Computers Boot Up

Reference :

How Computers Boot Up : Gustavo Duarte

The Kernel Boot Process : Gustavo Duarte

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The MBR itself contains the first stage of the boot loader. GRUB calls this stage 1

The MBR code plus code loaded in step 2 then read a file containing the second stage of the boot loader. In GRUB this is GRUB Stage 2, and in Windows Server this is c:\NTLDR. If step 2 fails in Windows you’d get a message like “NTLDR is missing”. The stage 2 code then reads a boot configuration file (e.g., grub.conf in GRUB, boot.ini in Windows). It then presents boot choices to the user or simply goes ahead in a single-boot system.

At this point the boot loader code needs to fire up a kernel. It must know enough about file systems to read the kernel from the boot partition. In Linux this means reading a file like “vmlinuz-2.6.22-14-server” containing the kernel, loading the file into memory and jumping to the kernel bootstrap code. In Windows Server 2003 some of the kernel start-up code is separate from the kernel image itself and is actually embedded into NTLDR. After performing several initializations, NTDLR loads the kernel image from file c:\Windows\System32\ntoskrnl.exe and, just as GRUB does, jumps to the kernel entry point.