BIOS boot partition

<h2 id="overview">Overview</h2>

<p>When used, the BIOS boot partition contains the second stage of the <a href="/facts/Boot_loader/6sNpcLMK">boot loader</a> program, such as the GRUB 2; the first stage is the code that is contained within the <a href="/facts/Master_Boot_Record/nEJerupp">Master Boot Record</a> (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 to 446 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.
</p><p>The <a href="/facts/Globally_unique_identifier/SUtkYJPe">globally unique identifier</a> (GUID) for the BIOS boot partition in the GPT scheme is 21686148-6449-6E6F-744E-65656445464916<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> (which, when written to a GPT in the required little endian fields, forms the ASCII string "Hah!IdontNeedEFI"). In the context of GPT on a BIOS-based computer, a BIOS boot partition is similar in some respects to the <a href="/facts/EFI_system_partition/Ul5sE2zY">EFI system partition</a>, which is used by systems based on <a href="/facts/Extensible_Firmware_Interface/w86VGmy2">EFI</a>. 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.
</p><p>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 <a href="/facts/1_MB_partition_alignment/aMHBy8gU">1 MiB partition alignment</a> policies used by most modern disk partitioning tools to provide optimum performance with <a href="/facts/Advanced_Format/57nHzxUV">Advanced Format</a> disks, <a href="/facts/Solid-state_drive/aMHBy8gU">SSD devices</a> and certain <a href="/facts/RAID/IpzWWgog">RAID</a> 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,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> but that is not very important since: (a) the alignment rule is "should" and not "must" (b) the rule is based on a performance consideration, which for a very small partition used only at system boot bears no real consequence.
</p>
<h2 id="creation">Creation</h2>
<p>The following utilities are known to support BIOS boot partitions:
</p>
<ul><li><a href="/facts/Cfdisk/tKhxEnAL">cfdisk</a></li>
<li><a href="/facts/Fdisk/S6hru6n4">fdisk</a></li>
<li><a href="/facts/GNU_Parted/pRCdmo13">GNU Parted</a> (2.0 or later).</li>
<li><a href="/facts/GParted/BTaRfAcn">GParted</a>, the front-end to GNU Parted.</li>
<li>gpt(8) partition editor in <a href="/facts/NetBSD/PTm5HWls">NetBSD</a> (5.0 or later).</li>
<li><a href="http://www.rodsbooks.com/gdisk/">gdisk</a>: GPT fdisk</li></ul>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Unified_Extensible_Firmware_Interface/w86VGmy2">Unified Extensible Firmware Interface</a> (UEFI)</li>
<li><a href="/facts/Windows_To_Go/HRJxsyFN">Windows To Go</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="https://www.gnu.org/software/grub/manual/html_node/BIOS-installation.html#BIOS-installation">BIOS installation</a>, part of the GRUB2 documentation</li>
<li><a href="http://www.funtoo.org/GUID_Booting_Guide">The Funtoo Linux GUID Booting Guide</a></li>
<li><a href="http://www.rodsbooks.com/gdisk/booting.html">Booting from GPT</a>, part of the GPT fdisk documentation</li>
<li><a href="http://www.rodsbooks.com/gdisk/bios.html">Legacy BIOS issues with GPT</a>, February 22, 2014, by Rod Smith</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"GUID Partition Table". archlinux.org. 2015-03-19. Retrieved 2015-03-05. <a href="https://wiki.archlinux.org/index.php/GUID_Partition_Table" target="_blank">https://wiki.archlinux.org/index.php/GUID_Partition_Table</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"GNU GRUB Installation, Section 3.4: BIOS installation". gnu.org. Retrieved 2014-06-26. <a href="https://www.gnu.org/software/grub/manual/html_node/BIOS-installation.html" target="_blank">https://www.gnu.org/software/grub/manual/html_node/BIOS-installation.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>"parttypes.cc in the gptfdisk source code". gptfdisk project on sourceforge.net. Retrieved 2023-05-03. <a href="https://sourceforge.net/p/gptfdisk/code/ci/master/tree/parttypes.cc" target="_blank">https://sourceforge.net/p/gptfdisk/code/ci/master/tree/parttypes.cc</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>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." <a href="https://uefi.org/specs/UEFI/2.10/05_GUID_Partition_Table_Format.html" target="_blank">https://uefi.org/specs/UEFI/2.10/05_GUID_Partition_Table_Format.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
</ol>

BIOS boot partition open-in-new

BIOS boot partition