Difference between pages "Windows Prefetch File Format" and "SuperFetch"

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A Windows Prefetch file consists of one file header and multiple file sections with different content. Not all content has an obvious forensic value.
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SuperFetch is a performance enhancement introduced in [[Microsoft]] [[Windows|Windows Vista]] to reduce the time necessary to launch applications. An expanded version of the [[Prefetch]] files found in Windows XP, they record usage scenarios and load resources into memory before they are needed. Those resources can be loaded into physical memory and extra memory provided by [[ReadyBoost]].
  
As far as have been possible to ascertain, there is no public description of the format. The description below has been synthesised from examination
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== Configuration ==
of multiple prefetch files.
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== Characteristics ==
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Because SuperFetch appears to leave a system with no available memory, some users turn it off to create the appearance of having more free memory. The feature can be configured by changing the <tt>HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters\EnableSuperfetch</tt> [[Registry]] key [http://www.codinghorror.com/blog/archives/000688.html]. A value of zero disables SuperFetch, one enables it for booting only, two for applications, and three for both applications and boot. This setting can also be changed using the Services console, <tt>services.msc</tt> [http://tiredblogger.wordpress.com/2007/03/27/superfetch-not-so-super-for-gaming/].
Integer values are stored in little-endian.
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Strings are stored as [http://en.wikipedia.org/wiki/UTF-16/UCS-2 UTF-16 little-endian] without a byte-order-mark (BOM).
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== File Formats ==
  
Timestamps are stored as [http://msdn2.microsoft.com/en-us/library/ms724284.aspx Windows FILETIME] in UTC.
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Data for SuperFetch is gathered by the <tt>%SystemRoot%\System32\Sysmain.dll</tt>, part of the Service Host process, <tt>%SystemRoot%\System32\Svchost.exe</tt>, and stored in a series of files in the <tt>%SystemRoot%\Prefetch</tt> directory [http://www.microsoft.com/technet/technetmag/issues/2007/03/VistaKernel/]. These files appear to start with the prefix <tt>Ag</tt> and have a <tt>.db</tt> extension. The format of these files is not fully known, there is available unofficial partial specification [http://blog.rewolf.pl/blog/?p=214] and open source (GPL) dumper for .db files [http://code.google.com/p/rewolf-superfetch-dumper/]. Some information can be gleaned from these files by searching for [[Unicode]] [[strings]] in them.
  
== File header ==
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The SuperFetch feature is seeded with some basic usage patterns when the operating system is installed [http://channel9.msdn.com/showpost.aspx?postid=242429].
 
