From: tcmay@got.net (Timothy C. May)
Date: Fri, 30 Aug 1996 04:37:33 +0800
To: cypherpunks@toad.com
Subject: "Undocumented Features"
Message-ID: <ae4b14ea00021004ad67@[205.199.118.202]>
MIME-Version: 1.0
Content-Type: text/plain


At 4:21 AM 8/29/96, Adamsc wrote:
>>> >looks like the DoJ have been looking around at the mirrors.. maybe to
>>> >legally hassle people about them....
>
>>> Or, just maybe, some DoJ employees wanted to see what folks on the outside
>>> were doing to tweek their bosses:-)? Reminds me of the *thousands* of hits
>>> the "Intel Secrets Page"(http://www.x86.org/) has gotten from users at
>>> intel.com
>
>Out of curiosity, has anyone used a decompiler to check if any of the
>undocumented stuff is getting used in shipping programs?

This has only marginal relevance to the list, but I'll mention in case it
helps a crypto coder out there: be wary of _ever_ using undocumented
features of a processor, compiler, or other system.

Why? Because "undocumented features" are not promises made by the vendor,
and may vanish in the next release (or, worse, change behavior in strange
and hard-to-detect ways).

We witnessed this several times at Intel with the x86 line. Various
customers discovered "undocumented features," made the mistake of
exploiting them, and then came crying to us when iterations of the
processor (what we call "steppings") took out the "features" or altered
their behavior. (And for a while there was even a rift between the Intel
versions and the NEC versions, which copied some Intel processors and
copied the undocumented features....when Intel was no longer supporting
them, the two processor families diverged, causing chaos.)

So, beware.

--Tim May

We got computers, we're tapping phone lines, I know that that ain't allowed.
---------:---------:---------:---------:---------:---------:---------:----
Timothy C. May              | Crypto Anarchy: encryption, digital money,
tcmay@got.net  408-728-0152 | anonymous networks, digital pseudonyms, zero
W.A.S.T.E.: Corralitos, CA  | knowledge, reputations, information markets,
Licensed Ontologist         | black markets, collapse of governments.
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