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85-438: A master boot record ( MBR ) is a type of boot sector in the first block 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 . The MBR holds the information on how the disc's sectors (aka "blocks") are divided into partitions, each partition notionally containing

170-653: A protective MBR . Specifically, it does not provide backward compatibility with operating systems that do not support the GPT scheme as well. Meanwhile, multiple forms of hybrid MBRs have been designed and implemented by third parties in order to maintain partitions located in the first physical 2 TiB of a disk in both partitioning schemes "in parallel" and/or to allow older operating systems to boot off GPT partitions as well. The present non-standard nature of these solutions causes various compatibility problems in certain scenarios. The MBR consists of 512 or more bytes located in

255-524: A 1 KB sector could be used with no problem. Like the MBR, a volume boot record normally expects to be loaded at address 0x0000 : 0x7C00 . This derives from the fact that the volume boot record design originated on unpartitioned media, where a volume boot record would be directly loaded by the BIOS boot procedure; as mentioned above, the BIOS treats MBRs and volume boot records (VBRs) exactly alike. Since this

340-403: A 64-bit integer, in little-endian notation, which are used to locate the byte offset of this partition. In this case, 00 7E corresponds to the hexadecimal value 0x7E00 ( 32,256 ). Under the assumption that the drive in question reports a sector size of 512 bytes, then dividing this byte offset by 512 results in 63, which is the physical sector number (or LBA) containing the first sector of

425-449: A VBR. If the device is a hard disk, that will be an MBR. It is the code in the MBR which generally understands disk partitioning, and in turn, is responsible for loading and running the VBR of whichever primary partition is set to boot (the active partition). The VBR then loads a second-stage bootloader from another location on the disk. Furthermore, whatever is stored in the first sector of

510-507: A Windows 2000/XP registry, is: If a disk's signature stored in the MBR was A8 E1 B9 D2 (in that order) and its first partition corresponded with logical drive C: under Windows, then the REG_BINARY data under the key value \DosDevices\C: would be: The first four bytes are said disk signature. (In other keys, these bytes may appear in reverse order from that found in the MBR sector.) These are followed by eight more bytes, forming

595-463: A crash when the BIOS passes execution to the boot sector assuming that it contains valid executable code. Nevertheless, some media for other platforms erroneously contain the signature, anyway, rendering this check not 100% reliable in practice. The signature is checked for by most system BIOSes since (at least) the IBM PC/AT (but not by the original IBM PC and some other machines). Even more so, it

680-399: A disk with a GPT, Intel's Extensible Firmware Interface specification requires that the tuple (1023, 255, 63) be used.) The 10-bit cylinder value is recorded within two bytes in order to facilitate making calls to the original/legacy INT 13h BIOS disk access routines, where 16 bits were divided into sector and cylinder parts, and not on byte boundaries. Due to the limits of CHS addressing,

765-424: A dummy boot loader which is safe to be executed, even if it may not be able actually to load an operating system. It does not indicate a particular (or even the presence of) file system or operating system, although some old versions of DOS 3 relied on it in their process to detect FAT -formatted media (newer versions do not). Boot code for other platforms or CPUs should not use this signature, since this may lead to

850-824: A file system. The MBR also contains executable code to function as a loader for the installed operating system—usually by passing control over to the loader's second stage, or in conjunction with each partition's volume boot record (VBR). This MBR code is usually referred to as a boot loader. The organization of the partition table in the MBR limits the maximum addressable storage space of a partitioned disk to 2  TiB (2 × 512 bytes) . Approaches to slightly raise this limit utilizing 32-bit arithmetic or 4096-byte sectors are not officially supported, as they fatally break compatibility with existing boot loaders, most MBR-compliant operating systems and associated system tools, and may cause serious data corruption when used outside of narrowly controlled system environments. Therefore,

935-519: A floppy diskette, USB device, hard disk or any other bootable storage device, is not required to immediately load any bootstrap code for an OS, if ever. The BIOS merely passes control to whatever exists there, as long as the sector meets the very simple qualification of having the boot record signature of 0x55, 0xAA in its last two bytes. This is why it is easy to replace the usual bootstrap code found in an MBR with more complex loaders, even large multi-functional boot managers (programs stored elsewhere on

