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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]
If the sector is read successfully, some BIOSes will also check for the boot sector signature 0x55 0xAA in the last two bytes of the sector (which is 512 bytes long), before accepting a boot sector and considering the device bootable. [b] When a bootable device is found, the BIOS transfers control to the loaded sector.
A bootable MBR device is defined as one that can be read from, and where the last two bytes of the first sector contain the little-endian word AA55h, [nb 6] found as byte sequence 55h, AAh on disk (also known as the MBR boot signature), or where it is otherwise established that the code inside the sector is executable on x86 PCs.
Drawbacks are that some older devices may not support USB booting and that the USB storage devices lifespan might be shortened. Ubuntu has included a utility for installing an operating system image file to a USB flash drive since version 9.10. [1] Windows support also has added a step by step on how to set up a USB device as a bootable drive. [2]
Despite Microsoft's radically different definition (see below), System Information, a utility app included in Windows NT family of operating systems, refers to it as "boot device". [ 2 ] [ 3 ] The system partition is the disk partition that contains the operating system folder, known as the system root .
The boot loader on the option ROM would attempt to boot from a disk, network, or other boot program source attached to or installed on the adapter card; if that boot attempt failed, it would pass control to the previous boot loader (to which INT 19h pointed before the option ROM hooked it), allowing the system to boot from another device as a ...
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 absence of moving parts in USB flash devices allows true random access, thereby avoiding the rotational latency and seek time (see also mechanical latency) of hard drives or optical media, meaning small programs will start faster from a USB flash drive than from a local hard disk or live CD.