Master boot record: Difference between revisions

{{About|aan IBM PC-specific type of [[boot sector]] on partitioned media|the first sector on non-partitioned media|volume boot record}}

The organization of the partition table in the MBR limits the maximum addressable storage space of a partitioned disk to 2&nbsp;[[Tebibyte|TiB]] {{nowrap|(2³²&nbsp;×&nbsp;512&nbsp;bytes)}}.<ref name="Microsoft_2013_2581408"/> Approaches to slightly raise this limit assumingutilizing 32-bit arithmeticsarithmetic or 4096-byte sectors are not officially supported, as they fatally break compatibility with existing boot loaders and, most MBR-compliant operating systems and associated system tools, and canmay cause serious data corruption when used outside of narrowly controlled system environments. Therefore, the MBR-based partitioning scheme is in the process of being superseded by the [[GUID Partition Table]] (GPT) scheme in new computers.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 [[floppy|floppies]], [[superfloppy|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 the master boot record (MBR), was introduced with IBM [[PC&nbsp;DOS]] 2.0 in March 1983 in order to support the 10&nbsp;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'', of which [[DOS]] could only use one. This did not change when [[FAT16]] was introduced as 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&nbsp;DOS, OS/2 and Windows were never enabled to boot off them, the MBR format and boot code remained almost unchanged in functionality, (except for in some third-party implementations,) throughout 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&nbsp;GB. ''Disk timestamps'' were also introduced.<ref name="Sedory_2004_Timestamp"/><!-- TBD: Recheck, if MBR LBA support was really added with 95B/7.1 only, since LBA support in general was added with 95A/7.0 in 1995 already IIRC. --> 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 the MBR, which enforce [[cylinder-head-sector|CHS]] access for [[FAT16B]] and [[FAT32]] partition types [[Partition type#PID 06h|{{mono|0x06}}]]/[[Partition type#PID 0Bh|{{mono|0x0B}}]], whereas LBA is used for [[Partition type#PID 0Eh|{{mono|0x0E}}]]/[[Partition type#PID 0Ch|{{mono|0x0C}}]].

MBR partition entries and the MBR boot code used in commercial operating systems, however, are limited to 32 bits.<ref name="Microsoft_2013_2581408"/> Therefore, the maximum disk size supported on disks using 512-byte sectors (whether real or emulated) by the MBR partitioning scheme (without 3332-bit arithmetic) is limited to 2&nbsp;TiB.<ref name="Microsoft_2013_2581408"/> 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 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&nbsp;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.

| style="text-align:center; background:#F2F2F2" | AAP physical drive (<code>0x80</code>-–<code>0xFE</code>; <code>0x00</code>: not used; <code>0x01</code>-–<code>0x7F</code>, <code>0xFF</code>: reserved)

| style="text-align:center; background:#F2F2F2" | LOADER physical drive and boot flag (<code>0x80</code>-–<code>0xFE</code>, <code>0x00</code>-–<code>0x7E</code>, <code>0xFF</code>, <code>0x7F</code>) (if not used, this and following 3 bytes must be all 0)

| style="text-align:center; background:#F2F2F2" | Reserved (default: <code>0x000000</code>)<!-- Already used, but it is not documented yet. -->

| {{anchor|PTE_OFS_8h}}{{mono|0x08}} || || 4&nbsp;bytes || [[logical block addressing|LBA]] of first absolute sector in the partition{{Efn|name="note-4"|The number of sectors is an index field; thus, the zero value is invalid, reserved and must not be used in normal partition entries. TheThis entry is used by operating systems in certain circumstances; in such cases the CHS addresses are ignored.<ref name="Microsoft_2000_LBA-Blocks"/>}}

| {{anchor|PTE_OFS_Ch}}{{mono|0x0C}} || || 4&nbsp;bytes || Number of sectors in partition{{Efn|name="note-45"|Zero is reserved and must not be used in normal partition entries. This entry is used by operating systems in certain circumstances; in such cases the CHS addresses are ignored.<ref name="Microsoft_2000_LBA-Blocks"/>}}

Since partitioning information 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&nbsp;TiB limit and the last one is assigned as starting at or close to block 2³²−1 and specify the size as up to 2³²−1, thereby defining a partition that requires 33 rather than 32 bits for the sector address to be accessed. However, in practice, only certain [[Logical block addressing#LBA48|LBA-48]]-enabled operating systems, including Linux, FreeBSD and Windows 7<ref name="Smith_2011_gdisk"/> 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 [[Logical block addressing|LBA-28]], often use 32-bit calculations, unless they are specifically designed to support 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.

On [[IBM PC-compatible]] computers, the [[bootstrapping]] [[firmware]] (contained within the [[read-only memory|ROM]] [[BIOS]]) loads and executes the master boot record.<ref name="OSDev_2011_MBR"/> The [[PC XT|PC/XT (type 5160)]] used an [[Intel 8088]] [[Computer processor|microprocessor]]. In order to remain compatible, all x86 BIOS architecture systems start with the microprocessor in an [[X86#Operating modes|operating mode]] referred to as [[real mode]]. The BIOS reads the MBR from the storage device into [[physical memory]], and then it directs the microprocessor to the start of the boot code. Since theThe BIOS runswill in real mode,switch the processor is into real mode, whenthen thebegin MBRto programexecute beginsthe toMBR executeprogram, and so the beginning of the MBR is expected to contain real-mode [[machine code]].<ref name="OSDev_2011_MBR"/>

[[File:Qtparted-usb-hdd-snapshot.png|thumb|300px|Information contained in the partition table of an external hard drive as it appears in the utility program [[QtParted]], running under Linux (with KDE)]]

* {{cn span|date=April 2021|text=[[DL register|DL]] = boot drive unit ([[fixed disk]]s / [[removable drive]]s: {{mono|0x80}} = first, {{mono|0x81}} = second, ..., {{mono|0xFE}}; [[floppy|floppies]] / [[superfloppy|superfloppies]]: {{mono|0x00}} = first, {{mono|0x01}} = second, ..., {{mono|0x7E}}; values {{mono|0x7F}} and {{mono|0xFF}} are reserved for ROM / remote drives and must not be used on disk<!-- by Microsoft/IBM and Digital Research/Novell/Caldera. -->).}}<ref name="Paul_1997"/><ref name="Paul_2017"/>

: MS-DOS 2.0-70–7.0 / PC &nbsp;DOS 2.0-60–6.3<!-- Not sure about PC DOS 7.0/2000 and 7.10 right now. --> MBRs do not pass on the DL value received on entry, but they rather use the boot status entry in the partition table entry of the selected primary partition as physical boot drive unit. Since this is, by convention, {{mono|0x80}} in most MBR partition tables, it won't change things unless the BIOS attempted to boot off a physical device other than the first fixed disk / removable drive in the row. This is also the reason why these operating systems cannot boot off a second hard disk, etc. Some FDISK tools allow to mark partitions on secondary disks as "active" as well. In this situation, knowing that these operating systems cannot boot off other drives anyway, some of them continue to use the traditionally fixed value of {{mono|0x80}} as active marker, whereas others use values corresponding with the currently assigned physical drive unit ({{mono|0x81}}, {{mono|0x82}}), thereby allowing booting from other drives, at least in theory. In fact, this will work with many MBR codes, which take a set bit 7 of the boot status entry as active flag rather than insisting on {{mono|0x80}}, however, MS-DOS/PC &nbsp;DOS MBRs are hard-wired to accept the fixed value of {{mono|0x80}} only. Storing the actual physical drive number in the partition table will also cause problems, when the BIOS assignment of physical drives changes, for example when drives are removed, added or swapped. Therefore, for a normal MBR accepting bit 7 as active flag and otherwise just using and passing on to the VBR the DL value originally provided by the BIOS allows for maximum flexibility. MS-DOS 7.1 - 81–8.0 MBRs have changed to treat bit 7 as active flag and any values {{mono|0x01}}..{{mono|0x7F}} as invalid, but they still take the physical drive unit from the partition table rather than using the DL value provided by the BIOS. DR-DOS 7.07 extended MBRs treat bit 7 as active flag and use and pass on the BIOS DL value by default (including non-standard values {{mono|0x00}}..{{mono|0x01}} used by some BIOSes also for partitioned media), but they also provide a special [[#NEWLDR|NEWLDR]] configuration block in order to support alternative boot methods in conjunction with LOADER and REAL/32 as well as to change the detail behaviour of the MBR, so that it can also work with drive values retrieved from the partition table (important in conjunction with LOADER and AAPs, see NEWLDR offset <code>[[#NEWLDR_OFS_00Ch|0x000C]]</code>), translate Wyse non-standard drive units {{mono|0x02}}..{{mono|0x7F}} to {{mono|0x80}}..{{mono|0xFD}}, and optionally fix up the drive value (stored at offset <code>[[Design of the FAT file system#EBPB_OFS_19h|0x019]]</code> in the [[Extended BIOS Parameter Block]] (EBPB)<!-- In OS/2, DOS 4.0 and NT. --> or at sector offset <code>[[Design of the FAT file system#BSIBM_OFS_1FDh|0x01FD]]</code> <!-- in DOS 3.2 to 3.31. --> ) in loaded VBRs before passing execution to them (see NEWLDR offset <code>[[#NEWLDR_OFS_014h|0x0014]]</code>)—this also allows other boot loaders to use NEWLDR as a chain-loader, configure its in-memory image on the fly and "tunnel" the loading of VBRs, EBRs, or AAPs through NEWLDR.

