Boot Record Signature AA55 Not Found (149877)
The information in this article applies to:
- Microsoft Windows 2000 Server
- Microsoft Windows 2000 Advanced Server
- Microsoft Windows 2000 Professional
- Microsoft Windows 2000 Datacenter Server
- Microsoft Windows NT Server 3.1
- Microsoft Windows NT Server 3.5
- Microsoft Windows NT Server 3.51
- Microsoft Windows NT Server 4.0
- Microsoft Windows NT Workstation 3.1
- Microsoft Windows NT Workstation 3.5
- Microsoft Windows NT Workstation 3.51
- Microsoft Windows NT Workstation 4.0
- Microsoft Windows NT Advanced Server
- Microsoft Windows for Workgroups
- Microsoft Windows 95
This article was previously published under Q149877 SUMMARY
Intel-based computers rely on the system BIOS to load and execute
bootstrap code. The BIOS bootstrap routine generates an int 0x19 which
loads the first sector of the floppy or hard disk (0:0:1 in CHS) in memory
at segment address 0000:7C00H. The first physical sector is called the
master boot record (MBR) and contains the primary bootstrap loader code.
After loading sector zero, the BIOS checks that the last two bytes of that
sector are 55AA as seen on the disk. This 55AA is called a boot record
signature and is kind of like an EOF when the sector is read. This is
something that the BIOS requires when booting. If the boot record
signature is not present, you will probably get a BIOS-dependent message:
Boot Record Signature AA55 Not Found, xxyy Found
Alternately, the message
Non-System or Non-Bootable Disk
or the message
will appear, or the system will stop responding.
If Windows is installed on a logical drive in an extended partition,
after the BOOT MENU choice and NTDETECT runs, this error message will
appear:
OS Loader 4.0
Boot record Signature AA55 Not Found, xxyy Found.
Windows could not start because of a computer disk hardware
configuration problem. Could not read from the selected boot disk.
Check boot path and disk hardware.
Please check the Windows documentation about hardware disk
Configuration and your hardware reference manuals for additional
information.
WORKAROUND
This problem is probably caused by a virus or a hardware malfunction. The
best way to determine how to recover is to use a disk editor to see if the
partition table entries are still intact. Each sector occupies 512 bytes.
The first 446 bytes of sector zero contain the MBR boot code followed by
the partition table entries, and ends with 55AA. If the partition table
entries are still intact at offsets 1BE through 1FD, manually record their
values, then write 55AA starting at offset 1FE. Once the signature 55AA is
written the MBR boot code can be regenerated by using the Fdisk.exe
program from MS-DOS version 5.0 or later.
To do this, run the following:
WARNING: This process will repair the bootstrap code and the 55AA
signature by rewriting sector zero but will also overwrite the partition
table entries with all zeros, rendering your logical drives useless
(unless, that is, the 55AA signature is manually entered using a disk
editor prior to your performing the FDISK /MBR).
If the partition table entries are not intact or were overwritten with
unreadable characters, the problem is more involved and entails locating
the master boot sector (MBS) for each partition and manually rebuilding
the partition table entries. This process is beyond the scope of this
article.
To speed recover from future MBR corruption, use the Windows Resource Kit utility Disksave.exe to save a copy of the MBR to a
floppy disk. This can be used if needed at some future date to restore the
MBR using Disksave.exe.
In the case where Windows is installed on a logical drive in an
extended partition, you will need a disk editing utility like Norton
Diskedit to examine each sector containing an extended partition logical
drive entry to make sure it ends with a 55AA. This process is beyond the
scope of this article.
Modification Type: | Major | Last Reviewed: | 6/3/2003 |
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Keywords: | KB149877 |
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