Re: How does linux boot

[Alvin Oga <aoga@ns.Linux-Consulting.com>]

hi ya ritesh

On Fri, 22 Oct 2004, Ritesh Raj Sarraf wrote:

> This is the way I understand.
> The computer is on. The BIOS loads the boot-loader. The boot-loader loads the
> kernel image. If the kernel image has modules, initrd also gets loaded so
> that appropriate modules can be loaded for the kernel to identify the
> hardware, filesystems etc etc...

not quite... close ...
 
> OS installation is done.. The kernel image, the initrd image etc are all
> stored on the disk.

easier concept is assume you boot from floppy.... lot easier to understand

> Now the boot loader loads and then loads the kernel image along with the
> initrd image.

initrd is NOT needed if the required drivers is built into the
kernel, ignore the initrd to avoid confusion too
 
> So, Is the boot-loader so smart and powerful

no ... boot-loader is the dumbest/simplest thing there is

the boot loader is usually 2 pieces ... ( 2.5 pieces if you use grub )

	the boot loader is 512 bytes TOTAL

	and of that 64bytes is used for the 4 primary partition
	and 2 more bytes for "boot flag" 

	so your total size of your boot loader is 512 - 64 -2 (446bytes)
	( that is simple, small and "dumb" or "super smart" )

	- its job is to figure out where to jump to that has
	the boot kernel ( usually stage 2 boot loader )
		grub plays silly and wants stage1.5 to know if its
		ext2 or ext3 or jfs, or xfs or reiserfs or foo-fs

	- usually the stage1 boot loader says where to jump to
	find the "main boot loader"

> (much more that the kernel) that
> it reads data from the disk without knowing the type of disk and the
> filesystem type ?

grub will try that ... to read the disk

lilo/other boot loaders all know how to read track 0 which
has all the info needed to boot

> I mean the kernel requires modules to be loaded to detect
> the type of disk (scsi or ide) ,

no ... the kernel does NOT need modules if the kernel was built
with the drivers compiled into it

> type of filesystem etc..

grub wants to know the filesystem on the disk ... 

> The boot-loader
> doesn't require anything ?  Amazing.

yeah... the "main" bootloader is small, simple, amazing, trivial
 
> If yes, the boot-loader is smart enough.

its NOT smart ... it's dumb .... it just does one thing,
where to find its next half (stage2) of the bootloader

> Why not use it's master-piece code into the kernel ? :-)

it IS already in the kernel

> If no, What have I missed to RTFM ? Any good docs ?

http://linux-boot.net/Boot.Sequence/

c ya
alvin