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.

Review URL: http://codereview.chromium.org/866005
3 files changed