Hardware Reference
In-Depth Information
the MBR) by checking the Boot Indicator bytes in each of the four partition table entries.
These bytes are at offsets 446 (1BEh), 462 (1CEh), 478 (1DEh), and 494 (1EEh) in the
MBR. These bytes indicate which of the four possible primary partition table entries con-
tains an active (bootable) partition. A value of 80h at any of these byte locations indicates
that particular table entry contains the active partition. Only one of the four Boot Indic-
ator bytes at a time is allowed to indicate an active partition (containing a value of 80h),
meaningthatifoneofthemis80h,theotherthreemustallcontain00h.Goingbytherules
initially set forth by IBM and Microsoft, only two possible conditions are allowed with
respect to all four Boot Indicator bytes:
• All four Boot Indicator bytes are 00h, indicating no Active (bootable) partitions.
• One Boot Indicator byte is 80h and the other three are 00h, indicating one Active
(bootable) partition.
If all four Boot Indicator bytes are 00h—which indicates that no active (bootable) parti-
tions exist—then the MBR returns control to the motherboard ROM, which will then nor-
mally display one of the error messages listed earlier. This would be an expected result if
you were to remove the existing partitions from a drive but had not created new partitions
on the drive, or if you had failed to make one of the partitions Active (bootable) when
partitioning or repartitioning the drive. For example, when creating multiple partitions on
a drive, it is possible to forget to set one of them as Active. If you do this by accident and
render your hard drive nonbootable, you can easily correct the problem by booting from
a floppy or CD with partitioning software such as FDISK or DISKPART and setting the
partition where your OS is installed as Active.
If only one of the Boot Indicator bytes is 80h and the rest are 00h (required for a properly
booting system), the system continues the standard boot process by loading the volume
boot record (VBR) from the active partition.
Invalid Partition Table
Any other Boot Indicator byte conditions are not allowed and, if present, indicate that the
partition table is corrupt, causing the MBR code to generate an error message and stop.
For example, if two or more of the Boot Indicator bytes are 80h (indicating multiple Act-
ive partitions, which is not allowed) or if any one of the Boot Indicator bytes is any value
other
than00hor80h,thepartitiontableisconsideredcorrupt;theMBRcodedisplaysthe
following error message and halts the system:
Invalid partition table
If you see this message, you can try rebooting from alternate media or a different drive
and then see if you can access the data on the problem drive. Depending on how much of
the partition table, MBR, or other sectors on the drive are corrupted, it may be easiest to
