Hardware Reference
In-Depth Information
options if you don't have backups and need the data back.
Error Loading Operating System
Once the MBR code makes it past the Boot Indicator byte check, and one of the partitions is marked
active (bootable), the MBR code continues by looking at the rest of the information in the active
partition table entry to determine the starting sector location for the active partition. The starting
sector location is a sector address indicated by both CHS (cylinder/head/sector) and LBA (logical
block address) values. The CHS value is used for drives 8.4GB or smaller in capacity, whereas the
LBA value is used for drives larger than 8.4GB (pretty much all drives today). The first sector of a
partition contains the volume boot record (VBR), which is also known as the
operating system boot
record
because it is installed on the drive during the OS partition format process. Once the VBR
address of the active partition has been determined, the MBR code reads that sector from the drive.
If the VBR of the active partition can't be read successfully due to a read error (physical problem
with the disk), a series of five retries occurs. If after all the retries the VBR still cannot be read, the
following message is displayed and the system stops:
Error loading operating system
This is a bad message to see because it generally indicates physical problems with the drive.
However, this message can also appear if the drive parameters are incorrectly entered in the BIOS
Setup, or if the VBR sector location as listed in the MBR partition table is invalid. For example, if
the MBR contains an invalid VBR address that points to a sector beyond the physical end of the drive,
this message results.
If you see this message unexpectedly, first check the BIOS Setup settings for the drive to be sure they
are correct, then check the partition table entries, and finally run a diagnostic test of the drive itself. If
the drive is bad or going bad and you need the data back, you may require the services of a
professional data-recovery outfit.
Missing Operating System
If the MBR code has successfully loaded the VBR with no read errors, the MBR code checks the last
two bytes of the VBR for the 55AAh signature. If the signature is not present, the MBR displays the
following error message and halts the system:
Missing operating system
This would be an expected occurrence if, for example, you had just created the partition and had not
yet done a high-level format, meaning that the partition has not yet been formatted by the OS, or no OS
is yet installed.
