Cylinder-head-sector

Last updated
Cylinder, head, and sector of a hard drive. Cylinder Head Sector.svg
Cylinder, head, and sector of a hard drive.

Cylinder-head-sector (CHS) is an early method for giving addresses to each physical block of data on a hard disk drive.

Contents

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 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 [1] 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. [2]

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. [2]

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

schematic of the hard drive geometry Hard drive geometry - English - 2019-05-30.svg
schematic of the hard drive geometry

CHS addressing is the process of identifying individual sectors (aka. physical block of data) on a disk by their position in a track, where the track is determined by the head and 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. [3] [4]

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 hard disks use 4096 bytes per physical sector (4Kn) [5] since 2010, but will also be able to emulate 512 byte sectors (512e) for a transitional period. [6]

Magneto-optical drives 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 (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 tracks 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 CH combination consisting ofSPT sectors, while a cylinder consists ofSPT×H sectors.

Cylinders

A cylinder is a division of data in a disk drive, as used in the CHS addressing mode of a Fixed Block Architecture disk or the cylinder–head–record (CCHHR) addressing mode of a CKD disk.

The concept is concentric, hollow, cylindrical slices through the physical disks (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 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 (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 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 CHS addressing supported in IBM-PC compatible BIOSes code used eight bits for a maximum of 256 heads counted as head 0 up to 255 (FFh). However, a bug in all versions of 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 . 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 code as well as old PC DOS 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. [7] 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 or other cylindrical disks. Old BIOS code supported ten bits in CHS addressing with up to 1024 cylinders (1024=210). Adding six bits for sectors and eight bits for heads 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. [8]

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. 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[ citation needed ] 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. [9] 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. [7] The 28=16+4+8 bits in the ATA-2 specification are also covered by Ralf Brown's Interrupt List, and an old working draft of this now expired standard was published. [10]

With an old BIOS limit of 1024 cylinders and the ATA limit of 16 heads [11] the combined effect was 1024×16×63=1032192 sectors, i.e., a 504  MB limit for sector size 512. BIOS 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. [7]

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, [12] displayed partition sizes using 1024-byte blocks.

Clusters are allocation units for data on various file systems (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 CH track, and end in a sector on the physically or logically next CH track.

CHS to LBA mapping

In 2002 the 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. [13] 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 = 214). [14]

CHS tuples can be mapped onto LBA addresses using the following formula:

where A is the LBA address, Nheads is the number of heads on the disk, Nsectors 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 [3] and ISO/IEC  9293:1994 [15] (superseding ISO 9293:1987 [16] ) standards for FAT file systems matches exactly the LBA formula given above: Logical Block Address and Logical Sector Number (LSN) are synonyms. [3] [15] [16] 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 (CKD) hard disks on IBM mainframes 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.[ citation needed ]

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 (31.25  MiB).[ citation needed ]

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 (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 ]

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 ]

For operating systems such as Microsoft DOS or older version of Windows, each partition must start and end at a cylinder boundary.[ citation needed ] Only some of the relatively modern operating systems (Windows XP included) may disregard this rule, but doing so can still cause some compatibility issues, especially if the user wants to perform dual booting on the same drive. Microsoft does not follow this rule with internal disk partition tools since Windows Vista. [17]

See also

Related Research Articles

<span class="mw-page-title-main">Disk storage</span> General category of storage mechanisms

Disk 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 a device implementing such a storage mechanism. Notable types are the hard disk drive (HDD) containing a non-removable disk, the floppy disk drive (FDD) and its removable floppy disk, and various optical disc drives (ODD) and associated optical disc media.

<span class="mw-page-title-main">Hard disk drive</span> Electro-mechanical data storage device

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 magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data when powered off. Modern HDDs are typically in the form of a small rectangular box.

File Allocation Table (FAT) is a file system developed for personal computers and was the default filesystem for MS-DOS and Windows 9x operating systems. Originally developed in 1977 for use on floppy disks, it was adapted for use on hard disks and other devices. The increase in disk drives capacity required three major variants: FAT12, FAT16 and FAT32. FAT was replaced with NTFS as the default file system on Microsoft operating systems starting with Windows XP. Nevertheless, FAT continues to be used on flash and other solid-state memory cards and modules, many portable and embedded devices because of its compatibility and ease of implementation.

