Apple II
File Type Notes
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                                                  Developer Technical Support

File Type:         $E0 (224)
Auxiliary Types:   $0002 & $0003

Full Name:     AppleDouble Header File (Auxiliary Type $0002)
               AppleDouble Data File (Auxiliary Type $0003)
Short Name:    AppleDouble Header (Auxiliary Type $0002)
               AppleDouble Data (Auxiliary Type $0003)

Revised by:    Matt Deatherage                                  November 1990
Written by:    Matt Deatherage                                     March 1989

Files of these types and auxiliary types contain file data in AppleDouble
format.
Changes since March 1990:  Added information about AppleDouble 2.0.
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AppleDouble is one of two standards (the other is AppleSingle) put forth by
Apple Computer, Inc. for representing files on foreign file systems while
preserving all attributes of the file's home system on file systems that do not
support the same attributes.

Experience indicated that a single format would be inadequate to cover all
cases.  Two closely related formats, however, can serve most needs.  Although
the primary impetus for developing these formats is storing extended files
(files with both resource and data forks) on file systems that do not support
the notion of two forks, the proposed formats are general enough that theycan be
used to represent a file from any file system on any other file system.

AppleDouble keeps the data fork as a separate file from the file attributes and
the resource fork, and this Note describes this file format.  AppleSingle keeps
all attributes and the contents of both forks in a single file in the foreign
file system, and is described in the File Type Note for File Type$E0, Auxiliary
Type $0001.

AppleSingle is intended to be used primarily as a storage format, especially for
cases where you must store an extended file on a foreign file system and later
reconstruct the extended file.  AppleDouble is more appropriate for applications
where the users of the foreign file system might want to modify the contents of
the file.  Since most applications keep file data in the data fork, AppleDouble
format saves the contents of the data fork in one file.All other file
attributes, including the resource fork, are kept in a separate file.


Reasons for Using AppleDouble

There are several reasons for supporting an interchange format between file
systems.  Perhaps the most germane is one of the least obvious:  handling
extended files on foreign file systems which do not support extended files.

For example, the ProDOS FST in GS/OS can create an extended file on a ProDOS
disk.  However, ProDOS 8 is unable to operate on the file, since it sees itas
having an unsupported storage type.  If a telecommunications program or other
utility capable of transferring files is operating under ProDOS 8 andattempts to
receive an extended file, it is unable to create the file.

At this point, the application could use READ_BLOCK and WRITE_BLOCK commands,
along with a knowledge of the ProDOS file system, to create the file on its own.
However, this is strongly discouraged.  The ProDOS file system format for
extended files is not documented and could change in the future.  In addition,
the program could be running on a eight-bit system.  If the disk is only used on
an eight-bit system, the extended files would not only be unwanted, but also
unremovable without using the disk on an Apple IIGS or later system running
GS/OS.

However, if the application is aware of the AppleDouble format, it canquickly
store an extended file in AppleDouble, leaving the conversion back to the
extended file to GS/OS, or another operating system.  This is the recommended
way for ProDOS 8 applications to create and handle extended files.  Useeither
AppleSingle or AppleDouble.


AppleDouble Format

AppleDouble consists of two files, an AppleDouble Header File and an AppleDouble
Data File.  The AppleDouble Header file contains a headerfollowed by data.  The
header consists of several fixed fields and a list of entry descriptors, each
pointing to an entry.  Apple defines these standardentries: Resource Fork, Real
Name (name in the home file system), Comment, Icon and File Info.  Each entry is
optional, so it may not appear in the file.  Wealso define the new entry Data
Pathname, pointing to the pathname of the AppleDouble Data File.  The Header
File has exactly the same format as an AppleSingle file, except it has no data
fork entry.  The AppleDouble DataFile consists of just the data fork of the
file, with no extra header at all.

Note:  All numeric entries, including entries representing ProDOS data
       structures (such as file type and auxiliary type) are Reverse
       ordered.  This is provided so any host CPU can attempt to
       interpret entries in the header without having to know the
       standard byte-ordering of the home file system.  Therefore, in
       this Note you see descriptive entries like "Rev. 4 Bytes."  This
       serves as a reminder that all header fields are stored high byte
       first, even though the notation Bytes does not imply any specific
       ordering in other File Type Notes.

Also note that ASCII strings are not stored in reverse order, just non-ASCII
constants.

