Hardware Reference
In-Depth Information
When your system boots, it reads the first sector from the hard disk—called the MBR—and runs the
code contained in that sector. The MBR code then reads the partition table (also contained in the
MBR) to determine which partition is bootable and where it starts. Then, it loads the first sector of
the bootable partition—called the
volume boot record
(VBR)—which contains the operating system-
specific boot code. However, before executing the VBR, the MBR checks to ensure that the VBR ends
with the signature bytes
55AAh
. The
Missing operating system
message is displayed by
the MBR if it finds that the first sector of the bootable partition (the VBR) does not end in
55AAh
.
Several things can cause this to occur, including the following:
•
The drive parameters entered in the BIOS Setup are incorrect or corrupted
—These are
the parameters defining your drive that you entered in the BIOS Setup, and they're stored in a
CMOS RAM chip powered by a battery on your motherboard. Incorrect parameters cause the
MBR program to translate differently and read the wrong VBR sector, thus displaying the
Missing operating system
message. A dead CMOS battery can also cause this
because it loses or corrupts the stored drive translation and transfer mode parameters. In fact, in
my experience, a dead battery is one of the more likely causes. To repair this, check and
replace the CMOS battery, run the BIOS Setup, go to the hard drive parameter screen, and enter
the correct drive parameters. Note that most drive parameters should be set to auto or
autodetect.
•
The drive is not yet partitioned and formatted on this system
—This is a normal error if you
try to boot the system from the hard disk before the OS installation is complete. Boot to an OS
startup disk (floppy or optical media) and run the SETUP program, which will prompt you
through the partitioning and formatting process during the OS installation.
•
The MBR or partition tables are corrupt
—This can be caused by boot sector viruses, among
other things. To repair this, cold boot (power off, then on) from a Windows XP or later install
disc and use the FixMBR command in the Recovery Console (XP) or Recovery Environment
(Vista or later), which recopies the MBR code but doesn't alter the partition table. Then reboot.
If the message persists and you need to recover the data on the drive, you then must either
rebuild the partition tables from scratch using a disk editor utility or hire a data recovery
specialist who can do this for you. If you don't need to recover the data on the drive, simply
reinstall the OS from scratch, which will prompt you through partitioning and formatting the
drive.
•
The MBR is corrupt
—To repair this, cold boot (power off, then on) from a Windows XP or
later install disc and use the
FixBOOT
command in the Recovery Console (XP) or Recovery
Environment (Vista or later), which rewrites the MBR.
The system is experiencing intermittent memory errors.
If the memory was recently added or some other change was made to the system, you should undo that
addition/change to see whether it is the cause. If it's not, remove and reseat all memory modules. If
the contacts look corroded, clean them with contact cleaner and then apply contact enhancer for
protection. Check the memory settings in the BIOS Setup; generally, all settings should be on
automatic. Next, upgrade to the latest BIOS for your motherboard, and remove all memory except one
bank. Then run only one bank of memory, but in the second or third bank position. A socket can
develop a problem, and most motherboards do not require that the sockets be filled in numerical
order. Also, replace the remaining module with one of the others that was removed, a new module, or
