[Info-vax] VMS Software Q1 '23 Update

[Dave Froble]

On 1/31/2023 12:47 PM, John Reagan wrote:
> On Monday, January 30, 2023 at 7:32:10 PM UTC-5, Arne Vajhøj wrote:
>> On 1/29/2023 11:16 PM, Dave Froble wrote:
>>> On 1/29/2023 7:17 PM, Arne Vajhøj wrote:
>>>> 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.
>> I wonder what Basic on Itanium does and what Basic on x86-64 does / will do.
>>
>> Convert to T and do T math? Or software emulation?
>
> Just like Itanium.  Convert G to T (same size exponent) or convert D to T (and round
> 3 bits); operate on the T; convert back to G (or D) as needed.  Nobody complained
> on Itanium.
>
> For VAX to Alpha, VEST/TIE would actually emulate all 56 bits of the D exponents.
> VEST/TIE also emulated VAX H-float.  Native Alpha rounded 3 bits for D to G,
> nobody complained.

Not 100% accurate, but complaining would do no good, so adequate work arounds 
were developed.

It just expounds on the concept that if one wants accuracy, then, don't use FP. 
If one has no reasonable choice, then, you get what you get.

> For Alpha, most compilers went from F/D on VAX to F/G on Alpha.  However, BASIC
> has stuck with D-floating for DOUBLE on Alpha and on Itanium.  At this point, we
> will probably continue that mapping (although I think they shouldn't have stuck with D
> for all that time).
>

If one begins a journey, and instead of continuing, continues to go back and 
start over, one never reaches the goal.

I and others knew that use of D-float for money wasn't an optimum solution, but 
once going down that road, if workable solutions were in place, it wasn't 
reasonable to go back and start over.

I tried one time with a customer.  The suggestion was to convert all monetary 
data to another format, including historical data, programs, everything.  It 
wasn't going to be cheap.  I was asked what the benefit would be.  Basically, 
maybe a penny here, a penny there.  I was almost thrown out of the CEO's office. 
  I'd already wasted more of his time than whatever accuracy could be saved.  I 
learned that lesson real quick, and well.

-- 
David Froble                       Tel: 724-529-0450
Dave Froble Enterprises, Inc.      E-Mail: davef at tsoft-inc.com
DFE Ultralights, Inc.
170 Grimplin Road
Vanderbilt, PA  15486