Booting - Boot Sequence of IBM-PC Compatibles

Boot Sequence of IBM-PC Compatibles

Upon starting, an IBM-compatible personal computer's x86 CPU runs, in real mode, the instruction located at the physical memory address FFFFFFF0h, usually pointing to the BIOS entry point inside the ROM. This memory location typically contains a jump instruction that transfers execution to the location of the BIOS start-up program. This program runs a power-on self-test (POST) to check and initialize required devices such as DRAM and the PCI bus (including running embedded ROMs). The most complicated step is setting up DRAM over SPI, made more difficult by the fact that at this point memory is very limited.

After initializing required hardware, the BIOS goes through a pre-configured list of non-volatile storage devices ("boot device sequence") until it finds one that is bootable. A bootable device is defined as one that can be read from, and where the last two bytes of the first sector contain the little-endian word AA55h, found as byte sequence 55h, AAh on disk (also known as the MBR boot signature), or where it is otherwise established that the code inside the sector is executable on x86 PCs.

Coreboot splits the initialization and boot services into distinct parts, supporting "payloads" such as SeaBIOS, TianoCore, GRUB, and Linux directly (from flash).

Once the BIOS has found a bootable device it loads the boot sector to linear address 7C00h (usually Segment:Offset 0000h:7C00h, but some BIOSes use 07C0h:0000h) and transfers execution to the boot code. In the case of a hard disk, this is referred to as the Master Boot Record (MBR) and is by definition not operating-system specific. The conventional MBR code checks the MBR's partition table for a partition set as bootable (the one with active flag set). If an active partition is found, the MBR code loads the boot sector code from that partition, known as Volume Boot Record (VBR), and executes it.

The VBR is often operating-system specific; however, in most operating systems its main function is to load and execute the operating system kernel, which continues startup.

If there is no active partition, or the active partition's boot sector is invalid, the MBR may load a secondary boot loader which will select a partition (often via user input) and load its boot sector, which usually loads the corresponding operating system kernel. In some cases, the MBR may also attempt to load secondary boot loaders before trying to boot the active partition. If all else fails, it should issue an INT 18h BIOS interrupt call (followed by an INT 19h just in case INT 18h would return) in order to give back control to the BIOS, which would then attempt to boot off other devices, attempt a remote boot via network or invoke ROM BASIC.

Some systems (particularly newer Macintoshes and new editions of Microsoft Windows) use Intel's EFI. Also coreboot allows a computer to boot without having the firmware/BIOS constantly running in system management mode. 16-bit BIOS interfaces are required by certain x86 operating systems, such as DOS and Windows 3.1/95/98 (and all when not booted via UEFI). However, most boot loaders retain 16-bit support BIOS call support.