Understanding The Boot Process

Understanding the PC Boot ProcessHow Linux and Windows start their dayBy Dominique Gerald Cimafranca dominique.cimafranca@gmail.comThis work is licensed under the Creative Commons Attribution-Share Alike 3.0 Philippines License. To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/3.0/ph/ or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA. The PC boot process in a nutshel1. Executes code from well-known location.2. Execute first-stage boot loader from MBR.3. Execute second-stage boot loader.4. Load the kernel.5. Load the first user space program. BIOS●BIOS – Basic Input/Output System●Located at memory location 0xFFFF0●Boot firmware designed to be run at startup●POST – Power-on Self-Test●Identifies, tests, and initializes system devices●Run-time services●Initial configuration●Selects which device to boot from●Alternatively, Extensible Firmware Interface (EFI) Stage 1 Boot Loader: MBR● MBR – Master Boot Record● Located on first sector of the boot disk● Size: 512 bytes● BIOS loads MBR to RAM, relinquishes control● Main job: load the second-stage boot loader Anatomy of the MBR●First 446 bytes●Primary boot loader●Code and error messages●Next 64 bytes●Partition information●Last 2 bytes●Magic number●Validation check for MBRImage from http://www.ibm.com/developerworks/linux/library/l-linuxboot/ Stage 2 Boot Loader●Loads the kernel●On Linux:●GRUB – Grand Universal Bootloader●LILO – Linux Loader●Others: SysLinux, ISOLinux, PXELinux●From Windows NT to Windows XP:●NTLDR●On Windows Vista:●Windows Boot Manager A closer look at GRUB● GRUB understands ext2 and ext3 file systems●LILO had to load raw sectors from the hard disk● GRUB displays a list of available kernels●On Ubuntu, defined in /boot/grub/menu.lst● More info: http://www.gnu.org/software/grub/ What does GRUB load?titleUbuntu 9.04, kernel 2.6.28­13­genericuuid0ef7b971kernel/boot/vmlinuz­2.6.28­13­generic root=UUID=0ef7b971 ro quiet splash initrd/boot/initrd.img­2.6.28­13­generic●kernel – a compressed kernel image●Performs initial minimal hardware setup●Decompresses the kernel image, puts it in memory●If present, loads RAM disk (see below)●initrd – initial RAM disk●Temporary root file system●Contains executables and drivers to load the real root Execution in the kernel●arch/i386/boot/head.S●performs basic hardware setup●cal s startup_32() of ./arch/i386/boot/compressed/head.S●arch/i386/boot/compressed/head.S●set up the basic environment●clear Block Started by Symbol●cal s decompress_kernel() found in ./arch/i386/boot/compressed/misc.c●calls startup_32 in ./arch/i386/kernel/head.S●arch/i386/kernel/head.S●also called swapper or process 0●initializes page tables and enables memory paging●detects CPU type●init/main.c●calls start_kernel()●calls kernel_thread to start init (process ID 1) initrd● Initial RAM disk – a small temporary file system● During stage 2 boot, initrd is copied into RAM and mounted● Allows the kernel to fully boot without having to mount any physical disks● Supports many hardware configurations through loadable modules● After kernel is booted, the real root file system is mounted Document Outline

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