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<html><head><title>Microsoft's HTML Help (.chm) format</title>
<meta name="author" content="Matthew T. Russotto"></head>
<body bgcolor=black text=lightgreen>
<h1> Microsoft's HTML Help (.chm) format</h1>
<h2 id="preface"> Preface</h2>
<p>
This is documentation on the .chm format used by Microsoft HTML Help.
This format has been reverse engineered in the past, but as far as I
know this is the first freely available documentation on it. One
Usenet message indicates that these .chm files are actually IStorage
files documented in the Microsoft Platform SDK. However, I have not
been able to locate such documentation.
</p>
<h3>Note</h3>
<p>
The word "section" is badly overloaded in this document. Sorry about
that.</p>
<p>
All numbers are in hexadecimal unless otherwise indicated in the
text. Except in tabular listings, this will be indicated by
$ or 0x as appropriate. All values within the file are Intel byte
order (little endian) unless indicated otherwise.
</p>
<h2 id="overview"> The overall format of a .chm file</h2>
<p>
The .chm file begins with a short ($38 byte) initial header. This is
followed by the header section table and the offset to the
content. Collectively, this is the "header".
</p>
<p>
The header is followed by the header sections. There are two header
sections. One header section is the file directory, the other
contains the file length and some unknown data. Immediately following
the header sections is the content.
</p>
<h2 id="header"> The Header</h2>
<p>
The header starts with the initial header, which has the following format
</p>
<pre>0000: char[4] 'ITSF'
0004: DWORD 3 (Version number)
0008: DWORD Total header length, including header section table and
following data.
000C: DWORD 1 (unknown)
0010: DWORD a timestamp.
Considered as a big-endian DWORD, it appears to contain
seconds (MSB) and fractional seconds (second byte).
The third and fourth bytes may contain even more fractional
bits. The 4 least significant bits in the last byte are
constant.
0014: DWORD Windows Language ID. The two I've seen
$0409 = LANG_ENGLISH/SUBLANG_ENGLISH_US
$0407 = LANG_GERMAN/SUBLANG_GERMAN
0018: GUID {7C01FD10-7BAA-11D0-9E0C-00A0-C922-E6EC}
0028: GUID {7C01FD11-7BAA-11D0-9E0C-00A0-C922-E6EC}
</pre>
<p>Note: a GUID is $10 bytes, arranged as 1 DWORD, 2 WORDs, and 8 BYTEs.</p>
<p>
It is followed by the header section table, which is 2 entries, where
each entry is $10 bytes long and has this format:
</p>
<pre>0000: QWORD Offset of section from beginning of file
0008: QWORD Length of section
</pre>
<p>
Following the header section table is 8 bytes of additional header
data. In Version 2 files, this data is not there and the content
section starts immediately after the directory.
</p>
<pre>0000: QWORD Offset within file of content section 0
</pre>
<h2 id="headersections">The Header Sections</h2>
<h3>Header Section 0</h3>
<p>
This section contains the total size of the file, and not much else
</p>
<pre>0000: DWORD $01FE (unknown)
0004: DWORD 0 (unknown)
0008: QWORD File Size
0010: DWORD 0 (unknown)
0014: DWORD 0 (unknown)
</pre>
<h3>Header Section 1: The Directory Listing</h3>
<p>
The central part of the .chm file: A directory of the files and information it
contains.
</p>
<h4>Directory header</h4>
<p>
The directory starts with a header; its format is as follows:
</p>
<pre>0000: char[4] 'ITSP'
0004: DWORD Version number 1
0008: DWORD Length of the directory header
000C: DWORD $0a (unknown)
0010: DWORD $1000 Directory chunk size
0014: DWORD "Density" of quickref section, usually 2.
0018: DWORD Depth of the index tree
1 there is no index, 2 if there is one level of PMGI
chunks.
001C: DWORD Chunk number of root index chunk, -1 if there is none
(though at least one file has 0 despite there being no
index chunk, probably a bug.)
0020: DWORD Chunk number of first PMGL (listing) chunk
0024: DWORD Chunk number of last PMGL (listing) chunk
0028: DWORD -1 (unknown)
002C: DWORD Number of directory chunks (total)
0030: DWORD Windows language ID
0034: GUID {5D02926A-212E-11D0-9DF9-00A0C922E6EC}
0044: DWORD $54 (This is the length again)
0048: DWORD -1 (unknown)
004C: DWORD -1 (unknown)
0050: DWORD -1 (unknown)
</pre>
<h4>The Listing Chunks</h4>
<p>
The header is directly followed by the directory chunks. There are two
types of directory chunks -- index chunks, and listing chunks. The
index chunk will be omitted if there is only one listing chunk. A
listing chunk has the following format:
</p>
<pre>0000: char[4] 'PMGL'
0004: DWORD Length of free space and/or quickref area at end of
directory chunk
0008: DWORD Always 0.
000C: DWORD Chunk number of previous listing chunk when reading
directory in sequence (-1 if this is the first listing chunk)
0010: DWORD Chunk number of next listing chunk when reading
directory in sequence (-1 if this is the last listing chunk)
0014: Directory listing entries (to quickref area) Sorted by
filename; the sort is case-insensitive.
