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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 ...
Prior to the development and ubiquitous adoption of the Plug and Play BIOS standard, an add-on device such as a hard disk controller or a network adapter card (NIC) was generally required to include an option ROM in order to be bootable, as the motherboard BIOS did not include any support for the device and so could not incorporate it into the BIOS's boot protocol.
Parts of coreboot becomes the systems BIOS and stays resident in memory after boot. Das U-Boot is a bootloader for embedded systems. It is used on systems that do not have a BIOS/UEFI but rather employ custom methods to read the bootloader into memory and execute it. Historical bootloaders, no longer in common use, include:
This is why the motherboard BIOS ROM is in segment F000 in the conventional memory map. During the POST flow of a contemporary BIOS, one of the first things a BIOS should do is determine the reason it is executing. For a cold boot, for example, it may need to execute all of its functionality.
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 — or even less, when additional features like more than ...
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 ROM is either placed on the same die as the CPU or is an external ROM chip. On modern systems, the boot ROM (whether integrated into CPU or external ROM chip) is usually use NOR flash which supports execute in place. The boot ROM will then initialize the hardware busses and peripherals needed to boot.
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.