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Date: Wed, 23 Jul 2003 16:18:59 -0400
From: Hyman Rosen <hyrosen@mail.com>
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Subject: Re: Ariane5 FAQ
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Date: 2003-07-23T16:18:59-04:00
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Robert I. Eachus wrote:
> As I said, this was not arbitrary behavior, it was required behavior.

*What* was required behavior? You have a piece of code that says
     integer_BH = convert(float_BH)
The analysis and specification of the Ariane 4 gave a physical
upper limit to float_BH, and the code was written in the "don't
care" way - if for some reason float_BH does exceed the limit,
let the code go ahead and do whatever the consequences of
violating that limit imply. If float_BH is larger than the limit
but smaller than the overflow value, the code keeps going,
possibly failing at a later point or possibly not causing any
harm. If float_BH is larget than the overflow value, the
machine generates an operand fault.

There's nothing wrong with having code like that if the
situation warrants it, which was the case in Ariane 4, where
they were trying to save the machine cycles that a limit
check would have cost. It's just that this kind of code is
brittle, so these dependencies on the state of external
data need to be made very clear, otherwise future reuse
attempts will stumble.