The MacBinary 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).

All values are stored in normal 68000 order, with Most Significant Byteappearing first in the file. 

The format of the header for MacBinary II is as follows:  Offset 000-Byte, old version number, must be kept at zero for compatibility  Offset 001-Byte, Length of filename (must be in the range 1-63)  Offset 002-1 to 63 chars, filename (only "length" bytes are significant).  Offset 065-Long Word, file type (normally expressed as four characters)  Offset 069-Long Word, file creator (normally expressed as four characters)  Offset 073-Byte, original 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.  Offset 074-Byte, zero fill, must be zero for compatibility  Offset 075-Word, file's vertical position within its window.  Offset 077-Word, file's horizontal position within its window.  Offset 079-Word, file's window or folder ID.  Offset 081-Byte, "Protected" flag (in low order bit).  Offset 082-Byte, zero fill, must be zero for compatibility  Offset 083-Long Word, Data Fork length (bytes, zero if no Data Fork).  Offset 087-Long Word, Resource Fork length (bytes, zero if no R.F.).  Offset 091-Long Word, File's creation date  Offset 095-Long Word, File's "last modified" date.  Offset 099-Word, length of Get Info comment to be sent after the resource             fork (if implemented, see below). *Offset 101-Byte, Finder Flags, bits 0-7. (Bits 8-15 are already in byte 73) *Offset 116-Long Word, Length of total files when packed files are unpacked.             This is only used by programs that pack and unpack on the fly,             mimicing a standalone utility such as PackIt.  A program that is             uploading a single file must zero this location when sending a             file.  Programs that do not unpack/uncompress files when             downloading may ignore this value. *Offset 120-Word, Length of a secondary header.  If this is non-zero,             Skip this many bytes (rounded up to the next multiple of 128)             This is for future expansion only, when sending files with             MacBinary, this word should be zero. *Offset 122-Byte, Version number of Macbinary II that the uploading program             is written for (the version begins at 129) *Offset 123-Byte, Minimum MacBinary II version needed to read this file             (start this value at 129 129) *Offset 124-Word, CRC of previous 124 bytes *This is newly defined for MacBinary II.All values are stored in normal 68000 order, with Most Significant Byteappearing first then (sic … within??) the file.  Any bytes in the header not defined aboveshould be set to zero.The original MacBinary format was ammended to include the sending of the FCMT(Get Info comment) after the resource fork was sent, if the length for suchcomment, given in offset 99, is not zero.  To the best of our knowledge, noprogram has implemented this feature, due to Apple's stated position that noprogram should read or write these comments.  The definition remains inMacBinary II, so that should Apple ever provide a documented way of reading andwriting these comments, terminal programs will be able to take advantage ofthis feature. All Finder flags and information would be uploaded, however, a downloadingprogram should clear the Finder flag bits of  0 - Set if file/folder is on the desktop (Finder 5.0 and later)  1 - bFOwnAppl (used internally)  8 - Inited (seen by Finder)  9 - Changed (used internally by Finder) 10 - Busy (copied from File System busy bit)Also, fdLocation and fdFldr should be zeroed
To determine if a header is a valid MacBinary header, check bytes 0 and 74 tobe both zero. If they are both zero, either (a) the CRC should match, whichmeans it is a MB II file, or (b) byte 82 is zero, which means it may be a MB Ifile. (Note that, at the current version level, byte 82 is kept zero tomaintain compatibility with MacBinary I. If at some point the MacBinaryversions change sufficiently that it is necessary to keep MacBinary I programsfrom downloading these files, we can change byte 82 to non-zero.)

If the header is a MB II header, the program will check the minimum versionbyte, to see if it knows enough to decode the file. If the minimum version inthe header is greater than the version that the terminal program was writtenfor, it will download the file as pure XModem (creating a "TEXT" file) andnotify the user that conversion is needed because the MacBinary version was toohigh.

If the header does NOT represent a valid MB II header, the program must atminimum check byte 82 to be zero--if it is not zero, the file is not a MB Ifile.  It is possible to write a much more robust routine, by checking thefollowing:  Offsets 101-125, Byte, should all be 0.  Offset 2, Byte, (the length of the file name) should be in the range of 1-63.  Offsets 83 and 87, Long Word, (the length of the forks) should be in the    range of 0-$007F FFFF.
If any of these tests fail, the file is not a valid MacBinary file. It maystill be desirable to distinguish between text files and foreign binary files(for stripping line feeds or similar helpful acts). 

Some tests that would prove useful include:   A quantity of bytes in the first block with the high bit set would point to     a binary file (though this could be fooled by files with many extended     ascii characters, such as generated by the option key on a Mac).   A large quantity of zero bytes (nulls) would also point to a binary file.------------------------------------------------------------------------ This proposal was adopted at two conferences attended by representatives fromCompuServe, Delphi and BIX networks, and many terminal software publishers.