula:

where
• mod is the modulo operation
Modulo operation
In computing, the modulo operation finds the remainder of division of one number by another.Given two positive numbers, and , a modulo n can be thought of as the remainder, on division of a by n...

, i.e. the remainder
Remainder
In arithmetic, the remainder is the amount "left over" after the division of two integers which cannot be expressed with an integer quotient....

, and
• is integer division, i.e. the quotient
Quotient
In mathematics, a quotient is the result of division. For example, when dividing 6 by 3, the quotient is 2, while 6 is called the dividend, and 3 the divisor. The quotient further is expressed as the number of times the divisor divides into the dividend e.g. The quotient of 6 and 2 is also 3.A...

of the division.

According to the ATA specifications, "If the content of words (61:60) is greater than or equal to 16,514,064 then the content of word 1 [the number of logical cylinders] shall be equal to 16,383." Therefore for LBA 16450559, an ATA drive may actually respond with the CHS tuple (16319, 15, 63), and the number of cylinders in this scheme must be much larger than 1024 allowed by INT 13H.

### OS dependencies

Operating systems that are sensitive to BIOS-reported drive geometry include Solaris, DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...

and Windows NT family, where NTLDR
NTLDR
NTLDR is the boot loader for all releases of Windows NT operating system up to and including Windows XP and Windows Server 2003. NTLDR is typically run from the primary hard disk drive, but it can also run from portable storage devices such as a CD-ROM, USB flash drive, or floppy disk...

(NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...

, 2000
Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...

, XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...

, Server 2003
Windows Server 2003
Windows Server 2003 is a server operating system produced by Microsoft, introduced on 24 April 2003. An updated version, Windows Server 2003 R2, was released to manufacturing on 6 December 2005...

Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...

, Server 2008, Windows 7 and Server 2008 R2
Windows Server 2008 R2
Windows Server 2008 R2 is a server operating system produced by Microsoft. It was released to manufacturing on July 22, 2009 and launched on October 22, 2009. According to the Windows Server Team blog, the retail availability was September 14, 2009. It is built on Windows NT 6.1, the same core...

) use Master boot record
Master boot record
A master boot record is a type of boot sector popularized by the IBM Personal Computer. It consists of a sequence of 512 bytes located at the first sector of a data storage device such as a hard disk...

which addresses the disk using CHS; x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...

and Itanium
Itanium
Itanium is a family of 64-bit Intel microprocessors that implement the Intel Itanium architecture . Intel markets the processors for enterprise servers and high-performance computing systems...

versions of Windows can partition the drive with GUID Partition Table
GUID Partition Table
In computer hardware, GUID Partition Table is a standard for the layout of the partition table on a physical hard disk. Although it forms a part of the Extensible Firmware Interface standard , it is also used on some BIOS systems because of the limitations of MBR partition tables, which restrict...

Some operating systems do not require any translation because they do not use geometry reported by BIOS in their boot loaders. Among these operating systems are BSD, Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

, Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

, OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...

and ReactOS
ReactOS
ReactOS is an open source computer operating system intended to be binary compatible with application software and device drivers made for Microsoft Windows NT versions 5.x and up...

.

Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.Though CHS...

• Block (data storage)
Block (data storage)
In computing , a block is a sequence of bytes or bits, having a nominal length . Data thus structured are said to be blocked. The process of putting data into blocks is called blocking. Blocking is used to facilitate the handling of the data-stream by the computer program receiving the data...

• Disk storage
Disk storage
Disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating disks...

• Disk formatting
Disk formatting
Disk formatting is the process of preparing a hard disk drive or flexible disk medium for data storage. In some cases, the formatting operation may also create one or more new file systems...

• Disk partitioning
Disk partitioning
Disk partitioning is the act of dividing a hard disk drive into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks. Partitions are also termed "slices" for operating systems based on BSD, Solaris or GNU Hurd...