Boot Record Signature AA55 Not Found

This article was previously published under Q149877
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Intel-based computers rely on the system BIOS to load and executebootstrap code. The BIOS bootstrap routine generates an int 0x19 whichloads the first sector of the floppy or hard disk (0:0:1 in CHS) in memoryat segment address 0000:7C00H. The first physical sector is called themaster boot record (MBR) and contains the primary bootstrap loader code.

After loading sector zero, the BIOS checks that the last two bytes of thatsector are 55AA as seen on the disk. This 55AA is called a boot recordsignature and is kind of like an EOF when the sector is read. This issomething that the BIOS requires when booting. If the boot recordsignature 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
Strike f1 to retry boot

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 willappear:
OS Loader 4.0
Boot record Signature AA55 Not Found, xxyy Found.

Windows could not start because of a computer disk hardwareconfiguration problem. Could not read from the selected boot disk.Check boot path and disk hardware.

Please check the Windows documentation about hardware diskConfiguration and your hardware reference manuals for additionalinformation.
The MBR consists of boot code that is used by the system BIOS to read thepartition table. From data contained in the partition table, the MBR candetermine which partition is set to be bootable (active) and also thestarting sector of that partition. Once that location is determined , theBIOS jumps to that sector and begins the next phase of the boot process byexecuting additional code that is operating-system specific.

If the only thing wrong with sector zero was that the last two bytes arenot 55AA, this could easily be fixed with a disk editor such as NortonDiskedit. However, this message is usually indicative of somethingoverwriting or destroying the entire boot sector (sector zero) includingthe partition table entries.

When you install Windows on a logical drive in an extended partitionOSLOADER needs to "walk the extended partition table" through BIOS callsin order to get to the partition you have Windows installed in. Each ofthese logical drives are addressed in a "daisy chain" of partition tables.Each sector that contains a partition table entry MUST end with a 55AA asthe last 2 bytes in the sector.

For additional information, please see the following articles in theMicrosoft Knowledge Base:

197295WinNT Fails to Boot to Partition That Starts More Than 4 GB
This problem is probably caused by a virus or a hardware malfunction. Thebest way to determine how to recover is to use a disk editor to see if thepartition table entries are still intact. Each sector occupies 512 bytes.The first 446 bytes of sector zero contain the MBR boot code followed bythe partition table entries, and ends with 55AA. If the partition tableentries are still intact at offsets 1BE through 1FD, manually record theirvalues, then write 55AA starting at offset 1FE. Once the signature 55AA iswritten the MBR boot code can be regenerated by using the Fdisk.exeprogram from MS-DOS version 5.0 or later.

To do this, run the following:

WARNING: This process will repair the bootstrap code and the 55AAsignature by rewriting sector zero but will also overwrite the partitiontable entries with all zeros, rendering your logical drives useless(unless, that is, the 55AA signature is manually entered using a diskeditor prior to your performing the FDISK /MBR).

If the partition table entries are not intact or were overwritten withunreadable characters, the problem is more involved and entails locatingthe master boot sector (MBS) for each partition and manually rebuildingthe partition table entries. This process is beyond the scope of thisarticle.

To speed recover from future MBR corruption, use the Windows Resource Kit utility Disksave.exe to save a copy of the MBR to afloppy disk. This can be used if needed at some future date to restore theMBR using Disksave.exe.

In the case where Windows is installed on a logical drive in anextended partition, you will need a disk editing utility like NortonDiskedit to examine each sector containing an extended partition logicaldrive entry to make sure it ends with a 55AA. This process is beyond thescope of this article.


Article ID: 149877 - Last Review: 12/04/2015 14:33:29 - Revision: 2.2

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