F.2. A Detailed Look at the Boot Process
The beginning of the boot process varies depending on the hardware platform being used. However, once the kernel is found and loaded by the boot loader, the default boot process is identical across all architectures. This chapter focuses primarily on the x86 architecture.
F.2.1. The Firmware Interface
F.2.1.1. BIOS-based x86 Systems
The Basic Input/Output System (BIOS) is a firmware interface that controls not only the first step of the boot process, but also provides the lowest level interface to peripheral devices. On x86 systems equipped with BIOS, the program is written into read-only, permanent memory and is always available for use. When the system boots, the processor looks at the end of system memory for the BIOS program, and runs it.
Once loaded, the BIOS tests the system, looks for and checks peripherals, and then locates a valid device with which to boot the system. Usually, it checks any optical drives or USB storage devices present for bootable media, then, failing that, looks to the system's hard drives. In most cases, the order of the drives searched while booting is controlled with a setting in the BIOS, and it looks on the master IDE on the primary IDE bus or for a SATA device with a boot flag set. The BIOS then loads into memory whatever program is residing in the first sector of this device, called the Master Boot Record (MBR). The MBR is only 512 bytes in size and contains machine code instructions for booting the machine, called a boot loader, along with the partition table. Once the BIOS finds and loads the boot loader program into memory, it yields control of the boot process to it.
This first-stage boot loader is a small machine code binary on the MBR. Its sole job is to locate the second stage boot loader (GRUB) and load the first part of it into memory.
F.2.1.2. UEFI-based x86 Systems
The Unified Extensible Firmware Interface (UEFI) is designed, like BIOS, to control the boot process (through boot services) and to provide an interface between system firmware and an operating system (through runtime services). Unlike BIOS, it features its own architecture, independent of the CPU, and its own device drivers. UEFI can mount partitions and read certain file systems.
When an x86 computer equipped with UEFI boots, the interface searches the system storage for a partition labeled with a specific globally unique identifier (GUID) that marks it as the EFI System Partition (ESP). This partition contains applications compiled for the EFI architecture, which might include bootloaders for operating systems and utility software. UEFI systems include an EFI boot manager that can boot the system from a default configuration, or prompt a user to choose an operating system to boot. When a bootloader is selected, manually or automatically, UEFI reads it into memory and yields control of the boot process to it.