The Header in the Header File:

Magic Number         Rev. Long    The Magic Number field is modeled after
                                  the feature in UNIX.  It is intended to be
                                  used in whatever way the foreign file
                                  system distinguishes a file as AppleDouble
                                  format.  See the section "Identifying
                                  AppleDouble Files."  The Magic Number for
                                  AppleDouble format is $00051607, which is
                                  stored reverse as $00 $05 $16 $07 (reverse
                                  of normal 65816/6502 order).
Version Number       Rev. Long    The version of AppleDouble format, in case
                                  the format evolves (more fields may be
                                  added to the header).  The version
                                  described here is $00010000, stored
                                  (reverse) as $00 $01 $00 $00.

Home File System     16 Bytes     A fixed-length, 16-byte ASCII string not
                                  preceded by a length byte, but possibly
                                  padded with blanks.  Apple has defined
                                  these values:
                                  ProDOS    $50726F444F5320202020202020202020
                                  Macintosh $4D6163696E746F736820202020202020
                                  MS-DOS    $4D532D444F5320202020202020202020
                                  Unix      $556E9878202020202020202020202020
                                  VAX VMS   $56415820564D53202020202020202020
                                  Apple welcomes suggestions for other file
                                  systems that should be included in this
                                  list.
Number of entries    Rev. Word    Tells how many different entries are
                                  included in the file.  This unsigned
                                  reverse word may be zero.  If it is non-
                                  zero, then that number of entry
                                  descriptors immediately follows this
                                  field.


For Each Entry:

Entry ID             Rev. Long    Identifies the entry.  Apple has defined
                                  the following Entry IDs and their values:
                                  1 = Data Fork
                                  2 = Resource Fork
                                  3 = Real Name (The file's name in the home
                                      file system)
                                  4 = Comment* (standard Macintosh comment)
                                  5 = Icon, B&W* (standard Macintosh black
                                      and white icon)
                                  6 = Icon, Color* (reserved for Macintosh
                                      color icon)
                                  7 = File Info (file attributes,dates, etc.)
                                  9 = Finder Info* (standard Macintosh
                                      Finder Info)
                                  Entry IDs marked with asterisks (*) are
                                  not used for most files created under
                                  ProDOS or GS/OS.  Furthermore, icon
                                  entries probably do not appear in most
                                  files since they are typically stored as a
                                  bundle in the application file's resource
                                  fork on the Macintosh.  Apple reserves the
                                  range of Entry IDs from $0 to $7FFFFFFF
                                  for future use.  The rest of the range is
                                  available for other systems to define
                                  their own entries.  Apple does not
                                  arbitrate the use of the rest of the
                                  range.
                                  Descriptions of the standard entries are
                                  given below.
Offset               Rev. Long    An unsigned reverse long which indicates
                                  the byte offset from the start of the file
                                  to the start of the entry.
Entry Length         Rev. Long    An unsigned reverse long which indicates
                                  the length of the entry in bytes.  The
                                  length may be zero.


Standard Entries:

The Real Name Entry:

The Real Name entry indicates the file's original filename in the host file
system.  This is not a Pascal or C string; it is just ASCII data.  The length is
indicated by the Entry Length field for the Real Name entry.

The File Info Entry:

The File Info entry (Entry ID = 7) is different for each home file system. For
ProDOS files, the entry is 16 bytes long and consists of the creationdate and
time and the modification date and time in ProDOS 8 (ProDOS 16/class zero GS/OS)
form, the access word, a two-byte file type and four-byte auxiliary type.  This
is detailed in standard format below, along with defined FileInfo entries for
some other file systems.

ProDOS:

Create Date         Rev. 2 Bytes  Creation date packed into standard
                                  ProDOS 8 format.
Create Time         Rev. 2 Bytes  Creation time packed into standard
                                  ProDOS 8 format.
Modification Date   Rev. 2 Bytes  Modification date packed into
                                  standard ProDOS 8 format.
Modification Time   Rev. 2 Bytes  Modification time packed into
                                  standard ProDOS 8 format.
Access              Rev. Word     The file's access.  This may be used
                                  directly in ProDOS 16 or GS/OS calls; only
                                  the low byte is significant to ProDOS 8.
File Type           Rev. Word     The file type of the original file.  Only
                                  the low byte is significant to ProDOS 8.
Auxiliary Type      Rev. Long     The auxiliary type of the original file.
                                  Only the low word is significant to ProDOS
                                  8.

Note:  Although the ProDOS Access field, File Type and Auxiliary Type are
       the same length as found in ProDOS 16 and GS/OS structures, the
       Create and Modification Dates and Times are stored in two-byte
       (albeit byte-reversed) ProDOS 8 format, not eight-byte Apple IIGS
       format.

