This appendix explains what happens right after users switch on their computers—that is, how a Linux kernel image is copied into memory and executed. In short, we discuss how the kernel, and thus the whole system, is “bootstrapped.”
Traditionally, the term bootstrap refers to a person who tries to stand up by pulling his own boots. In operating systems, the term denotes bringing at least a portion of the operating system into main memory and having the processor execute it. It also denotes the initialization of kernel data structures, the creation of some user processes, and the transfer of control to one of them.
Computer bootstrapping is a tedious, long task, since initially, nearly every hardware device, including the RAM, is in a random, unpredictable state. Moreover, the bootstrap process is highly dependent on the computer architecture; as usual, we refer to IBM’s PC architecture in this appendix.
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 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 a certain read-only, persistent
memory chip that is often called Read-Only Memory (ROM). The set of
programs stored in ROM is traditionally called Basic Input/Output
System (BIOS), 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 use 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), so they cannot share functions even if that would be beneficial.
The 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 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:
Executes a series of tests on the computer hardware to establish which devices are present and whether they are working properly. This phase is often called Power-On Self-Test (POST). During this phase, several messages, such as the BIOS version banner, are displayed.
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.
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, hard disk, and CD-ROM in the system.
As soon as a valid device is found, copies the contents of its first sector into RAM, starting from physical address
0x00007c00, and then jumps into that address and executes the code just loaded.
The rest of this appendix takes you from the most primitive starting state to the full glory of a running Linux system.