[time-nuts] Simulation

J. Forster 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.

-John

=============


> Hi
>
> 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....
>
> Bob
>
>
>
> 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
>> parts.
>>
>> 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.
>>
>> FWIW,
>>
>> -John
>>
>> ===============
>>
>>
>>
>>> J. Forster wrote:
>>>> FWIW, IMO any engineer who uses undocumented or uncontrolled
>>>> parameters
>>>> or
>>>> instructions in a production design is a fool.
>>>>
>>>> If you are that silly, you must fully specify the selection criteria.
>>>>
>>>> -John
>>>>
>>>
>>> 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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>
>






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