As of version 2.3.1 of the UEFI specification, a new global attribute flag bit is defined for entries in an EFI partition table. This flag marks those entries where relative sector #0 of the partition, the VBR, contains a PC/AT-style bootstrap program that can be loaded and run in the old PC/AT way.
MBRCODE-EFI.ASM is the
MBR
bootstrap program that is written by
the
NEWMBR command in the DASDPART utility
from the TAU system utilities.
The
NEWMBR command from
the OS/2 Command-Line Utilities
writes an older version of this program that is not compatible with the
EFI partition table scheme.
EFI partition table entries whose VBRs can be bootstrapped in the old
PC/AT way will be listed with
the HasVBR attribute
by the
the
INSPECT command in DASDPART.
This attribute can be turned on and off by
the
PARTATTRIB command.
Only one partition on any disc should be marked with this attribute. But it can be any partition. The bootstrap program looks for partition table entries that have this flag set, and loads and runs the VBR of the first flagged partition that it finds. Normally the partition so marked will be one of:
Warning: The bootstrap program makes no special provisions for Trusted Computing. It also requires, in order to operate, that the machine's firmware supports the INT 13h API extensions, since it contains no fallback code for the case where these extensions are not present. PC firmwares have generally supported these extensions since 1996.