Date   : Sun, 08 May 1983 19:58:00 EDT
From   : Keith Petersen <W8SDZ@mit-mc.arpa>
Subject: ITS .COM file format explained

(Thanks to GZ@MC for this excellent explanation of how .COM files are
stored on ITS).

COM files are stored on ITS as four 8-bit bytes per PDP-10 word, left
justified.  The first word is a file-type identifier and it is
446353300000 octal.  This particular packing of 8 bit bytes into
pdp-10 words is pretty standard, so if your FTP has a binary mode, it
will probably do the right thing, i.e. give you the correct sequence
of 8-bit bytes coming out of the FTP. If that is the case, the first 4
bytes should be: 223, 072, 330, 0 (octal). These come from the id word
above and are not part of the com file proper, i.e. should be
discarded before downloading to a CP/M system.

Now, if you don't have binary mode in your FTP, or if the binary mode
is such that you get something other 223,072,330,0 as first four bytes
when FTPing COM files (meaning that the server is using some other
byte packing), it might still be possible to FTP the files and
reconstruct the original from that. (The only problem might be CRLF
vs. LF transformations in ascii mode).  All you need to do figure out
how the server unpacks the bytes and then "undo" it.  I'd be happy to
help out if you have trouble figuring this out.  Just let me know what
the first few bytes are when you FTP a com file.  But before you go
writing any code, make sure there is no way to make FTP win directly.
--end--