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1020-456: A new file system with DOS 3.0. Support for an extended partition , a special primary partition type used as a container to hold other partitions, was added with DOS 3.2, and nested logical drives inside an extended partition came with DOS 3.30. Since MS-DOS, PC DOS, OS/2 and Windows were never enabled to boot off them, the MBR format and boot code remained almost unchanged in functionality (except some third-party implementations) throughout

1105-423: A partition table record is used to indicate an active partition. Standard-conformant MBRs will allow only one partition marked active and use this as part of a sanity-check to determine the existence of a valid partition table. They will display an error message, if more than one partition has been marked active. Some non-standard MBRs will not treat this as an error condition and just use the first marked partition in

1190-563: A partition that requires 33 rather than 32 bits for the sector address to be accessed. However, in practice, only certain LBA-48 -enabled operating systems, including Linux, FreeBSD and Windows 7 that use 64-bit sector addresses internally actually support this. Due to code space constraints and the nature of the MBR partition table to only support 32 bits, boot sectors, even if enabled to support LBA-48 rather than LBA-28 , often use 32-bit calculations, unless they are specifically designed to support

1275-462: A sector size of 512 bytes or are limited to 32-bit calculations, exceeding the 2 TiB limit could cause compatibility problems. In operating systems that support GPT-based boot through BIOS services rather than EFI, the first sector may also still be used to store the first stage of the bootloader code, but modified to recognize GPT partitions. The bootloader in the MBR must not assume a sector size of 512 bytes. The partition table header defines

1360-547: A small program that loads the Volume Boot Record (VBR) of the targeted partition. Control is then passed to this code, which is responsible for loading the actual operating system. This process is known as chain loading . Popular MBR code programs were created for booting PC DOS and MS-DOS , and similar boot code remains in wide use. These boot sectors expect the FDISK partition table scheme to be in use and scans

1445-492: A transition was made to using LBA, or logical block addressing . Both the partition length and partition start address are sector values stored in the partition table entries as 32-bit quantities. The sector size used to be considered fixed at 512 (2) bytes, and a broad range of important components including chipsets , boot sectors , operating systems , database engines , partitioning tools, backup and file system utilities and other software had this value hard-coded. Since

1530-578: Is a 32-bit value that is intended to identify uniquely the disk medium (as opposed to the disk unit—the two not necessarily being the same for removable hard disks). The disk signature was introduced by Windows NT version 3.5, but it is now used by several operating systems, including the Linux kernel version 2.6 and later. Linux tools can use the NT disk signature to determine which disk the machine booted from. Windows NT (and later Microsoft operating systems) uses

1615-611: Is also checked by most MBR boot loaders before passing control to the boot sector. Some BIOSes (like the IBM PC/AT) perform the check only for fixed disk/removable drives, while for floppies and superfloppies, it is enough to start with a byte greater or equal to 06h and the first nine words not to contain the same value, before the boot sector is accepted as valid, thereby avoiding the explicit test for 0x55 , 0xAA on floppies. Since old boot sectors (e.g., very old CP/M-86 and DOS media) sometimes do not feature this signature despite

1700-408: Is an issue. Where a data storage device has been partitioned with the GPT scheme, the master boot record will still contain a partition table, but its only purpose is to indicate the existence of the GPT and to prevent utility programs that understand only the MBR partition table scheme from creating any partitions in what they would otherwise see as free space on the disk, thereby accidentally erasing

1785-560: Is conflictive with the Advanced Active Partition feature of PTS-DOS 7 and DR-DOS 7.07, in particular if their boot code is located outside the first 8 GB of the disk, so that LBA addressing must be used. The MBR originated in the PC XT . IBM PC-compatible computers are little-endian , which means the processor stores numeric values spanning two or more bytes in memory least significant byte first. The format of

