[Info-vax] VMS Software Q1 '23 Update

[Arne Vajhøj]

On 1/29/2023 7:17 PM, Arne Vajhøj wrote:
> On 1/29/2023 5:41 PM, Dave Froble wrote:
>>       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.
> 
> The alternative is software emulation, which will be able to do VAX
> compatible D math but at a speed cost.
> 
> 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.

Experiment:

$ type d.bas
program d

declare double x, y, z

x = 12.34
y =  45.67
z = x + y

print z

end program
$ bas/noop/list/mach d
$ sear d.lis cvtdg,addg,cvtgd
57EA13CA     0060               CVTDG   F10, F10
57EB13CB     0068               CVTDG   F11, F11
554B140A     0070               ADDG    F10, F11, F10
57EA15AA     0074               CVTGD   F10, F10

Looks like G math to me.

(it does something smart with /OPT so that is why the /NOOP is used)

Arne