Difference between revisions of "Prefetch"

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Volume related information, volume path and volume serial number, are embedded into the Prefetch file. The precise offset for this information varies for each application ran. In the header at offset 0x6c, the location of the volume path is stored. The location is a 4-bytes (DWORD) value. The offset 0x6c is consistent between Windows XP and Windows 7.
 
Volume related information, volume path and volume serial number, are embedded into the Prefetch file. The precise offset for this information varies for each application ran. In the header at offset 0x6c, the location of the volume path is stored. The location is a 4-bytes (DWORD) value. The offset 0x6c is consistent between Windows XP and Windows 7.
  
At the location given from 0ffst 0x6c, a 4 byte value is stored which is the number of bytes from current offset (location from offset 0x6c) to the beginning of the volume path. The location from offset 0x6c, for ease, will be called the "volume path offset." The volume path is embedded as a NULL-terminating string.
+
At the location given from offset 0x6c, a 4 byte value is stored which is the number of bytes from current offset (location from offset 0x6c) to the beginning of the volume path. The location from offset 0x6c, for ease, will be called the "volume path offset." The volume path is embedded as a NULL-terminating string.
  
 
The length of the volume path is a 4-byte value is located at volume path offset + 0x4.
 
The length of the volume path is a 4-byte value is located at volume path offset + 0x4.

Revision as of 11:34, 3 July 2011

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Windows Prefetch files, introduced in Windows XP, are designed to speed up the application startup process. Prefetch files contain the name of the executable, a Unicode list of DLLs used by that executable, a count of how many times the executable has been run, and a timestamp indicating the last time the program was run. Although Prefetch is present in Windows 2003, by default it is only enabled for boot prefetching. The feature is also found in Windows Vista, where it has been augmented with SuperFetch, ReadyBoot, and ReadyBoost.

Up to 128 Prefetch files are stored in the %SystemRoot%\Prefetch directory [1]. Each file in that directory should contain the name of the application (up to eight (?) characters), a dash, and then an eight character hash of the location from which that application was run, and a .pf extension. The filenames should be all uppercase except for the extension. The format of hashes is not known. A sample filename for md5deep would look like: MD5DEEP.EXE-4F89AB0C.pf. If an application is run from two different locations on the drive (i.e. the user runs C:\md5deep.exe and then C:\Apps\Hashing\md5deep.exe), there will be two different prefetch files in the Prefetch folder.


Signature

Each Prefetch file has a signature in the first 8 bytes of the file. Windows XP will generate Prefetch files with the signature \x11\x00\x00\x00\x53\x43\x43\x41 (0x41434353 0x00000011). Windows 7 and Windows Vista Prefetch file's signature is \x17\x00\x00\x00\x53\x43\x43\x41 (0x41434353 0x00000017). The ASCII representation of these bytes will display "....SCCA".

Timestamps

Both the NTFS timestamps for a Prefetch file and the timestamp embedded in each Prefetch file contain valuable information. The timestamp embedded within the Prefetch file is a 64-bit (QWORD) FILETIME object The creation date of the file indicates the first time the application was executed. Both the modification date of the file and the embedded timestamp indicate the last time the application was executed.

Windows will store timestamps according to Windows epoch.

Creation Time

The creation time does not have a static offset on any Windows platform. The location of the creation time can be found using the offset 0x8 + length of Volume path offset.

Last Run Time

A timestamp of when the application was last ran is embedded into the Prefetch file. The offset to the "Last Run Time" is located at offset 0x78 from the beginning of the file on Windows XP. The offset for Windows Vista and Windows 7 is at 0x80.

MetaData

Header

In each Prefetch file, the size of the header is stored and can be found at offset 0x54 on Windows XP, Windows Vista, and Windows 7. The header size for Windows XP is 0x98 (152) and 0xf0 (240) on Windows Vista and Windows 7.

The Prefetch file will embed the application's name into the header at offset 0x10.

Run Count

The run count, or number of times the application has been run, is a 4-byte (DWORD) value located at offset 0x90 from the beginning of the file on Windows XP. On Windows Vista and Windows 7, the run time can be found at 0x98.

Volume

Volume related information, volume path and volume serial number, are embedded into the Prefetch file. The precise offset for this information varies for each application ran. In the header at offset 0x6c, the location of the volume path is stored. The location is a 4-bytes (DWORD) value. The offset 0x6c is consistent between Windows XP and Windows 7.

At the location given from offset 0x6c, a 4 byte value is stored which is the number of bytes from current offset (location from offset 0x6c) to the beginning of the volume path. The location from offset 0x6c, for ease, will be called the "volume path offset." The volume path is embedded as a NULL-terminating string.

The length of the volume path is a 4-byte value is located at volume path offset + 0x4.

The volume serial number is a 4-byte value that identifies a media storage. A serial number does not have a consistent offset within a Prefetch between Windows operating systems. The 4-byte value can be found eight (8) bytes from the creation time location. The vol command on Windows can verify the volume serial number.

End of File

The end of file (EOF) for each Prefetch file are located at offset 0xc.

See Also

External Links