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1870-413: Is expected to be. There are two definitions on how to store the information regarding the partitioning: The presence of an IBM PC compatible boot loader for x86-CPUs in the boot sector is by convention indicated by a two-byte hexadecimal sequence 0x55 0xAA (called the boot sector signature) at the end of the boot sector (offsets 0x1FE and 0x1FF ). This signature indicates the presence of at least

1955-400: Is located at the beginning of the "active" partition. The volume boot record will fit within a 512-byte sector, but it is safe for the MBR code to load additional sectors to accommodate boot loaders longer than one sector, provided they do not make any assumptions on what the sector size is. In fact, at least 1 KB of RAM is available at address 0x7C00 in every IBM XT- and AT-class machine, so

2040-405: Is not located in a partition; it is located at a first sector of the device (physical offset 0), preceding the first partition. (The boot sector present on a non-partitioned device or within an individual partition is called a volume boot record instead.) In cases where the computer is running a DDO BIOS overlay or boot manager , the partition table may be moved to some other physical location on

2125-464: Is some MBR replacement code that emulates EFI firmware's bootstrap, which makes non-EFI machines capable of booting from disks using the GPT partitioning scheme. It detects a GPT, places the processor in the correct operating mode, and loads the EFI compatible code from disk to complete this task. In addition to the bootstrap code and a partition table, master boot records may contain a disk signature . This

2210-489: Is still reserved in the GPT specification, but it is now used in a way that prevents MBR-based disk utilities from misrecognizing and possibly overwriting GPT disks. This is referred to as a protective MBR . A single partition of type EEh , encompassing the entire GPT drive (where "entire" actually means as much of the drive as can be represented in an MBR), is indicated and identifies it as GPT. Operating systems and tools which cannot read GPT disks will generally recognize

2295-424: Is stored in the MBR partition table using a beginning block address and a length, it may in theory be possible to define partitions in such a way that the allocated space for a disk with 512-byte sectors gives a total size approaching 4 TiB, if all but one partition are located below the 2 TiB limit and the last one is assigned as starting at or close to block 2−1 and specify the size as up to 2−1, thereby defining

2380-662: Is the MEMZ 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 (UUIDs), which are also known as globally unique identifiers (GUIDs). Forming a part of the Unified Extensible Firmware Interface (UEFI) standard ( Unified EFI Forum -proposed replacement for

2465-719: Is the nature and purpose of GUIDs and as per RFC 4122, no central registry is needed to ensure the uniqueness of the GUID partition type designators. The 64-bit partition table attributes are shared between 48-bit common attributes for all partition types, and 16-bit type-specific attributes: Microsoft defines the type-specific attributes for basic data partition as: Google defines the type-specific attributes for ChromeOS kernel as: Windows 7 and earlier do not support UEFI on 32-bit platforms, and therefore do not allow booting from GPT partitions. Limited to 128 partitions per disk. "Partition type GUID" means that each partition type

2550-450: Is the same location where the MBR is loaded, one of the first tasks of an MBR is to relocate itself somewhere else in memory. The relocation address is determined by the MBR, but it is most often 0x0000 : 0x0600 (for MS-DOS/PC DOS, OS/2 and Windows MBR code) or 0x0060 : 0x0000 (most DR-DOS MBRs). (Even though both of these segmented addresses resolve to the same physical memory address in real mode, for Apple Darwin to boot,

2635-529: Is unsuitable, and the MBR is not used as part of the system bootstrap. EFI firmware is instead capable of directly understanding the GPT partitioning scheme and the FAT filesystem format, and loads and runs programs held as files in the EFI System partition . The MBR will be involved only insofar as it might contain a partition table for compatibility purposes if the GPT partition table scheme has been used. There

Master boot record - Misplaced Pages Continue

2720-560: The BIOS Boot Specification (BBS), it is best practice for a modern MBR accepting bit 7 as active flag to pass on the DL value originally provided by the BIOS instead of using the entry in the partition table. Boot sector Usually, the first sector of the hard disk is the boot sector, regardless of sector size (512 or 4096 bytes) and partitioning flavor ( MBR or GPT ). The purpose of defining one particular sector as

