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Typically, the system firmware (UEFI or BIOS) will allow the user to configure a boot order. If the boot order is set to "first, the DVD drive; second, the hard disk drive", then the firmware will try to boot from the DVD drive, and if this fails (e.g. because there is no DVD in the drive), it will try to boot from the local hard disk drive.
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.
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]
When this bootsector is read and given control by BIOS, LILO loads the menu code and draws it then uses stored values together with user input to calculate and load the Linux kernel or chain-load any other boot-loader. GRUB 1 includes logic to read common file systems at run-time in order to access its configuration file. [15]
BIOS-style booting from MBR-partitioned disks is commonly called BIOS-MBR, regardless of it being performed on UEFI or legacy BIOS-based systems. Furthermore, booting legacy BIOS-based systems from GPT disks is also possible, and such a boot scheme is commonly called BIOS-GPT.
The BIOS Boot Specification (BBS) was developed by a consortium comprising Compaq, Intel and Phoenix Technologies to standardize the initialization sequence of Plug and Play (PnP) BIOS and Option ROMs. [3] This standard is supported by most post-2000 BIOSes. The standard presents the notion of a Boot Connection Vector (BCV) table and BCV ...
This information allows the boot loader in the MBR (or VBR, if passed on) to actively interact with the BIOS or a resident PnP / BBS BIOS overlay in memory in order to configure the boot order, etc., however, this information is ignored by most standard MBRs and VBRs.
The BIOS boot partition is a partition on a data storage device that GNU GRUB uses on legacy BIOS-based personal computers in order to boot an operating system, when the actual boot device contains a GUID Partition Table (GPT). Such a layout is sometimes referred to as BIOS/GPT boot.