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-=[ MacDown 1.0 Copyright 1988 Jason Harper ]=-
MacDown is a utility for converting some downloaded Macintosh
files into formats usable on Apple II-series computers. It
will run on any Apple II that supports ProDOS (in other
words, at least 64K memory), but the screen displays will
probably not be readable on an Apple ][ or ][+, even with a
lower-case adapter. The program is freeware, meaning that it
is a copyrighted work but I give users the right to freely
distribute it and upload it to computer services.
=> Why Mac files are a bit difficult to work with.
The Macintosh has a very unusual file system: its files,
instead of being a single sequence of bytes as most
computer's files are, consist of two completely separate
sequences of bytes plus a substantial amount of information
about the file, such as the format of data it contains and
the program that created it. The two components of a Mac
file are referred to as the 'data fork' and the 'resource
fork': the data fork contains arbitrary data much as other
computer's files do, and the resource fork contains a series
of discrete data items ("resources") such as icons. A Mac
file may be missing one or both forks: document files tend to
be mainly data, while program files tend to be mainly
resources. This file arrangement makes it impossible to
simply send a Mac file via modem: there must be some way to
recreate the two forks and the file's directory information.
Early Mac communication programs actually transmitted files
in three pieces which were glued back together at the
receiving end: this works when connected directly to another
Mac, but non-Mac host computer systems generally are not
prepared to handle such files. So, a standard was developed
to allow all the parts of a Mac file to be appended into a
single data stream, with imbedded information to allow the
original file to be recovered unchanged...
=> The MacBinary standard.
All current Mac communication programs now automatically
convert transmitted files into MacBinary format, and rebuild
received MacBinary files into an exact copy of the original
file. The format is quite similar to the Binary II format
used for transfer of files between Apple II computers (which
was based on MacBinary): the transmitted data starts with a
128-byte header containing the file's directory information
and the length of the two forks, followed by the data fork
(if any) rounded up to a multiple of 128 bytes, followed by
the resource fork (if any) also rounded up. There are
provisions for tacking on additional information at the end,
but this is seldom (if ever) implemented. Using MacBinary
files downloaded on an Apple II isn't very difficult: since
most Mac files that are meaningful on other computers contain
only data and no resources, just stripping off the first 128
bytes of the file is sufficient. This can be done (on files
at most 32K long) from ProDOS Basic with the commands:
BLOAD mac.file,Ttxt,A$1000
BSAVE mac.file,Ttxt,A$1080
replacing 'mac.file' and 'txt' with the downloaded file's
actual name and filetype. The process gets much more
complicated with longer files, and there are other problems
to worry about...
=> Compressed files.
Mac files tend to be pretty large, so it is a common practice
to apply some sort of compression to them. There are two
compression methods in current use: PackIt and StuffIt. Such
files can be easily identified because their names will end
with .PIT or .SIT, respectively. Another advantage of using
these schemes is that related files (a program and its
documentation, for example) can be packed into one file for
transmission: unlike Binary II, the current implementation of
MacBinary does not allow for multiple files. StuffIt is the
more recent and more efficient of the two formats:
unfortunately, its author has not seen fit to release enough
information on the format to allow decoders to be developed
for other computers. PackIt is, however, publicly defined
and MacDown will allow you to decompress such files for use
on an Apple II.
=> Mac file formats.
Mac files are identified by a 4-character filetype code and a
4-character creator code: for example, a MacPaint picture has
filetype=PNTG and creator=MPNT. A MacPaint-format picture
generated by some other program will still have a filetype of
PNTG but will have a different creator. Some common types: