GPT - (last) call for action

Rui Paulo rpaulo at fnop.net
Mon Jun 11 19:53:01 UTC 2007


At Mon, 11 Jun 2007 12:41:18 -0700,
Chuck Swiger wrote:
> 
> Hi, all--
> 
> On Jun 10, 2007, at 3:13 PM, Rui Paulo wrote:
> >> :Media sector size is 512
> >> :Warning: BIOS sector numbering starts with sector 1
> >> :Information from DOS bootblock is:
> >> :The data for partition 1 is:
> >> :sysid 238 (0xee),(EFI GPT)
> >> :    start 40, size 409600 (200 Meg), flag 0
> >> :        beg: cyl 0/ head 0/ sector 41;
> >> :        end: cyl 406/ head 6/ sector 14
> >>
> >>     I think I have it mostly figured out, but the 'start 40' in your
> >>     output can't be right.  The intel documentation says that the
> >>     starting LBA in a PMBR record must be set to 1 by definition
> >>     (table 11-7 in the 1.10 documentation).
> >
> > I don't know why Apple does that.
> 
> The offset of 40 sectors sounds like it is pointing to the first
> partition listed within the GPT?
> 
> A typical Intel Mac system using GPT ought to look something like this:
> 
> # fdisk /dev/rdisk0
> Disk: /dev/rdisk0       geometry: 9964/255/63 [160086528 sectors]
> Signature: 0xAA55
>           Starting       Ending
> #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
> ------------------------------------------------------------------------
> 1: EE 1023 254  63 - 1023 254  63 [         1 -  160086520] <Unknown ID>
> 2: 00    0   0   0 -    0   0   0 [         0 -          0] unused
> 3: 00    0   0   0 -    0   0   0 [         0 -          0] unused
> 4: 00    0   0   0 -    0   0   0 [         0 -          0] unused
> 
> # 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
>    159824344     262151
>    160086495         32         Sec GPT table
>    160086527          1         Sec GPT header

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.

Now, you don't seem to have used Boot Camp on your Mac, right?
If you ever use it, fdisk /dev/rdisk0 will show things differently.
The first partition with id 0xEE will should start at LBA 40 and end
at LBA 409640.

> The first, small partition is almost certainly a "boothfs" boot
> partition, as described in the man page for Apple's version of
> fdisk:

I don't think so.
The boothfs partition doesn't seem to be used on Intel Macs no
longer. The EFI boot loader that comes with Intel Macs can read HFS+
without any help (actually it's an EFI module), so bootufs/boothfs
partitions are no longer required.


--
Rui Paulo


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