* The contents of DH and ES:DI should be preserved by the MBR for full Plug-and-Play support (see above), however, many MBRs, including those of MS-DOS 2.0 - 80–8.0 / PC &nbsp;DOS 2.0 - 60–6.3<!-- Not sure about PC DOS 7.0/2000 and 7.1 right now. --> and Windows NT/2000/XP, do not. (This is unsurprising, since those versions of DOS predate the Plug-and-Play BIOS standard, and previous standards and conventions indicated no requirements to preserve any register other than DL.) Some MBRs<!-- DR-DOS 7.07, Windows Vista/7, but not Windows 2000/XP. --> set DH to 0.

* DS:SI = points to the 16-byte [[MBR partition table]] entry (in the relocated MBR) corresponding with the activated VBR. [[PC-MOS]] 5.1 depends on this to boot if no partition in the partition table is flagged as bootable. In conjunction with LOADER, [[Multiuser DOS]] and [[REAL/32]] boot sectors use this to locate the boot sector of the active partition (or another bootstrap loader like IBMBIO.LDR at a fixed position on disk) if the boot file (LOADER.SYS) could not be found. [[PTS-DOS]] 6.6<!-- TBD: not sure about 6.5x right now. --> and [[S/DOS]] 1.0 use this in conjunction with their [[Advanced Active Partition]] (AAP) feature. In addition to support for LOADER and AAPs, DR-DOS 7.07 can use this to determine the necessary INT 13h access method when using its dual CHS/LBA VBR code and it will update the boot drive / status flag field in the partition entry according to the effectively used DL value. [[Apple Darwin|Darwin]] bootloaders (Apple's <code>boot1h</code>, <code>boot1u</code>, and David Elliott's <code>boot1fat32</code>) depend on this pointer as well, but additionally they don't use DS, but assume it to be set to {{mono|0x0000}} instead.<ref name="Elliott_2009_Darwin"/> This will cause problems if this assumption is incorrect. The MBR code of OS/2<!-- TBD: Recheck! -->, MS-DOS 2.0 to 8.0, PC &nbsp;DOS 2.0 to 7.10 and Windows NT/2000/XP provides this same interface as well, although these systems do not use it.<!-- Editor's note: I'm not sure about PC DOS 7.1 right now. --> The Windows Vista/7 MBRs no longer provide this DS:SI pointer. While some extensions only depend on the 16-byte partition table entry itself, other extensions may require the whole 4 (or 5 entry) partition table to be present as well.

* DS:[[BP register|BP]] = optionally points to the 16-byte [[MBR partition table]] entry (in the relocated MBR) corresponding with the activated VBR. This is identical to the pointer provided by DS:SI (see above) and is provided by MS-DOS 2.0-80–8.0, PC &nbsp;DOS 2.0-70–7.10, Windows NT/2000/XP/Vista/7 MBRs<!-- TBD: Check OS/2 and early NT. -->. It is, however, not supported by most third-party MBRs.

<!-- <ref name="FOLDOC">{{cite web |author-first=Denis |author-last=Howe |title=master boot record |website=[[FOLDOC]] |date=2009-05-19 |orig-date=1985 |url=http://foldoc.org/master+boot+record |access-date=2015-05-02 |url-status=dead |archive-url=https://web.archive.org/web/20170824002628/https://foldoc.org/master%20boot%20record |archive-date=24 August 2017 }}</ref>-->