A direct-access storage device (DASD) 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, the main examples being drum memory and hard disk drives. Later, optical disc drives and flash memory units are also classified as DASD.

<span class="mw-page-title-main">SuperDisk</span> Storage medium from Imation

The SuperDisk LS-120 is a high-speed, high-capacity alternative to the 90 mm (3.5 in), 1.44 MB floppy disk. The SuperDisk hardware was created by 3M's storage products group Imation in 1997, with manufacturing chiefly by Matsushita.

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 file systems. The first part of the formatting process that performs basic medium preparation is often referred to as "low-level formatting". Partitioning is the common term for the second part of the process, dividing the device into several sub-devices and, in some cases, writing information to the device allowing an operating system to be booted from it. The third part of the process, usually termed "high-level formatting" most often refers to the process of generating a new file system. In some operating systems all or parts of these three processes can be combined or repeated at different levels and the term "format" is understood to mean an operation in which a new disk medium is fully prepared to store files. Some formatting utilities allow distinguishing between a quick format, which does not erase all existing data and a long option that does erase all existing data.

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

In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.

<span class="mw-page-title-main">TRSDOS</span> Operating system for Tandy TRS-80 computers

TRSDOS is the operating system for the Tandy TRS-80 line of eight-bit Zilog Z80 microcomputers that were sold through Radio Shack from 1977 through 1991. Tandy's manuals recommended that it be pronounced triss-doss. TRSDOS should not be confused with Tandy DOS, a version of MS-DOS licensed from Microsoft for Tandy's x86 line of personal computers (PCs).

<span class="mw-page-title-main">SpinRite</span>

SpinRite is a computer program for scanning RAS Random Access Storage devices such as hard disks, reading and rewriting data using proprietary programming methods to resolve and retrieve data that is unreadable by DOS or Windows. The first version was released in 1987 by Steve Gibson. The current version, 6.0, was released in 2004., with ongoing development open to the public at https://www.grc.com/dev/spinrite/

<span class="mw-page-title-main">GUID Partition Table</span> Computer disk partitioning standard

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 identifiers (GUIDs). Forming a part of the Unified Extensible Firmware Interface (UEFI) standard, it is nevertheless also used for some BIOSs, because of the limitations of master boot record (MBR) partition tables, which use 32 bits for logical block addressing (LBA) of traditional 512-byte disk sectors.

INT 13h 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 (CHS) addressing. Modern PC BIOSes also include INT 13h extension functions, originated by IBM and Microsoft in 1992, that provide those same disk access services using 64-bit LBA addressing; with minor additions, these were quasi-standardized by Phoenix Technologies and others as the EDD BIOS extensions.

The Logical Disk Manager (LDM) is an implementation of a logical volume manager for Microsoft Windows NT, developed by Microsoft and Veritas Software. It was introduced with the Windows 2000 operating system, and is supported in Windows XP, Windows Server 2003, Windows Vista, Windows 7, Windows 8, Windows 10 and Windows 11. The MMC-based Disk Management snap-in hosts the Logical Disk Manager. On Windows 8 and Windows Server 2012, Microsoft deprecated LDM in favor of Storage Spaces.

An extended boot record (EBR), or extended partition boot record (EPBR), is a descriptor for a logical partition under the common DOS disk drive partitioning system. In that system, when one partition record entry in the master boot record (MBR) is designated an extended partition, then that partition can be subdivided into a number of logical partitions. The actual structure of that extended partition is described by one or more EBRs, which are located inside the extended partition. The first EBR will always be located on the very first sector of the extended partition.

<span class="mw-page-title-main">Disk sector</span> Logical or physical division of storage media

In computer disk storage, a sector is a subdivision of a track on a magnetic disk or optical disc. For most disks, each sector stores a fixed amount of user-accessible data, traditionally 512 bytes for hard disk drives (HDDs) and 2048 bytes for CD-ROMs and DVD-ROMs. Newer HDDs and SSDs use 4096-byte (4 KiB) sectors, which are known as the Advanced Format (AF).

Track0 is the area located at the start of a computer disk and is predominantly used to store information regarding the layout of the disk and executable code needed to boot an operating system. Track0 typically consists of the first 64 sectors of the disk. Since the first sector of a disk is sector zero, the first 64 sectors are sectors 0 to 63.

