MS-DOS installation compression

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A pair of compression formats by Microsoft that appear on MS-DOS and early Windows (3.x) installation disks; the formats have no generally agreed-upon name. Compressed files may be generated by the COMPRESS.EXE program (appearing in Microsoft's Setup Toolkit for Visual C++), and uncompressed with the EXPAND.EXE program distributed with Windows and MS-DOS. Compressed files have the last character of the file extension replaced with an underscore, or sometimes a dollar sign.
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'''MS-DOS installation compression''' refers to a family of compressed file formats formats by Microsoft that appear on MS-DOS and early Windows (3.x) installation disks. The formats have no generally agreed-upon name. This article is primarily about the format whose files start with "SZDD". For the other formats, see [[Microsoft KWAJ installation compression]] and [[Microsoft SZ installation compression]].
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Compressed files may be generated by the COMPRESS.EXE program (appearing in Microsoft's Setup Toolkit for Visual C++), and decompressed with the EXPAND.EXE program distributed with Windows and MS-DOS. Compressed files have the last character of the file extension replaced with an underscore, or sometimes a dollar sign.
 
<!-- There are reports of the Google Chrome installer [http://festiva-yourchoice.blogspot.com/2010/01/expandexe-preventing-google-chrome-from.html needing the EXPAND.EXE program]. -->
 
<!-- There are reports of the Google Chrome installer [http://festiva-yourchoice.blogspot.com/2010/01/expandexe-preventing-google-chrome-from.html needing the EXPAND.EXE program]. -->
  
 
== See also ==
 
== See also ==
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* [[Microsoft KWAJ installation compression]]
 
* [[Microsoft SZ installation compression]]
 
* [[Microsoft SZ installation compression]]
  
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== Format details ==
 
== Format details ==
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There are two major formats, which can be referred to by their file signatures: "SZDD", and "KWAJ". SZDD is more common. Another variant, sometimes known as "SZ", or the "QBasic variant", is covered in a separate article: [[Microsoft SZ installation compression]].
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SZDD's compression format similar, but not identical, to [[LZSS (Haruhiko Okumura)|Okumura's LZSS]]. There is a byte at offset 8 that specifies the exact type of compression. Most sources say that the only known value for this byte is "A" (0x41), but some report[https://www.betaarchive.com/forum/viewtopic.php?t=26161] that there is also a type "B", used by Windows 3.1 builds 026 and 034e.
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=== SZDD ===
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SZDD's compression format similar to [[LZSS (Haruhiko Okumura)|Okumura's LZSS]]. There is a byte at offset 8 that specifies the exact type of compression. Most sources say that the only known value for this byte is "A" (0x41), but some report[https://www.betaarchive.com/forum/viewtopic.php?t=26161] that there is also a type "B", used by Windows 3.1 builds 026 and 034e.
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=== KWAJ ===
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KWAJ format is somewhat more sophisticated than SZDD. There is a 16-bit field at offset 8 that specifies the compression or encoding method.
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{| class="wikitable"
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!Compression code !! Description
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|-
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|0 || Uncompressed
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|-
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|1 || [[Exclusive Or|XOR]]
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|-
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|2 || [[LZSS]]
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|-
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|3 || [[LZ77 with Huffman coding|LZ77 + Huffman]]
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|-
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|4 || [[MSZIP]] (based on [[DEFLATE]])
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|}
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== Identification ==
 
== Identification ==
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File begin with bytes {{magic|53 5A 44 44 88 F0 27 33}} ("SZDD" variant), or {{magic|4B 57 41 4A 88 F0 27 D1}} ("KWAJ" variant).
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An SZDD file begins with bytes {{magic|53 5A 44 44 88 F0 27 33}}.
  
 
== Software ==
 
== Software ==

Revision as of 16:07, 7 January 2023

File Format
Name MS-DOS installation compression
Ontology
Extension(s) .??_, .??$
PRONOM fmt/462

MS-DOS installation compression refers to a family of compressed file formats formats by Microsoft that appear on MS-DOS and early Windows (3.x) installation disks. The formats have no generally agreed-upon name. This article is primarily about the format whose files start with "SZDD". For the other formats, see Microsoft KWAJ installation compression and Microsoft SZ installation compression.

Compressed files may be generated by the COMPRESS.EXE program (appearing in Microsoft's Setup Toolkit for Visual C++), and decompressed with the EXPAND.EXE program distributed with Windows and MS-DOS. Compressed files have the last character of the file extension replaced with an underscore, or sometimes a dollar sign.

Contents

See also

This is not the only compressed file format for which the last character of the file extension might be replaced with an underscore. Others include:

Format details

SZDD's compression format similar, but not identical, to Okumura's LZSS. There is a byte at offset 8 that specifies the exact type of compression. Most sources say that the only known value for this byte is "A" (0x41), but some report[1] that there is also a type "B", used by Windows 3.1 builds 026 and 034e.

Identification

An SZDD file begins with bytes 53 5A 44 44 88 F0 27 33.

Software

Sample files

Links

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