Macintosh Binary Transfer Format ("MacBinary") Standard Proposal

Dennis F. Brothers - 13 March 1985
(Revision 1 - 10 April 1985 - Miscellaneous clarifications)
(Revision 2 - 12 April 1985 - Corrected reversal of icon position v,h)
(Revision 3 -  6 May   1985 - Added processor ID, general cleanup)

The following notes are a proposal for a standard format for binary transfer
of arbitrary Macintosh documents via a telecommunication link. It is intended
for use both between Macintoshes running (possibly different) terminal
programs which adhere to the standard, and for use in uploading arbitrary
Macintosh documents to remote systems (where it is presumed that they will be
stored as an exact image of the data transmitted). A proposal is also made for
standard processing to be performed on text files transferred via a protocol,
to maximize the likelihood that text files transmitted to a remote system will
be usable by that system, and that text files received from a remote system
will be usable by the Macintosh.

It is recommended that the format and procedures described here be referred
to as "MacBinary", and that any terminal program implementing this format
and procedures be called "MacBinary-Compatible".

The binary format described is independent of the communication protocol used
to accomplish the transfer, and assumes only that an 8-bit transparent
transfer can be accomplished. Such protocols as Christensen (usually referred
to as XMODEM or MODEM7), Kermit, and CompuServe A or B meet this requirement.
Because of the proposed standard's MacTerminal/XMODEM heritage, there is a
requirement that the transmitted data be padded (with nulls) to a 128-byte
boundary at certain points, but this in no way implies that a block-oriented
protocol must be used. The basic format proposed is identical to that used by
MacTerminal, by Mike Boich and Martin Haeberli, and can be used with a version
of MacTerminal that has had a patch applied to "normalize" its implementation
of the XMODEM protocol.

In brief, the binary format consists of a 128-byte header containing all the
information necessary to reproduce the document's directory entry on the
receiving Macintosh; followed by the document's Data Fork (if it has one),
padded with nulls to a multiple of 128 bytes (if necessary); followed by the
document's Resource Fork (again, padded if necessary). The lengths of these
forks (either or both of which may be zero) are contained in the header.

The format of the 128-byte header is as follows (offsets and lengths are given
in decimal):

        Offset   Length   Contents
        ------   ------   --------
        000      1        Zero. This is a "version" byte.
        001      1        Length of filename.
        002      63       Filename (only "length" bytes are significant).
       (the following 16 bytes are a standard Finder Info record)
        065      4        File type.
        069      4        File creator.
        073      1        Finder flags:
                                 Bit 7 - Locked.
                                 Bit 6 - Invisible.
                                 Bit 5 - Bundle.
                                 Bit 4 - System.
                                 Bit 3 - Bozo.
                                 Bit 2 - Busy.
                                 Bit 1 - Changed.
                                 Bit 0 - Inited.
        074      1        Zero.
        075      2        File's vertical position within its window.
        077      2        File's horizontal position within its window.
        079      2        File's window or folder ID.
       (End of Finder Info)
        081      1        "Protected" flag (in low order bit).
        082      1        Zero.
        083      4        Data Fork length (bytes, zero if no Data Fork).
        087      4        Resource Fork lenth (bytes, zero if no R.F.).
        091      4        File's creation date.
        095      4        File's "last modified" date.
        099      27       Zero fill (reserved for expansion of standard).
	126      2        Reserved for computer type and OS ID
                          (this field will be zero for the current Macintosh).

Note that it is the responsibility of the receiving terminal program to
resolve file name conflicts, generally by somehow modifying the name of
received file if there already exists a file with the original name on the
target volume.

Note also that, while the original window or folder ID and position may be
transmitted, the receiving terminal program would not normally set these items
for the received file, but would instead accept the values that the File
Manager assigns when it creates the new file.

It is suggested that Macintosh terminal programs implement two modes of
protocol transfer: text and document. Text mode is used for unformatted files
of type TEXT (with only a data fork), and document mode (using the binary
format proposed here) is used for all other files. Document mode may also be
used on text files, of course, if it is desired to preserve such things as the
file's creator ID or creation date.

The intent of text mode is to provide compatibility with text files on other
systems. Toward that end, it is recommended that a linefeed be inserted after
each return character as the text file is transmitted, and that, in the case
of block-oriented protocols, the last block be explicitely padded with nulls
if the text does not end on a block boundary. When receiving in text mode,
linefeeds and trailing nulls should be stripped. If a CTRL-Z (Hex 1A) character
is received following all other text (and before any null padding), it should
also be stripped (Ctrl-Z is a common text terminator on CP/M and smoe other
systems). Note that the above discussion applies only to text files transferred
under some protocol, where an exact image of the transmitted data will be
stored in a file on the remote system.

