Compressed Macintosh resource data, as stored in resources with the "compressed" attribute.
Resource decompression is not documented by Apple. It is mostly used internally in System 7, some of Apple's own applications (such as ResEdit), and also by some third-party applications. Later versions of Classic Mac OS make less use of resource compression, but still support it fully for backwards compatibility. Carbon in Mac OS X no longer supports resource compression in any way.
The data of all compressed resources starts with a common header,
followed by the compressed data.
The data is decompressed using code in a 'dcmp' resource.
Some decompressors used by Apple are included in the System file,
but applications can also include custom decompressors.
The header of the compressed data indicates the ID of the 'dcmp' resource used to decompress the data,
along with some parameters for the decompressor.
This page hosts a formal specification of Compressed Macintosh resource using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
digraph {
rankdir=LR;
node [shape=plaintext];
subgraph cluster__compressed_resource {
label="CompressedResource";
graph[style=dotted];
compressed_resource__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="header_pos">0</TD><TD PORT="header_size">...</TD><TD>Header</TD><TD PORT="header_type">header</TD></TR>
<TR><TD PORT="compressed_data_pos">...</TD><TD PORT="compressed_data_size">⇲</TD><TD></TD><TD PORT="compressed_data_type">compressed_data</TD></TR>
</TABLE>>];
subgraph cluster__header {
label="CompressedResource::Header";
graph[style=dotted];
header__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="common_part_pos">0</TD><TD PORT="common_part_size">12</TD><TD>CommonPart</TD><TD PORT="common_part_type">common_part</TD></TR>
<TR><TD PORT="type_specific_part_raw_with_io_pos">12</TD><TD PORT="type_specific_part_raw_with_io_size">common_part.len_header - 12</TD><TD>BytesWithIo</TD><TD PORT="type_specific_part_raw_with_io_type">type_specific_part_raw_with_io</TD></TR>
</TABLE>>];
header__inst__type_specific_part [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="type_specific_part_pos">0</TD><TD PORT="type_specific_part_size">...</TD><TD>switch (common_part.header_type)</TD><TD PORT="type_specific_part_type">type_specific_part</TD></TR>
</TABLE>>];
header__inst__type_specific_part_raw [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>type_specific_part_raw</TD><TD>type_specific_part_raw_with_io.data</TD></TR>
</TABLE>>];
header__inst__type_specific_part_type_specific_part_switch [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#F0F2E4">case</TD><TD BGCOLOR="#F0F2E4">type</TD></TR>
<TR><TD>8</TD><TD PORT="case0">TypeSpecificPartType8</TD></TR>
<TR><TD>9</TD><TD PORT="case1">TypeSpecificPartType9</TD></TR>
</TABLE>>];
subgraph cluster__common_part {
label="CompressedResource::Header::CommonPart";
graph[style=dotted];
common_part__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="magic_pos">0</TD><TD PORT="magic_size">4</TD><TD>A8 9F 65 72</TD><TD PORT="magic_type">magic</TD></TR>
<TR><TD PORT="len_header_pos">4</TD><TD PORT="len_header_size">2</TD><TD>u2be</TD><TD PORT="len_header_type">len_header</TD></TR>
<TR><TD COLSPAN="4" PORT="len_header__valid">must be equal to 18</TD></TR>
<TR><TD PORT="header_type_pos">6</TD><TD PORT="header_type_size">1</TD><TD>u1</TD><TD PORT="header_type_type">header_type</TD></TR>
<TR><TD PORT="unknown_pos">7</TD><TD PORT="unknown_size">1</TD><TD>u1</TD><TD PORT="unknown_type">unknown</TD></TR>
<TR><TD COLSPAN="4" PORT="unknown__valid">must be equal to 1</TD></TR>
<TR><TD PORT="len_decompressed_pos">8</TD><TD PORT="len_decompressed_size">4</TD><TD>u4be</TD><TD PORT="len_decompressed_type">len_decompressed</TD></TR>
</TABLE>>];
}
subgraph cluster__type_specific_part_type_8 {
label="CompressedResource::Header::TypeSpecificPartType8";
graph[style=dotted];
type_specific_part_type_8__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="working_buffer_fractional_size_pos">0</TD><TD PORT="working_buffer_fractional_size_size">1</TD><TD>u1</TD><TD PORT="working_buffer_fractional_size_type">working_buffer_fractional_size</TD></TR>
<TR><TD PORT="expansion_buffer_size_pos">1</TD><TD PORT="expansion_buffer_size_size">1</TD><TD>u1</TD><TD PORT="expansion_buffer_size_type">expansion_buffer_size</TD></TR>
<TR><TD PORT="decompressor_id_pos">2</TD><TD PORT="decompressor_id_size">2</TD><TD>s2be</TD><TD PORT="decompressor_id_type">decompressor_id</TD></TR>
<TR><TD PORT="reserved_pos">4</TD><TD PORT="reserved_size">2</TD><TD>u2be</TD><TD PORT="reserved_type">reserved</TD></TR>
<TR><TD COLSPAN="4" PORT="reserved__valid">must be equal to 0</TD></TR>
</TABLE>>];
}
subgraph cluster__type_specific_part_type_9 {
label="CompressedResource::Header::TypeSpecificPartType9";
graph[style=dotted];
type_specific_part_type_9__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="decompressor_id_pos">0</TD><TD PORT="decompressor_id_size">2</TD><TD>s2be</TD><TD PORT="decompressor_id_type">decompressor_id</TD></TR>
<TR><TD PORT="decompressor_specific_parameters_with_io_pos">2</TD><TD PORT="decompressor_specific_parameters_with_io_size">4</TD><TD>BytesWithIo</TD><TD PORT="decompressor_specific_parameters_with_io_type">decompressor_specific_parameters_with_io</TD></TR>
</TABLE>>];
type_specific_part_type_9__inst__decompressor_specific_parameters [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>decompressor_specific_parameters</TD><TD>decompressor_specific_parameters_with_io.data</TD></TR>
</TABLE>>];
}
}
}
compressed_resource__seq:header_type -> header__seq [style=bold];
header__seq:common_part_type -> common_part__seq [style=bold];
common_part__seq:len_header_type -> header__seq:type_specific_part_raw_with_io_size [color="#404040"];
header__seq:common_part_size -> header__seq:type_specific_part_raw_with_io_size [color="#404040"];
header__seq:type_specific_part_raw_with_io_type -> bytes_with_io__seq [style=bold];
header__inst__type_specific_part:type_specific_part_type -> header__inst__type_specific_part_type_specific_part_switch [style=bold];
header__inst__type_specific_part_type_specific_part_switch:case0 -> type_specific_part_type_8__seq [style=bold];
header__inst__type_specific_part_type_specific_part_switch:case1 -> type_specific_part_type_9__seq [style=bold];
common_part__seq:header_type_type -> header__inst__type_specific_part:type_specific_part_type [color="#404040"];
bytes_with_io__seq:data_type -> header__inst__type_specific_part_raw [color="#404040"];
type_specific_part_type_9__seq:decompressor_specific_parameters_with_io_type -> bytes_with_io__seq [style=bold];
bytes_with_io__seq:data_type -> type_specific_part_type_9__inst__decompressor_specific_parameters [color="#404040"];
}