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Double boot (also known as cold double boot, double cold boot, double POST, power-on auto reboot, or fake boot) is a feature of the BIOS, and may occur after changes to the BIOS' settings or the system's configuration, or a power failure while the system was in one of certain sleep modes.
Typical POST screen (AMI BIOS) Summary screen after POST and before booting an operating system (AMI BIOS) A power-on self-test ( POST ) is a process performed by firmware or software routines immediately after a computer or other digital electronic device is powered on.
Before the operating system's first graphical screen is displayed, input and output are typically handled through BIOS. A boot menu such as the textual menu of Windows, which allows users to choose an operating system to boot, to boot into the safe mode, or to use the last known good configuration, is displayed through BIOS and receives ...
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]
Some boot loaders can also load other boot loaders; for example, GRUB loads BOOTMGR instead of loading Windows directly. Usually, a default choice is preselected with a time delay during which a user can press a key to change the choice; after this delay, the default choice is automatically run so normal booting can occur without interaction.
Grub+Linux also supports booting from a GUID partition table without UEFI. [19] The distribution Ubuntu added support for UEFI Secure Boot as of version 12.10. [122] Furthermore, the Linux kernel can be compiled with the option to run as an EFI bootloader on its own through the EFI boot stub feature.
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.
In all versions of Windows 9x except ME, it is also possible to load Windows by booting to a DOS prompt and typing "win". There are some command line switches that can be used with the WIN command: with the /D switch, Windows boots to safe mode , and with the /D:n switch, Windows boots to safe mode with networking.