Re: PC boot sequence (was: patch: MFSv3 support (libsa) for boot2 (i386))

[David Laight <david%l8s.co.uk@localhost>]

On Thu, Dec 29, 2011 at 03:10:06PM +0100, Edgar Fu? wrote:
> Am 29.12.2011 um 12:20 schrieb David Laight:
> 
> > The actual 'pc' boot sequence is:
> Ah brilliant, answers to all the questions I wanted to ask anyway.
> 
> > 4) The pbr code now reads the rest of the 8k 'boot code' area at the
> >   start of the partition
> This means "MBR partition", right?

Yes, nothing has found a netbsd disklabel yet.

> And this "boot code" is what is called "primary boot" in BSD terminology, 
> right?

That is what installboot(8) calls it :-)

> 
> > this is the rest of bootxx_xxxfs.
> What do you mean by "rest of"?

bootxx_ffxv1 is just under 8k, the first 512 sector is read by the mbr,
so that code has to read the rest of the sectors.

> 
> > 5) The bootxx code now tries to load the 'boot' program from filesystem,
> >   it checks the following places for a filesystem:
> I don't quite get why it has to look for a filesystem. I thought (and
> in only 8k it must have) the block numbers of /boot encoded into
> it by installboot? So, are these block numbers FS-relative?
> I thought they were disklabel-partition relative.

Nope, 8k is plenty of space for the code to parse the directory structure
(etc) in order to read a file.
That is why there are copies of bootxx for each fs type.
For ffsv1/v2 there is a lot of bloat caused by using access macros that
mimick the normal kernel ones - which is partially why there are
separate copies. It is also why you can't boot from fs with large
block/fragment sizes (difficult to have 64k buffers in real mode!).

> >   a) the start of the partition
> This again means "MBR partition", I suppose?

Yes.

        David

-- 
David Laight: david%l8s.co.uk@localhost