Path: utzoo!attcan!uunet!lll-winken!ncis.llnl.gov!helios.ee.lbl.gov!pasteur!ucbvax!bloom-beacon!mit-eddie!killer!elg
From: elg@killer.DALLAS.TX.US (Eric Green)
Newsgroups: comp.sys.amiga
Subject: Re: Atalk III 1.0e
Message-ID: <6872@killer.DALLAS.TX.US>
Date: 21 Jan 89 02:40:06 GMT
References: <5742@cbmvax.UUCP>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 37

in article <5742@cbmvax.UUCP>, ditto@cbmvax.UUCP (Michael "Ford" Ditto) says:
> In article <6842@killer.DALLAS.TX.US> elg@killer.DALLAS.TX.US (Eric Green) writes:
>>If I called your system (which creates the extra padding blocks) with
>>my CP/M system (a Commodore 128 running MEX), and uploaded a binary
>>file, then downloaded it back to myself, what I got back would be
>>incorrect.
> It shouldn't be incorrect if both sides use the spec'd "chopping"
> method.

Note that CP/M generally does not do "chopping", for the simpler
reason that it is not necessary (all files are already padded to a 128
byte boundary). Thus my recieved file would be 128 bytes longer than
it was when I transmitted it... Nasty.

>>CP/M is the ultimate test of whether an Xmodem protocol is "right" or
>>not. If it don't work under CP/M, it ain't Real Xmodem.
> Fine, but that doesn't mean that all files must become padded to
> a 128-byte boundary in the transfer.

This originally started as a conversation about flaws of the Xmodem
protocol, in particular, about the problem of binary files that are an
exact multiple of 128 bytes in length and have control-Z or NUL
charcters as their last bytes. "all files must become padded" is not
an accurate description of the problem. Your fix, creating an extra
block consisting solely of padding, will work, if the reciever does
normal auto-chopping. But if the reciever does not, e.g.  is a CP/M
system, you're breaking things. Breaking things is nasty.

The moral: If creating that extra "padding block", allow it to be
turned off for systems that don't need or want any kind of extra
padding (beyond that of the "fill the last block" variety).

--
Eric Lee Green    ..!{ames,decwrl,mit-eddie,osu-cis}!killer!elg
          Snail Mail P.O. Box 92191 Lafayette, LA 70509              
Netter A: In Hell they run VMS.
Netter B: No.  In Hell, they run MS-DOS.  And you only get 256k.