Re: Ariane5 FAQ

[Vinzent Hoefler <ada.rocks@jlfencey.com>]

Hyman Rosen wrote:

>If the value of the parameter had exceeded the Ariane 4 maximum but
>was less than the 16-bit maximum, there would have been no exception
>at all at the conversion point, and the too-high value would have
>gone through the rest of the system, doing something else.

It would have handled that value as correct reading from the sensor.

I don't see, how someone can avoid such problem. I mean, if, for
instance, I read an indoor-temperature sensor, it might be reasonable
to assume, that the read value will always be below 128 degrees
(Celsius) if the sensor is working, so that I can use a 8-bit-value to
store the reading. Now what, if we want to protect against a possible
overflow? How would you handle that?
Personally I think, just reading the value and calculating with it,
even if the reading says 95 degrees is a quite reasonable decision.
And it is also a very reasonable decision to say: If *that* reading
overflows my 8 bits within I can handle it, *then* the sensor must
definitely be broken. The room cannot get that warm. [Except in case
of fire perhaps... ;)].
So even if you specify an arbitrary lower limit (let's say, 60 degrees
Celsius) as reasonable maximum reading, you will *always* get some
sort of grey range, where you cannot really tell if the sensor is
broken or its reading is indeed correct. So what do you do, if you set
the reasonable limit to 60 and you read 59? Assume that the value read
is ok or already assume that the sensor must be broken? What, if you
then lower this "reasonable" limit accordingly?

Another example: If I count the flying time of an aircraft in a 32 Bit
value in seconds, I can tell with a very high probability, if that
counter overflows it *must* be an error, because no plane would be
able to fly 130+ years in a single session. But the decision for a
certain point when a particular flying time (24 hours?, 48 hours?,
three weeks?) makes sense or not is indefinitely harder to make.


Vinzent.

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