Macintosh:

Create Date         Rev. Long     Unsigned number of seconds between
                                  January 1, 1904, and the creation time of
                                  this file.
Modification Date   Rev. Long     Unsigned number of seconds between
                                  January 1, 1904, and the last modification
                                  of this file.
Last Backup Date    Rev. Long     Unsigned number of seconds between
                                  January 1, 1904, and the last backup time
                                  of this file.
Attributes          Rev. Long     32 boolean flags.  Once the bytes are
                                  unreversed, bit zero is the locked bit and
                                  bit one is the protected bit.

MS-DOS:

Modification Date    Rev. 4 Bytes  MS-DOS format modification date.
Attributes           Rev. 2 Bytes  MS-DOS attributes.

Unix:

Create Date/Time       Rev. 4 Bytes  Unix creation date and time.
Last Use Date/Time     Rev. 4 Bytes  Unix time for the last
                                     time this file was used.
Last Mod. Date/Time    Rev. 4 Bytes  Unix time for the last
                                     time this file was modified.


The Finder Info Entry:

The Finder Info entry (Entry ID = 9) is for files where the host file system is
Macintosh.  It consists of 16 bytes of Finder Info followed by 16 bytes of
Extended Finder Info.  These are the fields ioFlFndrInfo followed by
ioFlXFndrInfo, as described in Inside Macintosh, Volume IV-183.  Newlycreated
files have zeroes in all Finder Info subfields.  If you are creating an
AppleDouble file whose home system is Macintosh, you may zero all unknown
fields, but you may want to set the fdType and fdCreator subfields.


The Data Pathname Entry:

The Data Pathname entry (Entry ID = 100) is defined for the first time inthis
Note.  It consists of a class one GS/OS input string noting the pathname of the
AppleDouble Data File as originally created:

Path Length    Rev. Word    The length of the pathname.
Pathname       Bytes        ASCII pathname of the AppleDouble Data File
when created.

 For strategies on using this segment (or not using it) to find theAppleDouble
Data File, see the section "Finding the AppleDouble Data File."


The Entries in the Header File:

The entries themselves follow the header field and the entry descriptors.The
actual data representing each entry must be in a single, contiguous block. The
offset field in that entry's descriptor points to it.  The entries could appear
in any order, but since the data fork is the entry that is most commonly
extended, Apple strongly recommends that the data fork always bekept last in the
file to facilitate its extension.  Apple also recommends that those entries that
are most often read, such as Real Name, File Info (and Finder Info if present)
be kept as close as possible to the header tomaximize the probability that a
read of the first few blocks of the file retrieves these entries.

It is possible to have holes in the file (unused space between entries).  To
find the holes, you must take the list of entry descriptors and sort theminto
increasing offset order.  If the offset field of an entry is greater than the
offset plus the length of the previous entry (sorted), then a hole exists
between the entries.  You can make use of such holes; for example, if afile's
comment is ten bytes long, you could create a hole of 190 bytes after the
comment field to easily allow for the comment to later expand to its maximum
length of 200 bytes.  Because an AppleDouble file may contain holes, you must
find each entry by getting its offset from its entry descriptor, not by assuming
that it begins after the previous entry.

Byte ordering in file header fields follows 68000 convention, and each header
field has been so noted by the Reverse operator.


The AppleDouble Data File

The AppleDouble Data File is simply the data fork of the original file contained
in a file of its own.  You may create it with a File Type and Auxiliary Type
assignment best suited to it, if desired.  For example, if the program creating
the AppleDouble Data File knows that the data fork contains strictly ASCII text,
it could create the file with File Type $04 (Text File) so that other
applications can deal with it accordingly.

If the creating program wishes to make no assumptions about the content ofthe
data fork, it is encouraged to create the AppleDouble Data File with filetype
$E0 and auxiliary type $0003.  This identifies the file as an AppleDoubleData
File.


Identifying AppleDouble Files

As this is an interchange format, from a ProDOS directory entry there is no way
to guarantee which files are AppleDouble files.  Apple has allocated File Type
$E0, Auxiliary Type $0002 for files which are AppleDouble Header Files, and File
Type $E0, Auxiliary Type $0003 for files which are AppleDouble Data Files.  We
strongly encourage ProDOS 8 and GS/OS applications to use these file type and
auxiliary type assignments when creating AppleDouble files.

