jfor at quik.com
Sat Aug 14 16:15:16 UTC 2010
DEC code was a nightmare. Any DG Nova line code would run on any machine.
> 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
>> 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
>> mode). If a different production run has improved normal mode
>> the avalanche function may vanish.
>>> J. Forster wrote:
>>>> FWIW, IMO any engineer who uses undocumented or uncontrolled
>>>> 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
>>> 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
>>> 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
>>> 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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