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Date:      Sat, 10 Jun 2006 18:32:40 -0800
From:      Beech Rintoul <beech@alaskaparadise.com>
To:        freebsd-questions@freebsd.org
Cc:        Jerry McAllister <jerrymc@clunix.cl.msu.edu>, jekillen <jekillen@prodigy.net>
Subject:   Re: dual boot; Linux, FreeBSD
Message-ID:  <200606101832.56687.beech@alaskaparadise.com>
In-Reply-To: <200606110211.k5B2BTYD014987@clunix.cl.msu.edu>
References:  <200606110211.k5B2BTYD014987@clunix.cl.msu.edu>

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On Saturday 10 June 2006 18:11, Jerry McAllister wrote:
> > On Jun 10, 2006, at 8:00 AM, Jerry McAllister wrote:
> > >> Hello;
> > >> If I want to set up a dual boot of either Linux or FreeBSD, what is
> > >> the
> > >> best way to go about it?
> > >> Use Lilo, grub, or does FreeBSD have a boot loader that it likes
> > >> better
> > >> and Linux won't object to?
> > >
> > > Mabye you are using the term 'boot loader' for what I am used to seei=
ng
> > > called the 'MBR'.   The boot loader I am familiar comes later in the
> > > process and is unique to each OS.
> >
> > Thank you for correcting me on the terminology; and the info and advice.
> > So MBR is Master Boot Record? I remember when installing FreeBSD and
> > slicing and partitioning it asks if the slice should contain and MBR. I
> > assumed
> > that that was the part of the os that was os specific because the file
> > systems
> > are different, but I may have something to learn correctly.
> > Particularly the difference
> > between slices and labeling/partitioning. But this confusion hasn't
> > prevented me from installing successfully.
>
> OK.   Just a little more on MBR vs boot sector vs boot loader.
>
> The slice doesn't contain the MBR.  That is in sector 0 of the disk devic=
e.
> Then 'below' that, each slice contains a boot sector.
>
> In FreeBSd world, a slice is the primary division of the disk.  It is
> generally referred to as a primary partition in Microsloth land.  But
> that is the same.  Each slice (primary partition) can be bootable.
>
> To be bootable, a slice must be marked as bootable and have a boot
> record in its first sector.  Note that this is not the first sector on
> the disk device (eg sector 0) but the boot sector of the slice.   That
> boot record is unique to the particular OS living in that slice.
>
> In FreeBSD (as with most UNIXen) each slice can be further divided
> in to partitions.   In FreeBSD, the 'a' partition on the slice is
> generally assumed to be the system partition and is called 'root' and
> gets mounted as '/'.
>
> The sequence of events is essentially:
>   The system starts up, find and runs the BIOS.
>   The BIOS does some hardware checks, including the boot order.
>   A typical boot order can be:   floppy, CD, Hard disk.
>   The BIOS searches through its boot device list for one that has an MBR
>   The BIOS loads the first MBR it finds in its list and transfers control.
>   The MBR looks at its slice table and offers a choice of those that
>       are marked as bootable and have a recognizable boot record in its
>       boot sector.  All MBRs have some way of choosing a slice to boot by
>       default if you don't make any other choice.
>   The MBR loads up that boot sector and transfers control to it.
>   The boot sector code does some more checking and generally runs a boot
>   loader that is able to read up some sort of script that tells what
> features you want to be part of the system.
>   The boot record then finishes the boot and hands over control to the
> newly booted system, usually to a program called 'init' in UNIX world. So=
me
> more services might still be started after init begins to run.  In FreeBSD
> that is controlled by the rc.xxx scripts.   All that stuff that the boot
> loader looks at and the init program that is given control are normally
> somewhere in the root (/) partition, which is part of the slice being
> booted, which is a primary division of the disk device being booted.
>
> So, in a sense, there is a hierarchy:  BIOS, disk device, slice then
> partition Each has some part of the boot and tha is used in order.
>
> If there are bootable slices on more than one disk device, then each disk
> that has a bootable slice that you intend to use that way, must have an
> MBR. Each bootable slice on each bootable disk must have a boot record in
> the boot sector.  After that, it is up the the OS what comes next.
> The BIOS only deals with the first device in its boot list that has a
> proper MBR and hands control to it.   If there is more than one device
> to choose from, that MBR has to figure it out and give you that choice.
>
> Although it would be possible for that first MBR to read up all the
> slices that are marked as bootable on all disk devices and offer them
> all as choices right off the bat.   But, at least the FreeBSD MBR starts
> with the its own bootable slices and then just the other disks that
> have MBRs   If you want one of the other disks, you first select that
> disk (generally identified as choice 5 or F5) and then its MBR will
> put up its list of choices for you.   I haven't tried it with 3 disks
> with bootable slices.  I guess it will just continue on.
>
> Anyway, this all works just fine.  The MBR and initial boot record in
> the boot sector of each slice (or primary partition if you must degrade
> to MS terminology) have just enough standardization that the FreeBSD MBR
> or most any of the other more fancy ones, can initiate the boot for any
> of the OS-en commonly available to run on these machines.   Since the OS
> specific stuff really comes after it gets in to the slice boot record
> code in the boot sector, then generally any of them can boot any of them.
> The exception is MS MBRs.  I have heard that some more recent ones play
> better, but any I have had so far will not boot any slice except one
> for a MS OS.   I don't know what they screw up, but find it not surprisin=
g.
>
> So, there is the tome.
> All newbies, careful what you ask.  Someone may answer thusly with more
> than you every wanted to know.
>
> ////jerry

Maybe this ought to be included in the handbook. I've seen this question or=
=20
one like it 100's of times on these lists. That was the best answer I've=20
seen.

Just my $.02

Beech
>
> > > All of those you name will work as an MBR.
> > > I just stick with the FreeBSD MBR but I don't have any need for fancy
> > > features or display formatting that the others give you.
> > > The FreeBSD MBR should be able to start any of them.  FreeBSD can be
> > > started from any of those MBRs.   It is more an aesthetic thing.
> > > Advocates
> > > of each tend to get rabidly partisan.  But, the really meaningful
> > > differences
> > > are small.
> > >
> > > Past the MBR stage, use the boot sector and boot loader stuff that
> > > comes
> > > with the OS you put on each bootable slice.
> > >
> > > One thing you need to do is put the MBR on each disk if you are putti=
ng
> > > each OS on a separate disk.   The Bios will start the first one and
> > > the MBR should then give you a choice of any bootable slices on the
> > > first
> > > drive and also the choice of going to the second drive MBR.  If you
> > > then chose the second MBR, that one will give you the choice of all
> > > the bootable slices on that drive.   Probably you will make only one
> > > bootable slice on each drive, but could make up to 4.
>
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