The moment after a computer is powered on, it is practically useless because the RAM chips contain random data and no operating system is running. To begin the boot, a special hardware circuit raises the logical value of the RESET pin of the CPU. After RESET is thus asserted, some registers of the processor (including cs and eip) are set to fixed values, and the code found at physical address 0xfffffff0 is executed. This address is mapped by the hardware to some read-only, persistent memory chip, a kind of memory often called ROM (read-only memory). The set of programs stored in ROM is traditionally called BIOS (Basic Input/Output System), since it includes several interrupt-driven low-level procedures used by some operating systems, including Microsoft's MS-DOS, to handle the hardware devices that make up the computer.

Once initialized, Linux does not make any use of BIOS but provides its own device driver for every hardware device on the computer. In fact, the BIOS procedures must be executed in real mode, while the kernel executes in protected mode, so they cannot share functions even if that would be beneficial.

BIOS uses real mode addresses because they are the only ones available when the computer is turned on. A real mode address is composed of a seg segment and an off offset; the corresponding physical address is given by seg*16+off. As a result, no global descriptor table (GDT), local descriptor table (LDT), or paging table is needed by the CPU addressing circuit to translate a logical address into a physical one. Clearly, the code that initializes the GDT, LDT, and paging tables must run in real mode.

Linux is forced to use BIOS in the bootstrapping phase, when it must retrieve the kernel image from disk or from some other external device. The BIOS bootstrap procedure essentially performs the following four operations:

1. Executes a series of tests on the computer hardware, in order to establish which devices are present and whether they are working properly. This phase is often called POST (power-on self-test). During this phase, several messages, such as the BIOS version banner, are displayed.

2. Initializes the hardware devices. This phase is crucial in modern PCI-based architectures, since it guarantees that all hardware devices operate without conflicts on the IRQ lines and I/O ports. At the end of this phase, a table of installed PCI devices is displayed.

3. Searches for an operating system to boot. Actually, depending on the BIOS setting, the procedure may try to access (in a predefined, customizable order) the first sector (boot sector) of any floppy disk, any hard disk, and any CD-ROM in the system.

4. As soon as a valid device is found, copies the contents of its first sector into RAM, starting from physical address 0x00007c00, then jumps into that address and executes the code just loaded.

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