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In computing, BIOS (/ ˈ b aɪ ɒ s,-oʊ s /, BY-oss, -ohss; Basic Input/Output System, also known as the System BIOS, ROM BIOS, BIOS ROM or PC BIOS) is a type of firmware used to provide runtime services for operating systems and programs and to perform hardware initialization during the booting process (power-on startup). [1]
Once all the boot and system drivers have been loaded, the kernel starts the session manager (smss.exe), which begins the login process. After the user has successfully logged into the machine, winlogon applies User and Computer Group Policy setting and runs startup programs declared in the Windows Registry and in "Startup" folders. [5]
For a cold boot, for example, it may need to execute all of its functionality. If, however, the system supports power saving or quick boot methods, the BIOS may be able to circumvent the standard POST device discovery, and simply program the devices from a preloaded system device table.
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.
For this reason, it is possible (but not usual) for an option ROM to keep control and preempt the BIOS boot process. The BIOS (at least as originally designed by IBM) generally scans for and initializes (by executing) option ROMs in ascending address order at 2 KB address intervals within two different address ranges above address C0000h in the ...
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 ...
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:
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.