Boot Record Signature AA55 Not Found

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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
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 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.


The MBR consists of boot code that is used by the system BIOS to read the partition table. From data contained in the partition table, the MBR can determine which partition is set to be bootable (active) and also the starting sector of that partition. Once that location is determined , the BIOS jumps to that sector and begins the next phase of the boot process by executing additional code that is operating-system specific.

If the only thing wrong with sector zero was that the last two bytes are not 55AA, this could easily be fixed with a disk editor such as Norton Diskedit. However, this message is usually indicative of something overwriting or destroying the entire boot sector (sector zero) including the partition table entries.

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

For additional information, please see the following articles in the Microsoft 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. 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.


Article ID: 149877 - Last Review: February 21, 2007 - Revision: 2.2
  • Microsoft Windows 2000 Server
  • Microsoft Windows 2000 Advanced Server
  • Microsoft Windows 2000 Professional Edition
  • Microsoft Windows 2000 Datacenter Server
  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows NT Server 3.5
  • Microsoft Windows NT Server 3.51
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows NT Workstation 3.1
  • Microsoft Windows NT Workstation 3.5
  • Microsoft Windows NT Workstation 3.51
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows for Workgroups 3.2
  • Microsoft Windows 95

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