A PC/AT-style MBR bootstrap program for EFI partitioned discs


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.

© Copyright 2011 Jonathan de Boyne Pollard. "Moral" rights asserted.
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