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):




Zero. This is a "version" byte.


Length of filename.


Filename (only "length" bytes are significant)
(the following 16 bytes are a standard Finder Info record)


File type.


File creator.


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.




File's vertical position within its window.


File's horizontal position within its window.


File's window or folder ID.
(End of Finder Info)


"Protected" flag (in low order bit).




Data Fork length (bytes, zero if no Data Fork).


Resource Fork lenth (bytes, zero if no R.F.).


File's creation date.


File's "last modified" date.


Zero fill (reserved for expansion of standard).


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 discussion.
   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

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