BIOS boot partition: Difference between revisions

Browse history interactively

← Previous edit

Content deleted Content added

VisualWikitext

Revision as of 02:02, 2 May 2019 edit JJMC89 (talk \| contribs) Edit filter managers, Autopatrolled, Administrators 346,143 edits m Removing link(s): Wikipedia:Miscellany for deletion/7 pseudo-portal forks of outline pages (Xunlink) ← Previous edit		Latest revision as of 04:46, 19 August 2024 edit undo Artoria2e5 (talk \| contribs) Extended confirmed users, IP block exemptions 38,965 edits →Overview: full quote
(11 intermediate revisions by 11 users not shown)
Line 1: {{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 [[~~partition~~Partition (computing)\|partition]] on a [[data storage device]] that [[GNU~~ ~~ 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 Line 8 ⟶ 9: }}</ref> A BIOS boot partition is needed on GPT-partitioned storage devices to hold the second stages of GRUB. On traditional [[Master Boot Record\|MBR]]-partitioned devices, the [[disk sector]]s immediately following the first are usually unused, as the partitioning scheme does not designate them for any special purpose and partitioning tools avoid them for alignment purposes. On GPT-based devices, the sectors hold the actual partition table, necessitating the use of an extra partition. On 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 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 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 = https://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 ~~System~~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. The size requirements for a BIOS boot partition are quite low so it can be as small as about 30 KiB; however, as future boot loaders might require more space, 1 MiB might be a reasonable BIOS boot partition size. Due to the [[1 MB partition alignment\|1 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. (34–2047 for 512-byte sectors; 6–255 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 ~~but~~https://uefi.org/specs/UEFI/2.10/05_GUID_Partition_Table_Format.html, ~~that~~"GPT ispartitions ~~not~~should ~~very~~be ~~important{{cn\|date=June~~aligned ~~2017\|reason=Who~~to ~~says~~the itlarger isof: ~~unimportant?~~GPT Ispartitions ~~this~~should ~~just~~be ~~opinion~~aligned orto isthe itlarger ~~declared~~of: ina ~~the~~– ~~partitioning~~The ~~standard~~physical /block ~~bootloader's~~boundary, ~~documentation?~~if ~~Even~~any; ifb it– isThe ~~written~~optimal totransfer ~~rarely~~length granularity, itif isany."</ref> ~~read~~but ~~every~~that ~~boot,~~is ~~and~~not ~~anything~~very ~~that~~important ~~clobbers~~since: it(a) ~~will~~the ~~prevent~~alignment ~~the~~rule ~~system~~is ~~from~~"should" ~~being~~and ~~able~~not to"must" ~~boot.}}~~(b) ~~since~~the itrule is ~~written~~based toon a performance consideration, which for a very ~~infrequently~~small partition used only at system boot bears no real consequence. == Creation == Line 34 ⟶ 42: == See also == * [[Unified Extensible Firmware Interface]] (UEFI) * [[EFI System partition]] (ESP) * [[Windows To Go]] Line 44 ⟶ 50: == External links == * [https://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:Booting]]~~ [[Category:Disk partitions]]