Triple boot:XP,ubuntu&FC5 grub failed

[Jim Cornette]

Mike McCarty wrote:
> Jim Cornette wrote:
>> Mike McCarty wrote:
>>
>>> The BIOS does NOT search the partition table for an active partition.
>>> It is the code in the MBR which does that. All the BIOS looks for
>>> is the first physical sector on the physical volume having the BOOT
>>> RECORD signature (AA55) in the last two bytes. The BIOS accesses the
>>> disc in a physical manner (LBA translation aside). It knows nothing
>>> about partitions. Indeed, there are more than one way of partitioning
>>> discs, and the "traditional" method is not the only one. Anyway, if
>>> the BIOS finds the BR signature in the first physical sector, then it
>>> simply loads it to 0000:7C00 (IIRC) and jumps to it. [NB: The processors
>>> all reset to REAL mode, so this is not a selector address, but a
>>> segmented address.]

I wiped out a disk that did have win2k on it. I removed all of the 
partitions and set up 4 partitions. The first two were 100 MB partitions 
which I dedicated one for each installation that I was making. I made 
partition 1 active and then created an lvm for the first installation on 
partition 3. Afterwards, I toggled the active flag on partition 1 to off 
and toggled partition 2 to active. I created a separate lvm for the 
second installation on partition 4. (setup a /boot on /dev/hda1 for 
installation 1 and a /boot partition on /dev/hda2 for the second 
installation. For the lvm content, I just made a swap system and a / 
system for each of the two installations. For the seperate lvms for each 
installation, I made different vg and lv names for the swap and / for 
each installation)

Anyway, toggling either partition 1 or partition 2 to active would allow 
the desired installation to boot. The only drawback would be that you 
need to toggle the currently active partition to inactive followed by 
making the new desired partition active.

So you are saying above that the MBR has the default code to search for 
the active partition. The BIOS does not know about active partitions 
without instruction from the MBR. Now if I took a brand new disk with 
nothing on it, partitioned as described above and toggled the active 
partition, it would just come up confused without some OS writing 
information into the MBR?

It works if you wanted to simply toggle active partitions instead of 
chainloading installations. Grub has to be putting some information in 
the partitions for this to work. So you should be able to add entries 
within grub in order to chainload either installation and forget about 
adding grub to the MBR.

By the way, selecting advanced options during installation allows you to 
choose where you want to install grub. Otherwise, it defaults to 
installing grub into the MBR.


>>>
>>> Mike
>>
>>
>> So if I want to install grub onto partition 2 of my first drive and 
>> have no Grub installed into the MBR, the BIOS will not start loading 
>> whatever is in the first sector of my second partition?
> 
> I repeat: The BIOS looks at the absolute first sector on the disc,
> and if it ends with the BOOT RECORD marker AA55, then it loads it
> to location 0000:7C00 and jumps to that location. If it does not
> find the marker, then it looks at the next device in the list of
> bootable devices. This is true for both floppy disc drives and
> for fixed disc drives. What it does for CDROM and FLASH drives I dunno,
> not having delved into that.

So the technology is still a little primitive? Sort of like needing to 
access a specific address for the 8086 and MFM drives? (low level 
formatting anyway)

I guess BIOS is less capable than I supposed. No technical research on 
my part. I understand that going to the absolute first sector of the 
disk, grabbing information from the MBR (whatever it may contain) and 
getting information from the MBR as to where to shoot from next if it 
stores active partition information within it would work.

> 
>> I have not tried to load grub into a partition and then make the 
>> partition active. I thought that I read that someone else was loading 
>> grub from a partition.
> 
> That can be done. It just isn't done by the BIOS. 

OK, I'll buy that.

I do it, for example.
> My machine has the Windows XP Boot in the MBR. That bootstrap
> looks for the XP Boot Manager, which has the capability to load GRUB.
> In this case, not from the BR of another partition (although it is
> installed there, too, and that's where I got it from) but from a file
> in the XP file system. GRUB then loads stage 2, gets its config file
> from the Linux partition, and allows me to boot one of several revisions
> of FC. I also have some GRUB boot floppies, which are nice to have
> for emergency situations. I believe that GRUB can also be installed
> on a CDROM, though I have not done that.

I experienced messed up windows installations too many times to rely on 
installing a file to load grub via the XP bootloader. I rely on XP 
booting from the grub installed in the MBR. XP is of course on the 
active partition 1 and knows how to boot without the MBR being effected 
as well.

> 
>> I changed the active partition for the other OS and the OS which was 
>> on the active partition booted. When I changed the active partition to 
>> the other "other" OS, the secondary OS booted. I assumed that grub 
>> loaded in a similar way as the other OS to Load the Linux OS with the 
>> information grub puts into the selected partition.
> 
> GRUB gets loaded by whatever loads it. If it is in the MBR, then the
> BIOS loads it. If it is elsewhere, then something else loads it.
> The BIOS has no concept of partitions. GRUB is not an OS. It is an OS
> loader.

I knew grub was not an OS itself and simply locates the intended kernel 
to load.

  I want to try to run two different Linux
>> distributions and be able to toggle the active partition in my 
>> experimenting. (Grub installed in the selected partition, no grub in 
>> the MBR)
> 
> That is possible. It is even possible to chain load GRUB from GRUB.
> 
> I hope I'm not being overly terse. I have on occasion posted a
> description of the full bootstrap process from start to finish.

Thanks for the explanation. The end result is the same. I guess my main 
error was on thinking BIOS did this vs. information within the BR when 
nothing is intentionally installed to the MBR.

Jim


> 
> Mike


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