Difference between revisions of "Prefetch"

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Up to 128 Prefetch files are stored in the <tt>%SystemRoot%\Prefetch</tt> directory [http://blogs.msdn.com/ryanmy/archive/2005/05/25/421882.aspx]. Each file in that directory should contain the name of the application, a dash, and then an eight character hash of the location from which that application was run, and a <tt>.pf</tt> 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: <tt>MD5DEEP.EXE-4F89AB0C.pf</tt>. If an application is run from two different locations on the drive (i.e. the user runs <tt>C:\md5deep.exe</tt> and then <tt>C:\Apps\Hashing\md5deep.exe</tt>), there will be two different prefetch files in the Prefetch folder.
 
Up to 128 Prefetch files are stored in the <tt>%SystemRoot%\Prefetch</tt> directory [http://blogs.msdn.com/ryanmy/archive/2005/05/25/421882.aspx]. Each file in that directory should contain the name of the application, a dash, and then an eight character hash of the location from which that application was run, and a <tt>.pf</tt> 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: <tt>MD5DEEP.EXE-4F89AB0C.pf</tt>. If an application is run from two different locations on the drive (i.e. the user runs <tt>C:\md5deep.exe</tt> and then <tt>C:\Apps\Hashing\md5deep.exe</tt>), there will be two different prefetch files in the Prefetch folder.
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== File format ==
 
== File format ==
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For more information about the file format see: [[Windows Prefetch File Format]]
 
For more information about the file format see: [[Windows Prefetch File Format]]
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== Metadata ==
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The Prefetch file contains various metadata.
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* The executable's name, up to 29 characters.
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* The run count, or number of times the application has been run.
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* Volume related information, like volume path and volume serial number.
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* The size of the Prefetch file (sometimes referred to as end of file (EOF)).
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* The files and directories that were used doing the application's start-up.
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=== Timestamps ===
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The Prefetch file contains 2 types of timestamps
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* The time when the application was last ran (executed). Version 26 of the Prefetch format maintains 7 previous last run times.
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* The volume creation time (part of the volume information) of the volume the Prefetch file was created on.
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 +
The file system creation time of the Prefetch file indicates the first time the application was executed. Both the file system modification time of the Prefetch file and the embedded last run time indicate the last time the application was executed.
  
 
== Prefetch hash ==
 
== Prefetch hash ==
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     return hash_value  
 
     return hash_value  
 
</pre>
 
</pre>
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== Timestamps ==
 
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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) [http://msdn2.microsoft.com/en-us/library/ms724284.aspx 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.
 
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Windows will store timestamps according to a Windows [http://msdn.microsoft.com/en-us/library/ms724290%28VS.85%29.aspx file time].
 
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==== Creation Time ====
 
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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. See section Volume for more information.
 
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==== Last Run Time ====
 
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A timestamp of when the application was last ran is embedded into the Prefetch file.
 
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The offset from the beginning of the file to the "Last Run Time" is located:
 
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* at offset 0x78 on Windows XP and Windows 2003.
 
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* at offset 0x80 on Windows Vista, Windows 7, and Windows 8 (up to 8 entries for Windows 8).
 
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== Other MetaData ==
 
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The Prefetch file contains various other metadata.
 
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* The executable's name, up to 29 characters.
 
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* The run count, or number of times the application has been run.
 
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* Volume related information, like volume path and volume serial number.
 
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* The size of the Prefetch file (sometimes referred to as end of file (EOF))
 
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* The files and directories that were used doing the application's start-up.
 
  
 
== Registry Keys ==
 
== Registry Keys ==

Latest revision as of 04:37, 2 November 2013

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Please help to improve this article by expanding it.
Further information might be found on the discussion page.

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. For SSD drives Prefetch is disabled by default [1].

Up to 128 Prefetch files are stored in the %SystemRoot%\Prefetch directory [2]. Each file in that directory should contain the name of the application, 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.

Contents

File format

Each Prefetch file has a 4-byte signature (at offset 4) "SCCA" (or in hexadecimal notation 0x53 0x43 0x43 0x41). The signature is assumed to be preceded by a 4-byte format version indicator:

For more information about the file format see: Windows Prefetch File Format

Metadata

The Prefetch file contains various metadata.

  • The executable's name, up to 29 characters.
  • The run count, or number of times the application has been run.
  • Volume related information, like volume path and volume serial number.
  • The size of the Prefetch file (sometimes referred to as end of file (EOF)).
  • The files and directories that were used doing the application's start-up.

Timestamps

The Prefetch file contains 2 types of timestamps

  • The time when the application was last ran (executed). Version 26 of the Prefetch format maintains 7 previous last run times.
  • The volume creation time (part of the volume information) of the volume the Prefetch file was created on.

The file system creation time of the Prefetch file indicates the first time the application was executed. Both the file system modification time of the Prefetch file and the embedded last run time indicate the last time the application was executed.

Prefetch hash

There are multiple known hashing functions to be used for prefetch file filename hashing, namely:

  • SCCA XP hash function; used on Windows XP and Windows 2003
  • SCCA Vista hash function; used on Windows Vista
  • SCCA 2008 hash function; used on Windows 2008, Windows 7, (possibly: Windows 2012) and Windows 8 (including 8.1)

SCCA XP hash function

A Python implementation of the SCCA XP hash function: 

def ssca_xp_hash_function(filename):
    hash_value = 0
    for character in filename:
        hash_value = ((hash_value * 37) + ord(character)) % 0x100000000
        hash_value = (hash_value * 314159269) % 0x100000000
        if hash_value > 0x80000000:
            hash_value = 0x100000000 - hash_value

    return (abs(hash_value) % 1000000007) % 0x100000000

SCCA Vista hash function

A Python implementation of the SCCA Vista hash function: 

def ssca_vista_hash_function(filename):
    hash_value = 314159
    for character in filename:
        hash_value = ((hash_value * 37) + ord(character)) % 0x100000000
    return hash_value

SCCA 2008 hash function

A Python implementation of the SCCA 2008 hash function: 

def ssca_2008_hash_function(filename):
    hash_value = 314159
    filename_index = 0
    filename_length = len(filename)
    while filename_index + 8 < filename_length:
        character_value = ord(filename[filename_index + 1]) * 37
        character_value += ord(filename[filename_index + 2])
        character_value *= 37
        character_value += ord(filename[filename_index + 3])
        character_value *= 37
        character_value += ord(filename[filename_index + 4])
        character_value *= 37
        character_value += ord(filename[filename_index + 5])
        character_value *= 37
        character_value += ord(filename[filename_index + 6])
        character_value *= 37
        character_value += ord(filename[filename_index]) * 442596621
        character_value += ord(filename[filename_index + 7])
        hash_value = ((character_value - (hash_value * 803794207)) % 0x100000000)
        filename_index += 8

    while filename_index < filename_length: 
       hash_value = (((37 * hash_value) + ord(filename[filename_index])) % 0x100000000)
       filename_index += 1

    return hash_value

Registry Keys 

Key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters

The EnablePrefetcher Registry value can be used to disable prefetch.

See Also

External Links

Tools

Commercial

Free - Non Open Source

Open Source

  • prefetch-tool, Script to extract information from windows prefetch folder
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