Hardware Reference
In-Depth Information
strike F1 to retry boot, F2 for setup utility
Which of these two messages you see depends on whether no boot devices were found or readable,
or a boot sector could be read but was found not to have the proper signature bytes.
MBR Boot Error Messages
If the BIOS has successfully loaded the MBR and checked the signature, and if the signature bytes are
correct, the BIOS then passes control to the MBR by running the MBR code. The program code in the
MBR performs a test of the partition table (also contained in 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 contains 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 Indicator bytes at a time is allowed to indicate an active partition (containing a value of 80h),
meaning that if one of them is 80h, the other three must all contain 00h. Going by the rules 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) partitions exist—
then the MBR returns control to the motherboard ROM, which will then normally 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 Active partitions, which is not allowed)
or if any one of the Boot Indicator bytes is any value other than 00h or 80h, the partition table is
considered corrupt; the MBR code displays the 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 simply start over by removing and re-
creating all the partitions (which will lose any and all data they contain), or investigate data recovery
 
 
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