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O'Reilly Network: How Your Computer Boots; Excerpt from Understanding the Linux Kernel

Jan 27, 2001, 20:33 (1 Talkback[s])
(Other stories by Daniel P. Bovet)


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"In most cases, the Linux kernel is loaded from a hard disk, and a two-stage boot loader is required. The most commonly used Linux boot loader on Intel systems is named LILO (LInux LOader); corresponding programs exist for other architectures. LILO may be installed either on the MBR, replacing the small program that loads the boot sector of the active partition, or in the boot sector of a (usually active) disk partition. In both cases, the final result is the same: When the loader is executed at boot time, the user may choose which operating system to load."

"The LILO boot loader is broken into two parts, since otherwise it would be too large to fit into the MBR. The MBR or the partition boot sector includes a small boot loader, which is loaded into RAM starting from address 0x00007c00 by the BIOS. This small program moves itself to the address 0x0009a000, sets up the real mode stack (ranging from 0x0009b000 to 0x0009a200), and loads the second part of the LILO boot loader into RAM starting from address 0x0009b000. In turn, this latter program reads a map of available operating systems from disk and offers the user a prompt so she can choose one of them. Finally, after the user has chosen the kernel to be loaded (or let a time-out elapse so that LILO chooses a default), the boot loader may either copy the boot sector of the corresponding partition into RAM and execute it or directly copy the kernel image into RAM."

"The code of the setup( ) assembly language function is placed by the linker immediately after the integrated boot loader of the kernel, that is, at offset 0x200 of the kernel image file. The boot loader can thus easily locate the code and copy it into RAM starting from physical address 0x00090200. The setup( ) function must initialize the hardware devices in the computer and set up the environment for the execution of the kernel program. Although the BIOS already initialized most hardware devices, Linux does not rely on it but reinitializes the devices in its own manner to enhance portability and robustness."

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