Date   : Tue, 20 Aug 1985 16:01:15 GMT
From   : Dan Winkler <winkler@HARVARD.ARPA>
Subject: Getting binary files from Simtel20

Here's an explanation of the problem I was having with get20.  I was using
it to transfer binary files from Simtel20 to this Unix Vax.  When I did,
I would get strange files that had some recognizable text in them.  If I
ftp'd the same file in tenex mode, it arrived properly.  Here's why:

> From towson@AMSAA.ARPA Tue Aug 20 10:54:56 1985
> 
> Dan - SIMTEL20 is a DEC-20, which has a 36-bit word-length.  UNIX machines
> that I know of have either 16 or 32-bit words.  Thus, there is no way that a
> straight binary (i.e., image) ftp transfer can map correctly from the DEC-20
> to a UNIX machine.  Binary data are stored as four 8-bit bytes per 36-bit
> SIMTEL20 word, with the low-order four bits of each word filled with zeros.
> If such a file is interpreted as a contiguous string, as will happen if a
> straight binary transfer is made to a 16 or 32-bit UNIX machine, the
four zero
> filler-bits per 36-bit group will cause rather bizarre and frustrating
> results.  You will get alternate groups of four good bytes, four bad, four
> good, and so on.  String searches will turn up some recognizable text, but
> just enough to drive you nuts.  Tenex mode causes the DEC-20 to unpack the
> data into 8-bit chunks (with no filler bits) before sending it to the UNIX
> machine.
> 
> Good luck,
> Dave

So if you want to use get20 on a Unix machine, you better make sure that
it's using tenex mode.

Dan.