Since 2009 there are hard disks available with a sector size of 4096 bytes, called Advanced Format disks, but as of October 2013, such hard disks are still accessed in 512-byte sectors, using the 512e emulation. For a long time, the size of a sector has been 512 bytes.

This bootstrap program must be small because it has to fit in a single sector.

The MBR is the first sector of the hard disk. When a computer is turned on, its BIOS finds the primary bootable device (usually the computer's hard disk) and runs the initial bootstrap program from the master boot record (MBR). The GNU operating system uses GNU GRUB as its boot loader, as do most Linux distributions and the Solaris operating system on x86 systems, starting with the Solaris 10 1/06 release. It is predominantly used for Unix-like systems. GNU GRUB was developed from a package called the Grand Unified Bootloader (a play on Grand Unified Theory ).

GRUB is the reference implementation of the Free Software Foundation's Multiboot Specification, which provides a user the choice to boot one of multiple operating systems installed on a computer or select a specific kernel configuration available on a particular operating system's partitions. GNU GRUB (short for GNU GRand Unified Bootloader, commonly referred to as GRUB) is a boot loader package from the GNU Project. Linux, macOS, BSD, ( Solaris/ illumos (x86 port)), and Windows (through chainloading)

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