When receiving a file via a protocol, a terminal program may distinguish
between text and document modes by examining bytes 0, 74, and 82 of the first
128 bytes received. If they are each zero (and at least 128 bytes have been
received), then it is a safe assumption that a MacBinary-formatted document
is being received. Terminal programs implementing possible future versions of
the proposed standard would, of course, accept an appropriate set of version
numbers in byte 0. Note also that compatible extensions of Version 0 of the
proposed standard are possible (one such is suggested below) that involve
transmission of additional information following the information described here.
For this reason, a terminal program should be implemented so as to perform
the proper receive procedures for all data sent to it, but to ignore any data
that it does not know what to do with.

Since a text-mode document does not contain a file name, it is suggested that
when text-mode is detected, a file be opened under a dummy name on the
receiving Macintosh, the text written to that file, and the file renamed to a
name selected by the user (via an SFPutFile box) after the reception is
completed. This will avoid problems caused by indeterminate delays for name
selection at the beginning of the file transfer.

It is desirable to allow the user to specify the destination volume in advance
of the actual start of transfer for either type of transfer. Two methods are
suggested for this: provide a "Select Destination Volume" menu selection,
presumably in the menu containing the "Receive File" selection; or query the
user immediately after the "Receive File" menu selection is made. Either or
both of these methods could be implemented in a given terminal program - the
independent "Select Receive Volume" method is particularly desirable if the
ESC-a/ESC-b automatic receive facility (see below) is implemented. The volume
selection procedure should provide the same functions as the volume selection
portion of the SFGetFile and SFPutFile dialog boxes.

First proposed extension to the proposed standard:

It is proposed that the binary format described above be extended to allow the
transmission of descriptive text with a Macintosh document (specifically, the
descriptive text from the Finder's "Get Info" box for the file being
transferred). This is to be accomplished in a transparent manner by assigning
bytes 99 and 100 of the header described above to be used to hold the length
of the descriptive text (or zero, if there is none). The descriptive text, if
any, will begin on the 128-boundary immediately following the Resource Fork,
if present, else the Data Fork, if present, else immediately following the
header if neither fork is present. It is hoped that methods for reading and
setting a file's "Get Info" text will be made public at some point.

Notes on MacTerminal's XMODEM implementation, and a proposed extension:

Familiarity with the Christensen (XMODEM) protocol is assumed in the following

When doing "Mac-to-Mac" transfers, using the binary format described above,
unmodified MacTerminal does not use a true XMODEM protocol, and is therefore
incompatible with most non-Mac systems. The differences lie in two specifics:
the transmitting Macintosh initiates the transfer by sending the the two
characters ESCAPE (hex 1B) and "a" (hex 61); the receiving Macintosh is
expected to reply with the character ACK (hex 06). The transfer then proceeds
using normal XMODEM procedures, except that each of the header and the two
forks (if present) is treated as a separate XMODEM transfer, with the
transmitting Macintosh waiting for a NAK (hex 15), then sending the blocks of
that phase (beginning with a block number of one), then sending EOT (hex 04)
and waiting for an ACK (hex 06) from the receiving Macintosh.

It is proposed that a modified procedure be accepted as a standard, to be
implemented instead of or in addition to the above-described MacTerminal
"Mac-to-Mac" protocol in complying terminal programs. The modified procedure,
which is compatible with standard XMODEM implementations, functions as
follows: The transmitting Macintosh sends the two characters ESCAPE (hex 1B)
and "b" (hex 62). The receiving Macintosh may optionally reply with ACK (hex
06) (this is allowed so as to have minimum impact on existing
MacTerminal-compatible implementations). The transmitting Macintosh then
awaits receipt of a NAK (hex 15) (or optionally a "C" (hex 43), if the
receiving terminal program supports CRC checking), following which a single,
normal XMODEM transfer occurs. The transfer may be in text mode or document
mode, will begin with block number one, and block numbers will increment
uniformly (modulo 256) throughout the transfer.

It is expected that a patch to MacTerminal making it compatible with the above
proposed procedure will be available in the near future.

Responses to proposals:

Please address comments or questions on the above proposals to:

        Dennis F. Brothers
        197 Old Connecticut Path
        Wayland, MA 01778

        CompuServe: 70065,172
        Delphi:     DBROTHERS
        MCI Mail:   DBROTHERS

MacBinary Working Group:

An informal working group, consisting of Macintosh terminal program developers
and others with interests or expertise in the field of computer communications,
was formed during April, 1985 to discuss and refine this proposal. The group
met in the MAUG (Micro-networked Apple User's Group) Special Interest Group on
the CompuServe Information Service. The present form of the MacBinary format
standard represents a consensus of this group as a whole, but may not reflect
the opinion of a given individual member of the group.

The working group included:
   Christopher Allen
   William Bond
   Steve Brecher
   Dennis Brothers
   Ward Christensen
   Dan Cochran
   Mike Cohen
   Bill Cook
   Ed Edell
   Duane Harris
   Yves Lempereur
   Neil Shapiro
   Dan Smith
   Bill Steinberg
   Scott Watson