Fixed-block architecture (FBA) is an IBM term for the hard disk drive (HDD) layout in which each addressable block on the disk has the same size, utilizing 4 byte block numbers and a new set of command codes. FBA as a term was created and used by IBM for its 3310 and 3370 HDDs beginning in 1979 to distinguish such drives as IBM transitioned away from their variable record size format used on IBM's mainframe hard disk drives beginning in 1964 with its System/360.

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 MBRs was publicly introduced in 1983 with PC DOS 2.0.

Ranish Partition Manager is a freeware hard disk partition editor, disk cloning utility, and boot manager, that gives a high level of control for creating multi-boot systems. It is available on the freeware live CD SystemRescueCD and the Ultimate Boot CD. It runs under MS-DOS, PC DOS, DR-DOS, or FreeDOS.

The FAT file system is a file system used on MS-DOS and Windows 9x family of operating systems. It continues to be used on mobile devices and embedded systems, and thus is a well suited file system for data exchange between computers and devices of almost any type and age from 1981 through the present.

References

  1. ↑ "Overview and History of the IDE/ATA Interface". The PC Guide. 17 April 2001. Archived from the original on 4 February 2019.
  2. 1 2 Jonathan de Boyne Pollard (2011). "The gen on disc partition alignment" . Retrieved 21 November 2022.
  3. 1 2 3 "Volume and File Structure of Disk Cartridges for Information Interchange". Standard ECMA-107 (2nd ed., June 1995). ECMA. 1995. Retrieved 30 July 2011.
  4. ↑ "Standard Floppy Disk Formats Supported by MS-DOS". KB75131. Microsoft Knowledge Base. 12 May 2003. Archived from the original on 31 January 2009. Retrieved 4 June 2023.
  5. ↑ "Western Digital's Advanced Format: The 4K Sector Transition Begins". AnandTech. 18 December 2009. Retrieved 29 July 2011.
  6. ↑ "Advanced Format Technology Brief" (PDF). Hitachi. 2010. p. 1. Archived from the original (PDF) on 27 September 2011. Retrieved 1 August 2011. 512 byte emulation is sometimes referred to as 512e
  7. 1 2 3 Andries Brouwer (1 November 2004). "History of BIOS and IDE limits". Large Disk HOWTO v2.5. Retrieved 30 July 2011.
  8. ↑ "Windows NT 4.0 supports maximum of 7.8-GB system partition". Microsoft. 23 February 2007. Retrieved 30 July 2011.
  9. ↑ "5K500.B SATA OEM Specification Revision 1.2" (PDF). Hitachi. 17 March 2009. p. 51. Archived from the original (PDF) on 27 September 2011.
  10. ↑ "ATA-2" (PDF). X3T10/0948D. INCITS Technical Committee T13 AT Attachment. 18 March 1996. Archived from the original (PDF) on 28 July 2011.
  11. ↑ "ATA-1" (PDF). X3T10/791D. INCITS Technical Committee T10 SCSI Storage Interfaces. 1994. Archived from the original (PDF) on 21 March 2012.
  12. ↑ "Util-linux 2.25 Release Notes". The Linux Kernel Archives. Retrieved 24 March 2016.
  13. ↑ "ATA-6" (PDF). T13/1410D. INCITS Technical Committee T13 ATA Storage Interface. 2002. p. 22. Archived from the original (PDF) on 28 July 2011. Retrieved 30 July 2011. 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.
  14. ↑ "ATA-5" (PDF). T13/1321D. INCITS Technical Committee T13 ATA Storage Interface. 2000. p. 19. Archived from the original (PDF) on 28 July 2011. Retrieved 30 July 2011. 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.
  15. 1 2 "Information technology -- Volume and file structure of disk cartridges for information interchange". ISO/IEC 9293:1994. ISO catalogue. 1994. Retrieved 6 January 2012.
  16. 1 2 "Information processing -- Volume and file structure of flexible disk cartridges for information interchange". ISO 9293:1987. ISO catalogue. 1987. Retrieved 6 January 2012.
  17. ↑ "KB931760". Microsoft Windows XP Support. Microsoft Knowledge Base. 23 July 2009. Retrieved 30 July 2011.

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 standard to not use a head value of 255.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.