Re: linux bootup
From: Tauno Voipio (tauno.voipio_at_iki.INVALID.fi)
Date: 12/07/04
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Date: Tue, 07 Dec 2004 15:16:30 GMT
Kasper Dupont wrote:
> Uday Mullangi wrote:
>
>>Hi,
>>Could anyone answer few questions on bootup sequence of linux that i have.
>>
I guess that you mean on IBM PC-style hardware (Linux runs on many
other platforms, the boot on a Sparcstation or S/390 is vastly
different).
>>1.What is the typical size of a bootloader?
>
>
> There is no such thing as a typical size, it vary a lot.
> AFAIR syslinux is about 5KB. Grub is a litle more than
> 100KB.
>
>
>>Since the booloader code starts
>>from 0x7c00 and can touch till 0x9A000,
>
>
> I'm not sure what you think about here. The boot record
> on a PC is loaded on 0x7c00 and is 512 bytes long. (At
> least as long as you are booting from floppy or harddisk).
What happens after that is dependent only on the code
from the coot record. There is a limitation that the code
must run on real mode, so this limits it to 16 bit code.
This is due to the BIOS design handling only real-mode calls,
and the BIOS disk drivers have to be used, as there are no other
disk drivers in the system until the kernel is in and started.
>>Can i assume that the max size of
>>the bootloader could be 0x9a000-0x7c00=0x92400[ 585kb ].
>
>
> Who knows? Because of crappy hardware design, it is nice
> to keep the loader within that limit. How much would you
> want to put in a loader? When the Linux development was
> started you would have been considered insane if you
> wanted to use 100KB for your loader. Today people really
> like grub in spite of that size, and personally I think
> it have almost every feature I could ever want in a loader.
>
>
>>What if the
>>bootloader exceeds this size?
>
>
> If you want a loader larger than that, you will have to do
> something else. And if the loader try to use some memory
> that just isn't there, it will not work.
The loader could have its own disk drivers and boot up as a
kernel. Then the only limitation is that the booted-up target kernel
must fit in together with all the bootloader memory usage. The
current kernel start-up assumes that there is the BIOS available
and the entry is done in 16 bit real mode, so if your loader gets
over the one-megabyte boundary, it must be able to come back to
real mode.
>>2.Will the bootloader use the magic number of the(0xAA55) of the kernel
>>header? If so, when?
>
>
> Dunno. I believe that magic number originally existed
> because the first sector of a kernel image was in fact a
> loader (a primitve one).
>
0xaa55 is a signature needed by the BIOS boot to accept
the initial boot record.
>>3. Is the BIOS setting the IVT for loading linux standardised? (I asked the
>>because some vector numbers are used by the bootloader BIOS calls). If so,
>>where can i get the details?
>
>
> The loader will need to use some of these to access disk,
> keyboard, serial port, and screen. (Not all loaders need
> all of them). But Linux itself does not need them (except
> from stuff like VESA and ACPI).
>
One good source (1100+ pages) is:
Frank van Gilluwe, The Undocumented PC, Addison-Wesley,
ISBN 0-201-47950-8
-- Tauno Voipio tauno voipio (at) iki fi
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