2805-591: The PC BIOS ), 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 . All modern personal computer operating systems support GPT. Some, including macOS and Microsoft Windows on the x86 architecture, support booting from GPT partitions only on systems with EFI firmware, but FreeBSD and most Linux distributions can boot from GPT partitions on systems with either

2890-605: The boot sector is inter-operability between firmware and various operating systems. The purpose of chain loading first a firmware (e.g., the BIOS), then some code contained in the boot sector, and then, for example, an operating system, is maximal flexibility. On an IBM PC compatible machine, the BIOS selects a boot device, then copies the first sector from the device (which may be a MBR , VBR or any executable code), into physical memory at memory address 0x7C00. On other systems,

2975-464: The BIOS or the EFI firmware interface. The Master Boot Record (MBR) partitioning scheme, widely used since the early 1980s, imposed limitations for use of modern hardware. The available size for block addresses and related information is limited to 32 bits. For hard disks with 512‑byte sectors, the MBR partition table entries allow a maximum size of 2  TiB (2³² × 512‑bytes) or 2.20  TB (2.20 × 10¹² bytes). In

3060-405: The GPT header. The first 16 bytes of each entry designate the partition type's globally unique identifier (GUID). For example, the GUID for an EFI system partition is C12A7328-F81F-11D2-BA4B-00A0C93EC93B . The second 16 bytes are a GUID unique to the partition. Then follow the starting and ending 64 bit LBAs, partition attributes, and the 36 character (max.) Unicode partition name. As

3145-509: The GPT. On IBM PC-compatible computers, the bootstrapping firmware (contained within the ROM BIOS ) loads and executes the master boot record. The PC/XT (type 5160) used an Intel 8088 microprocessor . In order to remain compatible, all x86 BIOS architecture systems start with the microprocessor in an operating mode referred to as real mode . The BIOS reads the MBR from the storage device into physical memory , and then it directs

3230-535: The MBR boot sector code expects to be loaded at physical address 0x0000 : 0x7C00 , all the memory from physical address 0x0000 : 0x0501 (address 0x0000 : 0x0500 is the last one used by a Phoenix BIOS) to 0x0000 : 0x7FFF , later relaxed to 0x0000 : 0xFFFF (and sometimes up to 0x9000 : 0xFFFF )‍—‌the end of the first 640 KB ‍—‌is available in real mode. The INT 12h BIOS interrupt call may help in determining how much memory can be allocated safely (by default, it simply reads

3315-414: The MBR boot sector program must be small enough to fit within 446 bytes of memory or less. The MBR code examines the partition table, selects a suitable partition and loads the program that will perform the next stage of the boot process, usually by making use of INT 13h BIOS calls . The MBR bootstrap code loads and runs (a boot loader- or operating system-dependent) volume boot record code that

3400-406: The MBR copy. The primary validation for most BIOSes is the signature at offset 0x01FE , although a BIOS implementer may choose to include other checks, such as verifying that the MBR contains a valid partition table without entries referring to sectors beyond the reported capacity of the disk. To the BIOS, removable (e.g. floppy) and fixed disks are essentially the same. For either, the BIOS reads

3485-461: The MBR must be relocated to 0x0000 : 0x0600 instead of 0x0060 : 0x0000 , since the code depends on the DS:SI pointer to the partition entry provided by the MBR, but it erroneously refers to it via 0x0000 :SI only.) It is important not to relocate to other addresses in memory because many VBRs will assume a certain standard memory layout when loading their boot file. The Status field in

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3570-411: The MBR on media reflects this convention. Thus, the MBR signature will appear in a disk editor as the sequence 55 AA . The bootstrap sequence in the BIOS will load the first valid MBR that it finds into the computer's physical memory at address 0x7C00 to 0x7FFF . The last instruction executed in the BIOS code will be a "jump" to that address in order to direct execution to the beginning of

