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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.arch
Subject: Re: is Vax addressing sane today
Date: Sat, 21 Sep 2024 10:56:24 +0300
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On 2024-09-21 4:51, MitchAlsup1 wrote:
> On Sat, 21 Sep 2024 1:12:11 +0000, Lawrence D'Oliveiro wrote:
> 
>> On Fri, 20 Sep 2024 18:35:26 -0000 (UTC), Kent Dickey wrote:
>>
>>> 3) You want to clamp the value to a reasonable range and continue.  The
>>>     reasonable values need to be looked up somewhere.
>>
>> This won’t work. The values outside the range are by definition non-
>> representable, so comparisons against them are useless.
> 
> When a range is 0..10 both -1 and 11 are representable in
> the arithmetic of ALL computers, just not in the language
> specifying the range.


For "11" I agree, for "-1" disagree.

if the program was written (in whatever language) with the assumption 
that the data type in question is unsigned, then it cannot represent -1 
in the program's view of the bits. The bits that represent -1 in a 
signed two's complement view represent a large positive value in the 
unsigned view that the code uses.

Now if the error condition that was trapped or detected was an attempt 
to produce a negative value like -1 for an unsigned data type, that 
error condition is of course representable separately; it does not have 
to be encoded by an out-of-range value in the data type itself.