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Macintosh Resource File Format
This page describes the format of resource files used by the Mac OS Resource Manager. The Resource Manager was originally designed and implemented by Bruce Horn for use in the original Macintosh Operating System. It remains today as a feature unique to the Mac OS. Other operating systems have the concept of resources, but the Mac OS implementation remains unique.
A Mac OS resource file (or the resource fork of any file on an HFS or HFS+ file system) starts with four 32-bit integers pointing to the location of and indicating the length of the resource map section and the resource data section.
| Type | Description | Notes |
|---|---|---|
int32be |
resourceDataOffset |
offset from start of file to start of resource data section |
int32be |
resourceMapOffset |
offset from start of file to start of resource map section |
int32be |
resourceDataSize |
length of resource data section |
int32be |
resourceMapSize |
length of resource map section |
The resource data section usually starts at offset 0x0100 and consists of
an array of resource data records, each of which contains the length and data
of an individual resource. No word alignment is used.
| Type | Description | Notes |
|---|---|---|
data[] |
resourceData |
an array of resource data records |
int32be |
size |
length of data for this resource |
byte[] |
data |
data for this resource |
The resource map section follows immediately after the resource data section and begins with a header containing a copy of the information found at the beginning of the file.
| Type | Description | Notes |
|---|---|---|
int32be |
resourceDataOffset |
offset from start of file to start of resource data section |
int32be |
resourceMapOffset |
offset from start of file to start of resource map section |
int32be |
resourceDataSize |
length of resource data section |
int32be |
resourceMapSize |
length of resource map section |
int32be |
nextResourceMap |
used by the Resource Manager for internal bookkeeping; should be zero |
int16be |
fileRef |
used by the Resource Manager for internal bookkeeping; should be zero |
int16be |
attributes |
Bit 7 (0x0080) indicates that the resource map is read-only. Bit 6 (0x0040; needs compression) and bit 5 (0x0020; changed) are used by the Resource Manager for internal bookkeeping and should be zero. Other bits are reserved and should be zero. |
int16be |
typeListOffset |
offset from start of resource map to start of type list |
int16be |
nameListOffset |
offset from start of resource map to start of name list |
The use of 16-bit signed integers imposes a 32K limitation on the size of the resource map. This limitation is reached at 2,727 resources or fewer if the resources are of different types or have names.
The type list follows the header and contains an array of resource type records.
| Type | Description | Notes |
|---|---|---|
int16be |
typeCount |
The number of types, minus one. If there are no resources, this is 0xFFFF. If there is one resource type, this is 0x0000. If there are two resource types, this is 0x0001. And so on. |
type[] |
types |
an array of type records |
int32be |
type |
the resource type; a four-character constant (fcc) |
int16be |
resourceCount |
The number of resources of this type, minus one. If there is one resource of this type, this is 0x0000. If there are two resources of this type, this is 0x0001. And so on. |
int16be |
resourceListOffset |
offset from start of type list to start of resource list |
For each resource type in the type list, a resource list follows that consists of an array of resource records.
| Type | Description | Notes |
|---|---|---|
resource[] |
resourceList |
an array of resource records |
int16be |
id |
the resource id |
int16be |
nameOffset |
offset from start of name list to resource name; if the resource has no name, this is 0xFFFF
|
int8be |
attributes |
Bit 6 (0x40; system) indicates that this resource should be loaded in the system heap instead of the application heap. Bit 5 (0x20; purgeable) indicates that this resource may be paged out of memory. Bit 4 (0x10; locked) indicates that this resource may not be moved in memory. Bit 3 (0x08; protected) indicates that this resource is read-only. Bit 2 (0x04; preload) indicates that this resource should be loaded as soon as the file is opened. Bit 0 (0x01; compressed) indicates that the resource data is compressed. Bit 1 (0x02; changed) is used by the Resource Manager for internal bookkeeping and should be zero. Bit 7 (0x80) is reserved and should be zero. |
int24be |
dataOffset |
offset from start of resource data section to resource data |
int32be |
resourcePtr |
used by the Resource Manager for internal bookkeeping; should be zero |
The use of a 24-bit unsigned integer for the data offset imposes a 16MB restriction on the size of the resource file.
Finally, at the end of the file comes the name list which consists of an array of Pascal strings encoded in MacRoman. No word alignment is used.
| Type | Description | Notes |
|---|---|---|
name[] |
names |
an array of resource names |
int8be |
length |
length of resource name |
byte[] |
name |
resource name in MacRoman encoding |
If the resource id is in the range -16384 to -1 (0xC000 to 0xFFFF), the
resource is considered "owned" by another resource. The most significant five
bits (bitmask 0xF800) indicate the resource type of the owner, the middle six
bits (bitmask 0x07E0) indicate the resource id of the owner, and the least
significant five bits (bitmask 0x001F) indicate the "sub-id" of the owned
resource.
The Mac OS Resource Manager used this scheme to ensure that certain types of programs, themselves stored in resources, could find the other resources they needed even if the resources had to be renumbered to avoid conflicts. Utilities such as Font/DA Mover that were used to install and remove these programs used this scheme to ensure that all associated resources were installed or removed as well, and renumber the resources if necessary to avoid conflicts.
| MS5B of ID | Owner Type | Notes |
|---|---|---|
0xC000 |
DRVR |
a driver or desk accessory |
0xC800 |
WDEF |
a window definition: code to draw windows |
0xD000 |
MDEF |
a menu definition: code to draw menus |
0xD800 |
CDEF |
a control definition: code to draw UI widgets |
0xE000 |
PDEF |
a printer driver |
0xE800 |
PACK |
a package: a utility code library used by the Mac OS |
0xF000 |
cdev |
a control panel; owner id is set to 1 |
0xF800 |
reserved | reserved for future use |
The data for a compressed resource, indicated by bit 0 (0x01) set in the
resource's attributes field, follows a standard format, described below.
| Type | Description | Notes |
|---|---|---|
int32be |
compressedRsrcTag |
the magic number 0xA89F6572 indicating a compressed resource |
int32be |
typeFlags |
usually 0x00120801
|
int8be |
workingBufferFractionalSize |
length of compressed data relative to length of uncompressed data, out of 256 |
int8be |
expansionBufferSize |
greatest number of bytes compressed data will grow while being decompressed |
int16be |
dcmpID |
id number of a DCMP or dcmp resource used to decompress this resource |
byte[] |
compressedResourceData |
compressed data |
An empty resource file looks like:
0000: 00 00 01 00 00 00 01 00 00 00 00 00 00 00 00 1E
0010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00A0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0100: 00 00 01 00 00 00 01 00 00 00 00 00 00 00 00 1E
0110: 00 00 00 00 00 00 00 00 00 1C 00 1E FF FF