3655-472: The MBR partitioning scheme (without 32-bit arithmetic) is limited to 2 TiB. Consequently, a different partitioning scheme must be used for larger disks, as they have become widely available since 2010. The MBR partitioning scheme is therefore in the process of being superseded by the GUID Partition Table (GPT). The official approach does little more than ensuring data integrity by employing

3740-413: The MBR started to treat a set bit 7 as active flag and showed an error message for values 0x01 .. 0x7F only. It continued to treat the entry as physical drive unit to be used when loading the corresponding partition's VBR later on, thereby now also accepting other boot drives than 0x80 as valid, however, MS-DOS did not make use of this extension by itself. Storing the actual physical drive number in

3825-444: The MBR, which enforce CHS access for FAT16B and FAT32 partition types 0x06 / 0x0B , whereas LBA is used for 0x0E / 0x0C . Despite sometimes poor documentation of certain intrinsic details of the MBR format (which occasionally caused compatibility problems), it has been widely adopted as a de facto industry standard, due to the broad popularity of PC-compatible computers and its semi-static nature over decades. This

3910-555: The MBR-based partitioning scheme is in the process of being superseded by the GUID Partition Table (GPT) scheme in new computers. A GPT can coexist with an MBR in order to provide some limited form of backward compatibility for older systems. MBRs are not present on non-partitioned media such as floppies , superfloppies or other storage devices configured to behave as such, nor are they necessarily present on drives used in non-PC platforms. Support for partitioned media, and thereby

3995-592: The OS may refuse to manipulate the partition table. If the actual size of the disk exceeds the maximum partition size representable using the legacy 32-bit LBA entries in the MBR partition table, the recorded size of this partition is clipped at the maximum, thereby ignoring the rest of the disk. This amounts to a maximum reported size of 2 TiB, assuming a disk with 512 bytes per sector (see 512e ). It would result in 16 TiB with 4 KiB sectors ( 4Kn ), but since many older operating systems and tools are hard coded for

4080-484: The Partition Entry Array. Thus, on a disk with 512-byte sectors, at least 32 sectors are used for the Partition Entry Array, and the first usable block is at LBA 34 or higher, while on a 4,096-byte sectors disk, at least 4 sectors are used for the Partition Entry Array, and the first usable block is at LBA 6 or higher. For limited backward compatibility, the space of the legacy Master Boot Record (MBR)

4165-470: The base memory size in KB from segment :offset location 0x0040 : 0x0013 , but it may be hooked by other resident pre-boot software like BIOS overlays, RPL code or viruses to reduce the reported amount of available memory in order to keep other boot stage software like boot sectors from overwriting them). The last 66 bytes of the 512-byte MBR are reserved for the partition table and other information, so

4250-499: The boot manager code to load and to execute. (The first relies on behavior that is not universal across all disk partitioning utilities, most notably those that read and write GPTs. The last requires that the embedded list of disk locations be updated when changes are made that would relocate the remainder of the code.) On machines that do not use x86 processors, or on x86 machines with non-BIOS firmware such as Open Firmware or Extensible Firmware Interface (EFI) firmware, this design

4335-440: The device which can run without an operating system), allowing users a number of choices in what occurs next. With this kind of freedom, abuse often occurs in the form of boot sector viruses. Since code in the boot sector is executed automatically, boot sectors have historically been a common attack vector for computer viruses . To combat this behavior, the system BIOS often includes an option to prevent software from writing to

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4420-703: The device; e.g., Ontrack Disk Manager often placed a copy of the original MBR contents in the second sector, then hid itself from any subsequently booted OS or application, so the MBR copy was treated as if it were still residing in the first sector. By convention, there are exactly four primary partition table entries in the MBR partition table scheme, although some operating systems and system tools extended this to five (Advanced Active Partitions (AAP) with PTS-DOS 6.60 and DR-DOS 7.07), eight ( AST and NEC MS-DOS 3.x as well as Storage Dimensions SpeedStor ), or even sixteen entries (with Ontrack Disk Manager ). An artifact of hard disk technology from

4505-416: The disk as containing one partition of unknown type and no empty space, and will typically refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This minimizes accidental erasures. Furthermore, GPT-aware OSes may check the protective MBR and if the enclosed partition type is not of type EEh or if there are multiple partitions defined on the target device,