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{| class="wikitable"
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|-
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! Field
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! Offset
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! Length
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! Type
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! Notes
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|-
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| H1
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| 0x0000
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| 4
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| DWORD
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| Format version (see format version section below)
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|-
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| H2
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| 0x0004
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| 4
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| DWORD
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| Signature 'SCCA' (or in hexadecimal representation 0x53 0x43 0x43 0x4)
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|-
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| H3
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| 0x0008
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| 4
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| DWORD?
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| Unknown - Values observed: 0x0F - Windows XP, 0x11 - Windows 7, Windows 8.1
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|-
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| H4
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| 0x000C
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| 4
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| DWORD
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| Prefetch file size (or length) (sometimes referred to as End of File (EOF)).
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|-
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| H5
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|0x0010
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| 60
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| USTR
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| The name of the (original) executable as a Unicode (UTF-16 litte-endian string), up to 29 characters and terminated by an end-of-string character (U+0000). This name should correspond with the one in the prefetch file filename.
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|-
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| H6
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|0x004C
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|4
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|DWORD
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|The prefetch hash. This hash value should correspond with the one in the prefetch file filename.
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|-
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| H7
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|0x0050
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|4
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|?
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| Unknown (flags)? Values observed: 0 for almost all prefetch files (XP); 1 for NTOSBOOT-B00DFAAD.pf (XP)
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|-
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|}
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It's worth noting that the name of a carved prefetch file can be restored using the information in field H5 and H6, and its size can be determined by field H4.
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=== Format version ===
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{| class="wikitable"
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|-
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! Value
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! Windows version
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|-
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| 17 (0x11)
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| Windows XP, Windows 2003
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|-
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| 23 (0x17)
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| Windows Vista, Windows 7
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|-
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| 26 (0x1a)
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| Windows 8.1 (note this could be Windows 8 as well but has not been confirmed)
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|-
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|}
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=== File information ===
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The format of the file information part of the file header is version dependent.
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==== File information - version 17 ====
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It is sometime considered part of the file header. Below the structure for format version 17.
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{| class="wikitable"
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|-
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! Field
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! Offset
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! Length
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! Type
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! Notes
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|-
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| H8
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| 0x0054
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| 4
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| DWORD
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| The offset to section A. The offset is relative from the start of the file.
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|-
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| H9
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| 0x0058
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| 4
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| DWORD
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| The number of entries in section A.
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|-
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| H10
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| 0x005C
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| 4
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| DWORD
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| The offset to section B. The offset is relative from the start of the file.
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|-
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| H11
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| 0x0060
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| 4
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| DWORD
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| The number of entries in section B.
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|-
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| H12
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| 0x0064
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| 4
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| DWORD
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| The offset to section C. The offset is relative from the start of the file.
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|-
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| H13
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| 0x0068
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| 4
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| DWORD
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| Length of section C.
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|-
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| H14
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| 0x006C
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| 4
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| DWORD
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| Offset to section D. The offset is relative from the start of the file.
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|-
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| H15
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| 0x0070
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| 4
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| DWORD
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| The number of entries in section D.
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|-
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| H16
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| 0x0074
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| 4
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| DWORD
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| Length of section D.
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|-
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| H17
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| 0x0078
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| 8
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| FILETIME
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| Latest execution time (or run time) of executable (FILETIME)
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|-
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| H18
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| 0x0080
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| 16
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| ?
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| Unknown ? Possibly structured as 4 DWORD. Observed values: /0x00000000 0x00000000 0x00000000 0x00000000/, /0x47868c00 0x00000000 0x47860c00 0x00000000/ (don't exclude the possibility here that this is remnant data)
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|-
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| H19
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| 0x0090
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| 4
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| DWORD
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| Execution counter (or run count)
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|-
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| H20
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| 0x0094
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| 4
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| DWORD?
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| Unknown ? Observed values: 1, 2, 3, 4, 5, 6 (XP)
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|-
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|}
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==== File information - version 23 ====
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==== File information - version 26 ====
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== Section A ==
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This section contains an array with 20 byte (version 17) or 32 byte (version 23 and 26) entry records.
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The actual format and usage of these entry records is currently not known.
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== Section B ==
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This section contains an array with 12 byte (version 17, 23 and 26) entry records.
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The actual format and usage of these entry records is currently not known.
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== Section C (filename strings) ==
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This section contains filenames strings, it consists of an array of UTF-16 little-endian formatted strings with end-of-string characters (U+0000).
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At the end of the section there seems to be alignment padding that can contain remnant values.
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== Section D - Volume information (block) ==
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Section D contains one or more subsections. The number is (most likely) determined by the DWORD at file offset 0x0070. Each subsection refers to directories on an identified volume.
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In this section, all offsets are assumed to be counted from the start of the D section.
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If all the executables and libraries referenced in the C section are from one single disk volume, there will be only one section in the D section. If multiple volumes are referenced by section C, section D will contain multiple sections.  (A simple way to force this situation is to copy, say, NOTEPAD.EXE to a USB drive, and start it from that volume. The corresponding prefetch file will have one D header referring to, e.g. \DEVICE\HARDDISK1\DP(1)0-0+4 (the USB drive), and one to, e.g. \DEVICE\HARDDISKVOLUME1\ (where the .DLLs and other support files were found).
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=== Volume information - version 17 ===
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The following values are version dependent. Below the structure for format version 17.
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{| class="wikitable"
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|-
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! Field
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! Offset
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! Length
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! Type
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! Notes
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|-
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| DH1
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| +0x0000
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| 4
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| DWORD
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| Offset to volume device path (Unicode, terminated by U+0000)
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|-
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| DH2
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| +0x0004
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| 4
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| DWORD
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| Length of volume device path (nr of characters, including terminating U+0000)
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|-
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| DH3
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| +0x0008
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| 8
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| FILETIME
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| Volume creation time.
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|-
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| DH4
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| +0x0010
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| 4
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| DWORD
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| Volume serial number of volume indicated by volume string
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|-
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| DH5
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| +0x0014
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| 4
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| DWORD
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| ? Offset to section DHS1
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|-
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| DH6
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| +0x0018
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| 4
+
| DWORD
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| ? Length of section DHS1 (in bytes)
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|-
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| DH7
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| +0x001C
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| 4
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| DWORD
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| ? Offset to section DHS2
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|-
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| DH8
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| +0x0020
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| 4
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| DWORD
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| ? Nr of strings in section DHS2
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|-
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| ?
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| +0x0024
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| ?
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| ?
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| ? additional 4 bytes
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|}
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=== Volume information - version 23 and 26 ===
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The following values are version dependent. Below the structure for format version 23 and 26.
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<b>TODO</b>
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== See Also ==
 
