Out of Space

[Gene Heskett]

On Wednesday 10 August 2016 14:03:40 Oliver Grawert wrote:

> hi,
>
> On Mi, 2016-08-10 at 13:05 -0400, Gene Heskett wrote:
> > On Wednesday 10 August 2016 07:02:49 Oliver Grawert wrote:
> > 
> >
> > > it definitely has not ... thats a bootloader thing ...
> > >
> > > ciao
> > > 	oli
> >
> > Oli, I think that needs an explanation.
> > ...
> >
> > So please shine a light in that crevice holding the secret sauce
> > that 
> > makes all this work.
> >
> > I am sure I am not the only one with a visible curiosity bump, :)
> >  
>
> well, the point here is that the BIOS never actually accesses your
> partition table (no matter what kind of partition table that is) 
>
> let us take a look at the MBR and the boot process a BIOS does:
>
> wether you use MSDOS or GPT, the following is identical for both:
>
> a disk has an MBR that is 512 bytes big and can contain the first
> stage bootloader starting at byte 0 of your disk (usually with grub
> that is the "stage1" binary blob) and ending at byte 440 ... your BIOS
> will look if it finds a bootloader signature in the first bytes of the
> HDD ... and fire it up in case it finds the desired byte signature ...
>
> so now you are in your bootloader, past the BIOS...
>
> lets take a look at the rest of these 512 bytes of the MBR:
>
> in case of an MSDOS partition table you have 4 entries each 16 bytes
> big starting at byte 446 ... each of these 16byte blocks is an entry
> for a possible primary (or extended) partition ...
>
> in case of a GPT partition table your partiton table actually starts
> in the first logical block *after* the 512 bytes the MBR occupies, the
> byte 446 to 510 block that normally holds the MSDOS table is actually
> just marking the disk as "occupied" so that the disk cant be trashed
> if you use a non GPT partitioning tool on it (you can easily try it by
> running something like fdisk on a system with GPT). along with that
> there is always a safety copy of your GPT at the very end of the disk.
>
> in either case, the BIOS only looks at the MBR and is not dealing with
> the partition table at all, your bootloader (and later in the boot the
> kernel) does that.
>
> that said ... if you use a dual boot with windows it will only boot
> from a GPT disk when there is UEFI ... the same thing is true if you
> use secure boot, there GPT and UEFI are tied together. so in both of
> these cases the claim that you need UEFI for GPT is true.
>
> but if you have a plain linux only machine it does not matter if you
> use UEFI or BIOS, you can defintely use a GPT instead of an MSDOS
> partiton table even with a BIOS based machine (at least when using a
> bootloader that can understand GPT like grub2).
>
> ciao
> 	oli
>
Thank you Oli.  So before I try the live boot again, I'll change that 
disk to a GPT disk first though.


>  


Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>