AppleDouble files which do not have file type $E0 and auxiliary type $0002 or
$0003 can most easily be identified by opening them and attempting to interpret
them.  If it is not an AppleDouble Header File, the Magic Number is not
contained in the first four bytes of the file.  The chances that the file would
begin with those four bytes and not be an AppleDouble Header File, on a purely
random basis, are 4,294,967,295 to 1.  The chances that both the Magic Number
and the Version bytes would be the same in a non-AppleSingle file are roughly
1.8 x 10^19 to 1.


Finding the AppleDouble Data File

Since the AppleDouble Data File can be stored anywhere, with any file typeand
auxiliary type, a program may have to make an effort to find it.  Werecommend
the following steps:

1.  If the Data Pathname segment exists, use that pathname.  If the
    path specified in the segment does not exist, extract the file
    name from the end of the pathname and look in the current
    directory for that file name.
2.  If Step 1 does not find the file (or if the Data Pathname segment
    does not exist), perform the appropriate Home File System
    algorithm (described below) to generate the name of the
    AppleDouble Data File from the AppleDouble Header File.
3.  If none of the file names generated in Step 2 are found, ask the
    user where the AppleDouble Data File is located.


Filename Conventions:

Apple proposes the following standard for identifying AppleDouble Header File
names and AppleDouble Data File names from the file's real name.

ProDOS:

To generate the AppleDouble Data File name, use character substitution or
deletion to remove illegal characters, and use truncation if necessary to
reduce the length of the name to two characters less than the maximum file
name length.  This would be a maximum of 13, since the maximum file name
length is 15.

To generate the AppleDouble Header File name, prefix the AppleDouble DataFile
name with the characters "R." (uppercase R period).

For example, the file name "This is a Foo File" could translate to an
AppleDouble Data File Name of "THIS.IS.A.FOO."  The AppleDouble Header File
name would then be "R.THIS.IS.A.FOO."

Unix:

To generate the AppleDouble Data File name, use character substitution to
replace any illegal characters with an underscore (_).  Since different Unix
systems have different requirements on maximum file name length, do not
explicitly truncate the name to a specific length.  Rather, allow the
truncation to be done by the Unix functions create(), open(), etc.

To generate the AppleDouble Header File name, A/UX (Apple's implementation of
Unix for Macintosh computers) prefixes a percent sign (%) to the AppleDouble
Data File name.  If necessary, truncate the last character to keep the file name
within the legal length range.  Other Unix systems may prefix adirectory name
(e.g., ".AppleDouble/") to the AppleDouble Data File name to create the name of
the AppleDouble Header File.  In this scheme, all AppleDouble Header Files
corresponding to AppleDouble Data files are kept together in a single
subdirectory.

MS-DOS:

To generate the AppleDouble Data File name, use character substitution or
deletion to remove illegal characters, and use truncation if necessary to
reduce the length of the name to eight characters.  Then add the MS-DOS
extension that is most appropriate to the file (such as "TXT" for a pure text
file).

To generate the AppleDouble Header File name, add the extension ".ADF" to the
eight-character file name.

In any instance, most programs probably wish to display the names being used
for both AppleDouble files, so that the user may keep track of them on disk.

AppleDouble name derivations will be defined for all other file systems of
interest.  This allows applications running on the foreign file system (and
users as well) to see easily which files are AppleDouble pairs.Knowledgeable
users, if they know the derivation, could rename or move the files so as to
preserve the connection between the two.  However, there is no guaranteed way to
prevent one file of the pair from being inconsistently renamed, moved, or
deleted.


About AppleDouble 2.0

AppleDouble 2.0 is a revision to the original AppleDouble specification
described in this Note.  AppleDouble 2.0 comes closer to the ideal of an
interchange format by allowing file information for multiple file systems in the
same AppleDouble file.

AppleDouble 2.0 basically replaces the File Info entry (ID = 7) with a File
Dates entry (ID = 8) and one or more host file system entries, such as a
Macintosh File Info entry (ID = 10), a ProDOS File Info entry (ID = 11), or an
MS-DOS File Info entry (ID = 12).  Information on these entries and AppleDouble
2.0 can be found in the AppleSingle/AppleDouble Formats for Foreign Files
Developer's Note, available from APDA, AppleLink, and the Developer CD series.


Further Reference
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  o  Inside Macintosh, Volume IV
  o  ProDOS 8 Technical Reference Manual
  o  GS/OS Reference
  o  AppleSingle/AppleDouble Formats for Foreign Files Developer's Note