</pre>
<p>
The quickref area is written backwards from the end of the chunk. One
quickref entry exists for every n entries in the file, where n is
calculated as 1 + (1 << quickref density). So for density = 2, n = 5.
</p>
<pre>Chunklen-0002: WORD Number of entries in the chunk
Chunklen-0004: WORD Offset of entry n from entry 0
Chunklen-0008: WORD Offset of entry 2n from entry 0
Chunklen-000C: WORD Offset of entry 3n from entry 0
...
</pre>
<p>
The format of a directory listing entry is as follows
</p>
<pre> ENCINT: length of name
BYTEs: name (UTF-8 encoded)
ENCINT: content section
ENCINT: offset
ENCINT: length
</pre>
<p>
The offset is from the beginning of the content section the file is
in, after the section has been decompressed (if appropriate). The
length also refers to length of the file in the section after decompression.
</p>
<p>
There are two kinds of file represented in the directory: user data and
format related files. The files which are format-related have names which begin
with '::', the user data files have names which begin with "/".
</p>
<h4>The Index Chunk</h4>
<p>
An index chunk has the following format
</p>
<pre>0000: char[4] 'PMGI'
0004: DWORD Length of quickref/free area at end of directory chunk
0008: Directory index entries (to quickref/free area)
</pre>
<p>
The quickref area in an PMGI is the same as in an PMGL
</p>
<p>
The format of a directory index entry is as follows
</p>
<pre> ENCINT: length of name
BYTEs: name (UTF-8 encoded)
ENCINT: directory listing chunk which starts with name
</pre>
<p>
When higher-level indexes exist (when the depth of the index tree is 3 or
higher), presumably the upper-level indexes will contain the numbers
of lower-level index chunks rather than listing chunks
</p>
<h4>Encoded Integers</h4>
<p>
An ENCINT is a variable-length integer. The high bit of each byte
indicates "continued to the next byte". Bytes are stored most
significant to least significant. So, for example, $EA $15 is
(((0xEA&0x7F)<<7)|0x15) = 0x3515.
</p>
<h2 id="content">The Content</h2>
<p>
In Version 3, the content typically immediately follows the header
sections, and is at the location indicated by the DWORD following the
header section table. In Version 2, the content immediately follows
the header.
All content section 0 locations in the directory are relative to that
point. The other content sections are stored WITHIN content section
0.
</p>
<h3>The Namelist file</h3>
<p>
There exists in content section 0 and in the directory a file called
"::DataSpace/NameList". This file contains the names of all the
content sections. The format is as follows:
</p>
<pre>0000: WORD Length of file, in words
0002: WORD Number of entries in file
Each entry:
0000: WORD Length of name in words, excluding terminating null
0002: WORD Double-byte characters
xxxx: WORD 0
</pre>
<p>
Yes, the names have a length word AND are null terminated; sort of a
belt-and-suspenders approach. The coding system is likely UTF-16 (little endian).
</p>
<p>
The section names seen so far are
</p>
<ul>
<li>Uncompressed</li>
<li>MSCompressed</li>
</ul>
<p>
"Uncompressed" is self-explanatory. The section "MSCompressed" is
compressed with Microsoft's LZX algorithm.
</p>
<h3>The Section Data</h3>
<p>
For each section other than 0, there exists a file called
'::DataSpace/Storage/<Section Name>/Content'. This file contains the
compressed data for the section. So, conceptually,
getting a file from a nonzero section is a multi-step process. First
you must get the content file from section 0. Then you decompress (if
appropriate) the section. Then you get the desired file from your decompressed section.
</p>
<h3>Other section format-related files</h3>
<p>
There are several other files associated with the sections
</p>
<ul>
<li>
::DataSpace/Storage/<SectionName>/ControlData
<p>
This file contains $20 bytes of information on the compression.
The information is partially known:
</p>
<pre>0000: DWORD Number of DWORDs following 'LZXC', must be 6 if version is 2
0004: ASCII 'LZXC' Compression type identifier
0008: DWORD Version (Must be <=2)
000C: DWORD The LZX reset interval
0010: DWORD The window size
0014: DWORD The cache size
0018: DWORD 0 (unknown)
</pre>
<p>
Reset interval, window size, and cache size are in bytes if version is
1, $8000-byte blocks if version is 2.
</p>
</li>
<li>::DataSpace/Storage/<SectionName>/SpanInfo
<p>
This file contains a quadword containing the uncompressed length of
the section.
</p>
</li>
<li>::DataSpace/Storage/<SectionName>/Transform/List
<p>
It appears this file was intended to contain a list of GUIDs belonging
to methods of decompressing (or otherwise transforming) the section.
However, it actually contains only half of the string representation
of a GUID, apparently because it was sized for characters but contains
wide characters.
</p>
</li>
</ul>
<h2 id="compression">Appendix: The Compression</h2>
<p>
The compressed sections are compressed using LZX, a compression
method Microsoft also uses for its cabinet files. To ensure this, check the
second DWORD of compression info in the ControlData file for the
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