4590-411: The disk partitioning in an inconsistent state. As an example, the malware NotPetya attempts to gain administrative privileges on an operating system, and then would attempt to overwrite the boot sector of a computer. The CIA has also developed malware that attempts to modify the boot sector in order to load additional drivers to be used by other malware. Another Malware that overwrites boot sector

4675-585: The disk signature as an index to all the partitions on any disk ever connected to the computer under that OS; these signatures are kept in Windows Registry keys, primarily for storing the persistent mappings between disk partitions and drive letters. It may also be used in Windows NT BOOT.INI files (though most do not), to describe the location of bootable Windows NT (or later) partitions. One key (among many), where NT disk signatures appear in

4760-399: The disk, which it assumes to be "free" space that is not allocated to any disk partition, and executes it. A MBR program may interact with the user to determine which partition on which drive should boot, and may transfer control to the MBR of a different drive. Other MBR code contains a list of disk locations (often corresponding to the contents of files in a filesystem ) of the remainder of

4845-650: The drive is forced to perform two read-modify-write operations to satisfy a single misaligned 4,096‑byte write operation. Since April 2014, enterprise-class drives without emulation technology ( 4K native ) have been available on the market. Readiness of the support for 4 KB logical sectors within operating systems differs among their types, vendors and versions. For example, Microsoft Windows supports 4K native drives since Windows 8 and Windows Server 2012 (both released in 2012) in UEFI . Like MBR, GPT uses logical block addressing (LBA) in place of

4930-403: The end of 2009, disk drives employing 4096-byte sectors ( 4Kn or Advanced Format ) have been available, although the size of the sector for some of these drives was still reported as 512 bytes to the host system through conversion in the hard-drive firmware and referred to as 512 emulation drives ( 512e ). Since block addresses and sizes are stored in the partition table of an MBR using 32 bits,

5015-579: The era of the PC XT , the partition table subdivides a storage medium using units of cylinders , heads , and sectors ( CHS addressing). These values no longer correspond to their namesakes in modern disk drives, as well as being irrelevant in other devices such as solid-state drives , which do not physically have cylinders or heads. In the CHS scheme, sector indices have (almost) always begun with sector 1 rather than sector 0 by convention, and due to an error in all versions of MS-DOS/PC DOS up to including 7.10,

5100-522: The eras of DOS and OS/2 up to 1996. In 1996, support for logical block addressing (LBA) was introduced in Windows 95B and MS-DOS 7.10 (Not to be confused with IBM PC-DOS 7.1) in order to support disks larger than 8 GB. Disk timestamps were also introduced. This also reflected the idea that the MBR is meant to be operating system and file system independent. However, this design rule was partially compromised in more recent Microsoft implementations of

5185-471: The fact that they can be booted successfully, the check can be disabled in some environments. If the BIOS or MBR code does not detect a valid boot sector and therefore cannot pass execution to the boot sector code, it will try the next boot device in the row. If they all fail it will typically display an error message and invoke INT 18h. This will either start up optional resident software in ROM ( ROM BASIC ), reboot

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5270-623: The first sector of the drive. It may contain one or more of: IBM PC DOS 2.0 introduced the FDISK utility to set up and maintain MBR partitions. When a storage device has been partitioned according to this scheme, its MBR contains a partition table describing the locations, sizes, and other attributes of linear regions referred to as partitions. The partitions themselves may also contain data to describe more complex partitioning schemes, such as extended boot records (EBRs), BSD disklabels , or Logical Disk Manager metadata partitions. The MBR

5355-565: The first byte of physical sector 20,498,940 . Starting with Windows Vista , the disk signature is also stored in the Boot Configuration Data (BCD) store, and the boot process depends on it. If the disk signature changes, cannot be found or has a conflict, Windows is unable to boot. Unless Windows is forced to use the overlapping part of the LBA address of the Advanced Active Partition entry as pseudo-disk signature, Windows' usage

