🚀 go-pugleaf

"Nick Maclaren" <nmm1@cus.cam.ac.uk> wrote in message
news:bla4k2$nas$1@pegasus.csx.cam.ac.uk...
> In article <Rh0eb.632709$Ho3.121617@sccrnsc03>,
> Glen Herrmannsfeldt <gah@ugcs.caltech.edu> wrote:
> >
> >Well, you would get SDR 2,2 and LE 2,A in that case.  The compiler does
have
> >to keep track of which registers have single and which have double
values.
> >Leaving low bits from previous calculations in would seem a bad idea.
> >Though according to other posts it may be legal in Fortran.
>
> It certainly is legal, provided they are from an appropriate calculation,
> but that is not what is going on here.
>
> The specification of IBM floating-point register operations is precisely
> that the unused low-order bits are ignored.  Those bits are therefore
> NOT being left in the value; they are being left OUTSIDE the value.
> That is why it is quite safe for single precision multiply to create
> them and to leave them where they are.
>
> Similarly, conversion from single to double precision has to clear those
> bits, whether the operand being converted has been created by multiply
> or anything else.

What I was saying is that if conversion from single to double did not clear
those bits you would get whatever was left there, possibly from an unrelated
calculation.   I believe that would be a bad idea, but some posts by others
seem to indicate that it would be legal Fortran.  Someone else posted:

|Item (6) listed in Section 1.4 of the Fortran standard explicitly
|states that the standard does not specify
|
|    (6)  The physical properties of the representation of quantities and
|         the method of rounding, approximating, or computing numeric
|         values on a particular processor.

The belief is that this statement could include leaving garbage bits in for
low bits.

-- glen