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== Overview ==
[[File:GNU GRUB components.svg|thumb|440x440px|Example 2 stores [[GRUB 2]]'s 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]] for the BIOS boot partition in the GPT scheme is <tt>21686148-6449-6E6F-744E-656564454649</tt><ref>{{cite web▼
| url = http://www.gnu.org/software/grub/manual/html_node/BIOS-installation.html
| title = GNU GRUB Installation, Section 3.4: BIOS installation
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| website = gnu.org
}}</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.
▲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 size requirements for a BIOS boot partition are quite small, so it can be as small as about 30 KiB. Though, future boot loaders might require more space, so creating a larger BIOS boot partition is advisable; 1 MiB might be a sensible 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. If created by utilizing that free space, the BIOS boot partition would be out of the GPT alignment specification, but that is not very important since it is written to very infrequently.
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