== See Also ==
 
* [[Prefetch]]
 
* [[Prefetch]]
 +
* [[Windows]]
  
 
== External Links ==
 
== External Links ==
* [https://googledrive.com/host/0B3fBvzttpiiSbl9XZGZzQ05hZkU/Windows%20Prefetch%20File%20(PF)%20format.pdf Windows Prefetch File (PF) format], by the [[libssca|libssca project]]
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* [http://en.wikipedia.org/wiki/Windows_Vista_I/O_technologies#SuperFetch Wikipedia: Windows Vista I/O technologies - SuperFetch]
 
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* [http://channel9.msdn.com/showpost.aspx?postid=242429 Channel 9 Interview with Michael Fortin of Microsoft on SuperFetch]
[[Category:File Formats]]
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* [http://www.informationweek.com/news/showArticle.jhtml?articleID=196902178 Microsoft Predicts The Future With Vista's SuperFetch] from Information Week
 +
* [http://jessekornblum.com/presentations/dodcc08-2.pdf DC3 Presentation: My You Look SuperFetching]
 +
* [http://code.google.com/p/rewolf-superfetch-dumper/ Open source Ag*.db files dumper]
 +
* [http://blog.rewolf.pl/blog/?p=214 Windows SuperFetch file format – partial specification]

Revision as of 13:25, 14 April 2014

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

SuperFetch is a performance enhancement introduced in Microsoft Windows Vista to reduce the time necessary to launch applications. An expanded version of the Prefetch files found in Windows XP, they record usage scenarios and load resources into memory before they are needed. Those resources can be loaded into physical memory and extra memory provided by ReadyBoost.

Configuration

Because SuperFetch appears to leave a system with no available memory, some users turn it off to create the appearance of having more free memory. The feature can be configured by changing the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters\EnableSuperfetch Registry key [1]. A value of zero disables SuperFetch, one enables it for booting only, two for applications, and three for both applications and boot. This setting can also be changed using the Services console, services.msc [2].

File Formats

Data for SuperFetch is gathered by the %SystemRoot%\System32\Sysmain.dll, part of the Service Host process, %SystemRoot%\System32\Svchost.exe, and stored in a series of files in the %SystemRoot%\Prefetch directory [3]. These files appear to start with the prefix Ag and have a .db extension. The format of these files is not fully known, there is available unofficial partial specification [4] and open source (GPL) dumper for .db files [5]. Some information can be gleaned from these files by searching for Unicode strings in them.

The SuperFetch feature is seeded with some basic usage patterns when the operating system is installed [6].

See Also

External Links