5440-404: The first physical sector of the media into RAM at absolute address 0x7C00 , checks the signature in the last two bytes of the loaded sector, and then, if the correct signature is found, transfers control to the first byte of the sector with a jump (JMP) instruction. The only real distinction that the BIOS makes is that (by default, or if the boot order is not configurable) it attempts to boot from

5525-420: The first removable disk before trying to boot from the first fixed disk. From the perspective of the BIOS, the action of the MBR loading a volume boot record into RAM is exactly the same as the action of a floppy disk volume boot record loading the object code of an operating system loader into RAM. In either case, the program that the BIOS loaded is going about the work of chain loading an operating system. While

5610-400: The first sector of any attached hard drives; it could thereby protect the master boot record containing the partition table from being overwritten accidentally, but not the volume boot records in the bootable partitions. Depending on the BIOS, attempts to write to the protected sector may be blocked with or without user interaction. Most BIOSes, however, will display a popup message giving

5695-469: The full address range of LBA-48 or are intended to run on 64-bit platforms only. Any boot code or operating system using 32-bit sector addresses internally would cause addresses to wrap around accessing this partition and thereby result in serious data corruption over all partitions. For disks that present a sector size other than 512 bytes, such as USB external drives , there are limitations as well. A sector size of 4096 results in an eight-fold increase in

5780-420: The hard disk will no longer be bootable, unless used with a custom BIOS that defines a non-damaged sector as the boot sector. However, since the very first sector additionally contains data regarding the partitioning of the hard disk, the hard disk will become entirely unusable except when used in conjunction with custom software. A disk can be partitioned into multiple partitions and, on conventional systems, it

5865-456: The historical cylinder-head-sector (CHS) addressing. The protective MBR is stored at LBA 0, and the GPT header is in LBA 1, with a backup GPT header stored at the final LBA. The GPT header has a pointer to the partition table ( Partition Entry Array ), which is typically at LBA 2. Each entry on the partition table has a size of 128 bytes. The UEFI specification stipulates that a minimum of 16,384 bytes, regardless of sector size, are allocated for

5950-570: The late 1990s, Intel developed a new partition table format as part of what eventually became the Unified Extensible Firmware Interface (UEFI). The GUID Partition Table is specified in chapter 5 of the UEFI 2.8 specification. GPT uses 64 bits for logical block addresses, allowing a maximum disk size of 2 sectors. For disks with 512‑byte sectors, the maximum size is 8  ZiB (2  × 512‑bytes) or 9.44  ZB (9.44 × 10²¹ bytes). For disks with 4,096‑byte sectors

6035-413: The list of partitions in the MBR's embedded partition table to find the only one that is marked with the active flag . It then loads and runs the volume boot record (VBR) of the active partition. There are alternative boot code implementations, some of which are installed by boot managers , which operate in a variety of ways. Some MBR code loads additional code for a boot manager from the first track of

6120-562: The master boot record (MBR), was introduced with IBM PC DOS 2.0 in March 1983 in order to support the 10 MB hard disk of the then-new IBM Personal Computer XT , still using the FAT12 file system. The original version of the MBR was written by David Litton of IBM in June 1982. The partition table supported up to four primary partitions . This did not change when FAT16 was introduced as

6205-522: The maximum size is 64  ZiB (2  × 4,096‑bytes) or 75.6  ZB (75.6 × 10²¹ bytes). In 2010, hard-disk manufacturers introduced drives with 4,096‑byte sectors ( Advanced Format ). For compatibility with legacy hardware and software, those drives include an emulation technology ( 512e ) that presents 512‑byte sectors to the entity accessing the hard drive, despite their underlying 4,096‑byte physical sectors. Performance could be degraded on write operations, when

6290-403: The maximum size, as well as the highest start address, of a partition using drives that have 512-byte sectors (actual or emulated) cannot exceed 2 TiB −512 bytes ( 2 199 023 255 040 bytes or 4 294 967 295 (2−1) sectors × 512 (2) bytes per sector). Alleviating this capacity limitation was one of the prime motivations for the development of the GPT. Since partitioning information

