Understanding the Linux Kernel

Prehistoric Age: The BIOS

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 (see Section 2.2 in Chapter 2), 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, Local Descriptor Table, or paging table ...