BIOS boot partition: Difference between revisions

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{{Short description|Partition used by GNU GRUB on BIOS to boot an operating system with GUID Partition Table}}

{{Refimprove|date=June 2014}}

The '''BIOS boot partition''' is a [[partitionPartition (computing)|partition]] on a [[data storage device]] that [[GNU&nbsp; GRUB]] uses on legacy [[BIOS]]-based [[personal computer]]s in order to [[Booting|boot]] an [[operating system]], when the actual [[boot device]] contains a [[GUID Partition Table]] (GPT). Such a layout is sometimes referred to as BIOS/GPT boot.<ref>{{cite web

| url = https://wiki.archlinux.org/index.php/GUID_Partition_Table

| title = GUID Partition Table

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}}</ref>

A BIOS boot partition is needed becauseon GPT-partitioned usesstorage devices to hold the [[disksecond sector]]sstages immediatelyof followingGRUB. theOn traditional [[Master Boot Record|MBR]]-partitioned (MBR)devices, tothe hold[[disk sector]]s immediately following the actualfirst partitionare tableusually unused, whereasas the traditional MBR-based partitioning scheme does not designate them for any special purpose. and Aspartitioning atools result,avoid therethem isfor notalignment enoughpurposes. unusedOn spaceGPT-based availabledevices, forthe storingsectors secondhold stagesthe actual partition table, necessitating the use of [[bootan loader]]sextra onpartition. GPTOn MBR-partitioned disks;, boot loaders are usually implemented so the portion of their code stored within the MBR, which cannot hold more than 512 bytes, operates as a first stage that serves primarily to load a more sophisticated second stage, which is, for example, capable of reading and loading an [[operating system kernel]] from a [[file system]].

== Overview ==

[[File:GNU GRUB components.svg|thumb|upright=2|In the example 2 above, [[GRUB&nbsp;2]] stores its {{Mono|core.img}} in a BIOS boot partition.]]

When used, the BIOS boot partition contains the second stage of the [[boot loader]] program, such as the [[GRUB&nbsp;2]]; the first stage is the code that is contained within the [[Master Boot Record]] (MBR). Use of this partition is not the only way BIOS-based boot can be performed while using GPT-partitioned hard drives; however, complex boot loaders such as [[GRUB 2]] cannot fit entirely within the confines of the MBR's 398<!-- MBR with disk timestamp, disk signature, AAP and NEWLDR support --> to 446<!-- classic MBR without any extensions --> bytes of space, thus they need an ancillary storage space. On MBR disks, such boot loaders typically use the sectors immediately following the MBR for this storage; that space is usually known as the "MBR gap". No equivalent unused space exists on GPT disks, and the BIOS boot partition is a way to officially allocate such space for use by the boot loader.

The [[globally unique identifier]] (GUID) for the BIOS boot partition in the GPT scheme is <tt>{{samp|21686148-6449-6E6F-744E-656564454649<sub>16</ttsub>}}<ref>{{cite web

| url = httphttps://www.gnu.org/software/grub/manual/html_node/BIOS-installation.html

| title = GNU GRUB Installation, Section 3.4: BIOS installation

| accessdate = 2014-06-26

| website = gnu.org

}}</ref>

}}</ref> (which, when written to a GPT in the required little endian fields, forms the ASCII string "<tt>Hah!IdontNeedEFI</tt>").<!-- See talk page for some notes on endianness confusion. --> In the context of GPT on a BIOS-based computer, a BIOS boot partition is similar in some respects to the [[EFI System partition]], which is used by systems based on [[Extensible Firmware Interface|EFI]]. The EFI System partition holds a filesystem and files used by the UEFI, while the BIOS boot partition is used in BIOS-based systems and accessed without a filesystem by holding raw binary code.

<ref>

{{cite web

| url = https://sourceforge.net/p/gptfdisk/code/ci/master/tree/parttypes.cc

| title = parttypes.cc in the gptfdisk source code

| accessdate = 2023-05-03

| website = gptfdisk project on sourceforge.net

}}</ref> (which, when written to a GPT in the required little endian fields, forms the ASCII string "<ttcode>Hah!IdontNeedEFI</ttcode>").<!-- See talk page for some notes on endianness confusion. --> In the context of GPT on a BIOS-based computer, a BIOS boot partition is similar in some respects to the [[EFI Systemsystem partition]], which is used by systems based on [[Extensible Firmware Interface|EFI]]. The EFI System partition holds a filesystem and files used by the UEFI, while the BIOS boot partition is used in BIOS-based systems and accessed without a filesystem by holding raw binary code.

The size requirements for a BIOS boot partition are quite low so it can be as small as about 30&nbsp;KiB; however, as future boot loaders might require more space, 1&nbsp;MiB might be a reasonable BIOS boot partition size. Due to the [[1 MB partition alignment|1&nbsp;MiB partition alignment]] policies used by most modern disk partitioning tools to provide optimum performance with [[Advanced Format]] disks, [[Solid-state drive|SSD devices]] and certain [[RAID]] configurations, some room is left allowing the placement of a BIOS boot partition between the GPT and the first partition aligned that way. (34&ndash;2047 for 512-byte sectors; 6&ndash;255 for 4096-byte sectors). If created by utilizing that free space, the BIOS boot partition would be out of the GPT alignment specification,<ref>See https://uefi.org/specs/UEFI/2.10/05_GUID_Partition_Table_Format.html, "GPT partitions should be aligned to the larger of: GPT partitions should be aligned to the larger of: a – The physical block boundary, if any; b – The optimal transfer length granularity, if any."</ref> but that is not very important since: it(a) the alignment rule is written"should" toand not "must" (b) the rule is based on a performance consideration, which for a very infrequentlysmall partition used only at system boot bears no real consequence.

== Creation ==

The following utilities are known to support BIOS boot partitions:

* [[cfdisk]]

* [[fdisk]]

* [[GRUB 2]] (1.97~beta1 or later): when a BIOS boot partition is found during installation, GRUB will embed itself in it.

* [[GNU Parted]] (2.0 or later).

* [[GParted]], the front-end to GNU Parted.

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== See also ==

{{Portal|Computing}}

* [[Unified Extensible Firmware Interface]] (UEFI)

* [[EFI System partition]] (ESP)

* [[Windows To Go]]

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== External links ==

* [httphttps://www.gnu.org/software/grub/manual/html_node/BIOS-installation.html#BIOS-installation BIOS installation], part of the GRUB2 documentation

* [http://www.funtoo.org/wiki/GUID_Booting_Guide The Funtoo Linux GUID Booting Guide]

* [http://www.rodsbooks.com/gdisk/booting.html Booting from GPT], part of the GPT fdisk documentation

* [http://www.rodsbooks.com/gdisk/bios.html Legacy BIOS issues with GPT], February 22, 2014, by Rod Smith

[[Category:BIOS]]

[[Category:BootingDisk partitions]]