[Info-vax] VMS Software Q1 '23 Update

[Dave Froble]

On 1/29/2023 7:17 PM, Arne Vajhøj wrote:
> On 1/29/2023 5:41 PM, Dave Froble wrote:
>> On 1/29/2023 2:41 PM, John Dallman wrote:
>>> In article <tr6dje$2s9rm$1 at dont-email.me>, arne at vajhoej.dk (Arne Vajhøj)
>>> wrote:
>>>> Didn't VAX to Alpha migration cause some issues with D floating?
>>>>
>>>> If I remember correct then Alpha did not fully support D.
>>>
>>> It didn't support D at all, as best I know. VAX had:
>
>>> D format is double-precision, with 8 exponent bits, 1 sign bit, and 55
>>> fraction bits, approximately plus or minus 2.9E-39 to plus or minus
>>> 1.7E+38, with a precision of approximately 16 decimal digits. It's F
>>> format with more precision, but no more range. This format was not
>>> carried forwards into Alpha, and does not seem to have been much used.
>>>
>>> G format is also double-precision, with 11 exponent bits, 1 sign bit, and
>>> 52 fraction bits, approximately plus or minus 5.6E-309 to plus or minus
>>> 0.9E+308, with a precision of approximately 15 decimal digits. The
>>> exponent has a bias of 1,024 (not 128). This format was carried forwards
>>> into Alpha.
>
>>       gfloat, sfloat, tfloat, and xfloat.  These  formats  correspond  to  the
>>       SINGLE, DOUBLE, GFLOAT, SFLOAT, TFLOAT, and XFLOAT keywords.
>>
>>       Keyword             Range                                  Precision
>
>>       DOUBLE (64-bit)    .29 * 10^-38 to 1.7 * 10^38             16 digits
>>       GFLOAT (64-bit)    .56 * 10^-308 to .90 * 10^308           15 digits
>
>> Now, it has been my impression that that DOUBLE is D-float.
>>
>> Walks like a D-float
>> Quacks like a D-float
>> Is it a D-float?
>
> Good question.
>
> It can certainly take D float as input and provide D float
> as output.
>
> The question is how.
>
> The Alpha architecture manual says that it only
> supports:
> - load D from memory
> - convert from D to G
> - do G math
> - convert back from G to D
> - store D in memory
> which sort of operate on D float but loose 3 bits precision.

Correct, that is how it's provided.

> The alternative is software emulation, which will be able to do VAX
> compatible D math but at a speed cost.

Would you think it would do better than the loss of the 3 bits?  Not an issue 
most of the time.  However,  I'm aware of a few instances where things aren't so 
good.  I seem to recall the 5.9 to 6.0 range having an issue.

But, it was rather nice to move massive amounts of data from a VAX and have it 
"just work" on Alpha.

> So either hardware G math loosing 3 bits of precision (which is what
> I assume when I asked the question above) or software full D math.
>
> Arne
>
>


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David Froble                       Tel: 724-529-0450
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