BIOS boot partition

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The BIOS boot partition is a partition on a data storage device that may be used by legacy BIOS-based systems in order to boot, when the actual boot device contains a GUID Partition Table (GPT).

It must be utilized because there is not enough unused space available for the second stages of boot loaders on GPT disks. On MBR-partitioned disks, boot loaders are occupying unused sectors immediately following the Master Boot Record (MBR) for that purpose, while there is no equivalent for those on GPT disks.

Overview[edit]

The Globally Unique Identifier for the BIOS boot partition in the GPT scheme is 21686148-6449-6E6F-744E-656564454649[1] (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 EFI System partition, which is used by systems based on 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 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.

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 MiB partition alignment policies used by most modern disk partitioning tools to provide optimum performance with Advanced Format disks, 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.

Creation[edit]

The following utilities are known to support BIOS boot partitions:

  • 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.
  • gpt(8) partition editor in NetBSD (5.0 or later).
  • gdisk: GPT fdisk

See also[edit]

References[edit]

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