GPT - (last) call for action

[Rui Paulo]

At Mon, 11 Jun 2007 13:33:42 -0700 (PDT),
Matthew Dillon wrote:
> 
> 
> :>> # gpt -r show /dev/rdisk0
> :>>        start       size  index  contents
> :>>            0          1         PMBR
> :>>            1          1         Pri GPT header
> :>>            2         32         Pri GPT table
> :>>           34          6
> :>>           40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-
> :>> xxxxxxxxxxxx
> :>>       409640  159414704      2  GPT part - 48465300-0000-11AA-AA11-
> :>> xxxxxxxxxxxx
> :...
> :> Well, what's happening is that Boot Camp syncs the BIOS partition
> :> table with the GPT table, so the first partition should start at 40,
> :> just like the GPT.
> :>
> :> Why does it start at 40 ? Because you need room for the PMBR, the
> :> Primary GPT header and the Primary GPT table.
> :
> :Agreed, you need about 32 sectors for the GPT header+table.
> 
>     It makes sense for them to point the first MBR slice at the first
>     partition in the GPT, even though the standard says something else.
> 
>     It really sounds like they are making an accomodation for BIOS
>     booting or older Windows booting... or *something* of that sort.  The
>     fact that the bootability bit is not set in the MBR (I'm not sure about
>     that, is it set or not?)... that seems to imply a compatibility issue
>     with other OS's like Windows in a multi-boot environment.
> 
>     They are just doing it all with a single slice instead of having
>     two slices.
> 
>     I'll bet they found that the two-slice method doesn't work in some
>     cases and the one-slice method does.  The standard document doesn't
>     allow either method but it does seem to be a bit less insistent on
>     the starting sector for slice 1 then it does on there only being
>     one slice in the MBR, period.  I can also see some OS's / disk managers
>     barfing on having two slices which overlap each other.
> 
>     So it really does make sense for them to point the MBR at sector 40.
>     The more I think about it, the more sense it makes.

And also, if they used two partitions that would mean you would only
have one partition left for installing Windows.

--
Rui Paulo