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A modern PC is configured to attempt to boot from various devices in a certain order. If a computer is not booting from the device desired, such as the floppy drive, the user may have to enter the BIOS Setup function by pressing a special key when the computer is first turned on (such as Delete, F1, F2, F10 or F12), and then changing the boot order. [6]
The boot partition (or boot volume) [5] is the disk partition that contains the operating system folder, known as the system root or %systemroot% in Windows NT. [ 6 ] : 174 Before Windows 7 , the system and boot partitions were, by default, the same and were given the "C:" drive letter .
The boot sector code is the first-stage boot loader. It is located on fixed disks and removable drives, and must fit into the first 446 bytes of the Master Boot Record in order to leave room for the default 64-byte partition table with four partition entries and the two-byte boot signature, which the BIOS requires for a proper boot loader ...
When GRUB is installed on a hard disk, boot.img is written into the boot sector of that hard disk. boot.img has a size of only 446 bytes. A boot sector is the sector of a persistent data storage device (e.g., hard disk , floppy disk , optical disc , etc.) which contains machine code to be loaded into random-access memory (RAM) and then executed ...
The MBR is not located in a partition; it is located at a first sector of the device (physical offset 0), preceding the first partition. (The boot sector present on a non-partitioned device or within an individual partition is called a volume boot record instead.)
On partitioned devices, it is the first sector of an individual partition on the device, with the first sector of the entire device being a Master Boot Record (MBR) containing the partition table. The code in volume boot records is invoked either directly by the machine's firmware or indirectly by code in the master boot record or a boot manager.
Microsoft boot sectors, therefore, traditionally imposed certain restrictions on the boot process. For example, the boot file had to be located at a fixed position in the root directory of the file system and stored within consecutive sectors, [7] [8] conditions taken care of by the SYS command and slightly relaxed in later versions of DOS.
The boot loader is responsible for accessing the file system on the boot drive, starting ntoskrnl.exe, and loading boot-time device drivers into memory. Once all the boot and system drivers have been loaded, the kernel starts the session manager (smss.exe), which begins the login process.