Western Digital T13/2132-D - Manual (Page 86)

T13/2132-D Revision 3 June 23, 2010
74 Working Draft Enhanced Disk Drive - 4 (EDD-4)
A.5 Hybrid VBR boot code
Hybrid VBR boot code is responsible for booting the OS, and is usually OS-specific. If EAX is set to "!GPT", then
hybrid VBR boot code uses the GPT Partition Entry field to identify its partition.
NOTE 6 — In contrast, legacy VBR boot code only recognizes the MBR disk layout and uses the
Starting LBA field and Size In LBA field to identify its partition.