Describe PMBR in terms of syslinux GPT extension.

Describe PMBR in terms of syslinux GPT extension.

syslinux provides a GPT-aware MBR that interprets the MBR content like this:

Offset Size Contents
--------------------------------------------------
0x000/0 424 PMBR boot code
0x1a8/424 16 GUID of the boot partition
0x1b8/440 4 MBR-compatible disk ID
0x1bc/444 2 Magic number: 1D 9A
0x1be/446 16 PMBR protective entry
0x1ce/462 48 PMBR null entries
0x1fe/510 2 Boot signature: 55 AA


This change modifies the gpt program to understand that format:

1) Expect no more than 424 bytes of executable code for the 'boot' command.

2) Display the boot partition GUID when the PMBR has the magic numbers 1D 9A at offset 0x1bc for the 'show' command.

[tedbo, 2010-03-12 22:32:47]

LGTM

Thanks! A minor request for comment below but this change looks very nice.

http://codereview.chromium.org/866005/diff/1/3
File gpt.h (right):

http://codereview.chromium.org/866005/diff/1/3#newcode60
gpt.h:60: struct mbr {
The original struct represented a generic MBR with up to 446 byes of code data.
The new struct is specific to the syslinux GPT new protocol (as was the code to
make it bootable in boot.c). Maybe add a comment here that explains that this is
specific to the Syslinux GTP MBR boot protocol?