lists at rtty.us
Sat Aug 14 16:11:01 UTC 2010
The PDP-8 had so much code that depended on un-documented instructions that they had to include them in later versions of the machine....
On Aug 14, 2010, at 12:01 PM, "J. Forster" <jfor at quik.com> wrote:
> I think you missinterpret what I meant. Two examples:
> I've seen programmers who use "instructions" that are not part of a uP
> instruction set and are undocumented, just to be "clever". If a different
> brand of chip, or even a different rev., the chip does something
> completely different. These guys should be strung up by their tender
> I've also seen transistors used as avalanche switches (basically a failure
> mode). If a different production run has improved normal mode performance,
> the avalanche function may vanish.
>> J. Forster wrote:
>>> FWIW, IMO any engineer who uses undocumented or uncontrolled parameters
>>> instructions in a production design is a fool.
>>> If you are that silly, you must fully specify the selection criteria.
>> Or, has their back against the wall and can't do it any other way.
>> How is this any different than using trimpots or hand select?
>> For years, folks have hand selected matched pairs of devices, since the
>> circuit requires tighter tolerances than the mfr guarantees.
>> Many, many RF designs have "select at test" pads to set levels or tuning
>> stubs depending on what the actual gain or impedance properties of the
>> active devices are, or for trimming temperature dependencies.
>> Would you say that the engineer is a fool for not just specifying
>> tighter tolerances.. the tighter tolerances may not be available from
>> the mfr (who has to respond to many customers, most of which will be
>> happy with the standard performance). It's sort of a tradeoff.. do you
>> go to the mfr and say, I need a better grade of part, or do you buy the
>> run-of-the-mill part, and sort them.
>> You might decide to do the latter for competitive reasons, e.g. rather
>> than the mfr producing a better grade of part, and potentially selling
>> it to your competitors too, you keep the "secret sauce" in house.
>> (Granted you could have the mfr make/select a proprietary part for you..
>> that's basically changing who does the work, but doesn't change the
>> underlying design)
>> Even manufacturers do this, for instance with speed grades on things
>> like microprocessors. They don't have enough process control to
>> guarantee a particular speed, so they make em all, and then sort them.
>> The other thing is that the selection criteria might not be knowable in
>> a standalone sense. That is, you have to put the part into the circuit
>> and see if it works, rather than measuring some device parameter. I
>> would agree that to a certain extent, this implies that you don't really
>> know how the circuit works, but it might also be that the most cost
>> effective approach is to use empiricism, rather than analysis.
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