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Before Windows 7, the system and boot partitions were, by default, the same and were given the "C:" drive letter. [7]: 971 Since Windows 7, however, Windows Setup creates, by default, a separate system partition that is not given an identifier and therefore is hidden. The boot partition is still given "C:" as its identifier.
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]
A type of UEFI application is an OS boot loader such as GRUB, rEFInd, Gummiboot, and Windows Boot Manager, which loads some OS files into memory and executes them. Also, an OS boot loader can provide a user interface to allow the selection of another UEFI application to run. Utilities like the UEFI Shell are also UEFI applications.
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 ...
On system with BIOS firmware, the BIOS invokes MBR boot code from a hard disk drive at startup. The MBR boot code and the VBR boot code are OS-specific. In Microsoft Windows, the MBR boot code tries to find an active partition (the MBR is only 512 bytes), then executes the VBR boot code of an active partition.
The BIOS uses the boot devices set in Nonvolatile BIOS memory , or, in the earliest PCs, DIP switches. The BIOS checks each device in order to see if it is bootable by attempting to load the first sector (boot sector). If the sector cannot be read, the BIOS proceeds to the next device.
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 BIOS reads the MBR from the storage device into physical memory, and then it directs the microprocessor to the start of the boot code. The BIOS will switch the processor to real mode, then begin to execute the MBR program, and so the beginning of the MBR is expected to contain real-mode machine code. [23]