[syslinux] Custom MBR

YYZ yyz01 at yahoo.com
Sat Dec 13 16:12:37 PST 2008

--- On Fri, 12/12/08, Jeffrey Hutzelman <jhutz at cmu.edu> wrote:

> From: Jeffrey Hutzelman <jhutz at cmu.edu>
> Subject: Re: [syslinux] Custom MBR
> To: yyz01 at yahoo.com, "For discussion of Syslinux and tftp-hpa" <syslinux at zytor.com>
> Cc: jhutz at cmu.edu
> Date: Friday, December 12, 2008, 11:26 PM
> --On Friday, December 12, 2008 01:11:23 PM -0800 YYZ
> <yyz01 at yahoo.com> wrote:
> > and they don’t rely on the active flag in MBR at all
> ... unless the bootloader is not in the MBR, in which case
> they rely on an MBR to find and load the bootloader, which
> it does by loading it from the boot sector of the partition
> whose active flag is set.

The first stage loader IS in the MBR and it doesn't care which partition is active.

> > The catch is that
> > whenever you start your system, the control should
> somehow get
> > transferred to the boot manager wherever it resides
> (unused sectors of
> > track0, a dedicated boot partition, etc.).
> And that happens - the first-stage bootloader either _is_
> the MBR, or is located in the boot sector of some other
> partition, and is located by the MBR in the traditional
> fashion.

As I said, first stage loader is the MBR and knows where to load the next stage from.

> > This is what I’m asking for
> > – I would like to install syslinux on any partition
> of my choice and
> > would like to boot into syslinux
> You can have that.  Just set the active flag on the
> partition where you installed syslinux.  That's what
> it's for.

What if it's a logical partition?

> > regardless of which partition is active.
> Why?  What is wrong with how it works today?
> What are you trying to do that can't be done with a
> standard MBR?

Because the way it works today is pretty lame and has several limitations.
It's not interactive and doesn't work with logical partitions.
Unfortunately, until M$ decides to go with a gpt based design, most of us
are stuck with this. Using MBR as first stage to load a more advanced
multi-boot manager that lets users decide interactively which partition to
boot is a better solution in the meantime.

> What are you using the active flag for, other than to tell
> the MBR where to find a bootloader?

You don't have to use this flag at all and you can boot from logical
partitions too, where this flag has little meaning.

> -- Jeff


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