6375-422: The microprocessor to the start of the boot code. The BIOS will switch the processor to real mode, then begin to execute the MBR program, and so the beginning of the MBR is expected to contain real-mode machine code . Since the BIOS bootstrap routine loads and runs exactly one sector from the physical disk, having the partition table in the MBR with the boot code simplifies the design of the MBR program. It contains

6460-399: The number of heads is generally limited to 255 instead of 256. When a CHS address is too large to fit into these fields, the tuple (1023, 254, 63) is typically used today, although on older systems, and with older disk tools, the cylinder value often wrapped around modulo the CHS barrier near 8 GB, causing ambiguity and risks of data corruption. (If the situation involves a "protective" MBR on

6545-404: The partition (unlike the sector count used in the sectors value of CHS tuples, which counts from one , the absolute or LBA sector value starts counting from zero ). If this disk had another partition with the values 00 F8 93 71 02 following the disk signature (under, e.g., the key value \DosDevices\D: ), it would begin at byte offset 0x00027193F800 ( 10,495,457,280 ), which is also

6630-442: The partition table does not normally cause backward compatibility problems, since the value will differ from 0x80 only on drives other than the first one (which have not been bootable before, anyway). However, even with systems enabled to boot off other drives, the extension may still not work universally, for example, after the BIOS assignment of physical drives has changed when drives are removed, added or swapped. Therefore, per

6715-498: The process may be quite different. The UEFI (not legacy boot via CSM) does not rely on boot sectors, UEFI system loads the boot loader ( EFI application file in USB disk or in the EFI system partition ) directly. Additionally, the UEFI specification also contains "secure boot", which basically wants the UEFI code to be digitally signed . In case a boot sector receives physical damage,

6800-453: The row. Traditionally, values other than 0x00 (not active) and 0x80 (active) were invalid and the bootstrap program would display an error message upon encountering them. However, the Plug and Play BIOS Specification and BIOS Boot Specification (BBS) allowed other devices to become bootable as well since 1994. Consequently, with the introduction of MS-DOS 7.10 (Windows 95B) and higher,

6885-675: The size of a partition that can be defined using MBR, allowing partitions up to 16 TiB (2 × 4096 bytes) in size. Versions of Windows more recent than Windows XP support the larger sector sizes, as well as Mac OS X, and Linux has supported larger sector sizes since 2.6.31 or 2.6.32, but issues with boot loaders, partitioning tools and computer BIOS implementations present certain limitations, since they are often hard-wired to reserve only 512 bytes for sector buffers, causing memory to become overwritten for larger sector sizes. This may cause unpredictable behaviour as well, and therefore should be avoided when compatibility and standard conformity

6970-441: The system via INT 19h after user confirmation or cause the system to halt the bootstrapping process until the next power-up. Systems not following the above described design are: On IBM PC compatible machines, the BIOS is ignorant of the distinction between VBRs and MBRs, and of partitioning. The firmware simply loads and runs the first sector of the storage device. If the device is a floppy or USB flash drive , that will be

7055-407: The usable blocks on the disk. It also defines the number and size of the partition entries that make up the partition table (offsets 80 and 84 in the table). After the primary header and before the backup header, the Partition Entry Array describes partitions, using a minimum size of 128 bytes for each entry block. The starting location of the array on disk, and the size of each entry, are given in

7140-420: The user a chance to override the setting. The BIOS option is disabled by default because the message may not be displayed correctly in graphics mode and blocking access to the MBR may cause problems with operating system setup programs or disk access, encryption or partitioning tools like FDISK , which may not have been written to be aware of that possibility, causing them to abort ungracefully and possibly leaving

7225-433: Was even to the extent of being supported by computer operating systems for other platforms. Sometimes this was in addition to other pre-existing or cross-platform standards for bootstrapping and partitioning. MBR partition entries and the MBR boot code used in commercial operating systems, however, are limited to 32 bits. Therefore, the maximum disk size supported on disks using 512-byte sectors (whether real or emulated) by

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