uunet!cwi.nl!Dik.Winterauunet.uu.netRe: Extended precision

[uunet!ichips.intel.com!jmarshal]

	 > The x87 is 100% IEEE compatible in its default mode.
	
	Not having access to my references I wonder, but I think not.  Giving your
	example (which by coincidence exactly matches my example as I posted
	earlier), there are double precision numbers a and b such that when x
	is double:
	  x = a + b
	will not yield the nearest result when using the default mode.
	It gives 2^53+4, nearest would be 2^53+2.
	
	dik

Actually, if x is a double and the x87 precision control word is set to
double precision, the x87 architecture will deliver a 2^53+2 IEEE 
result. If the precision control word is set to extended precision, then
the 2^64+2(depending on operands) rounding from the arithmetic calculation
happens, then a 2^53+2 rounding will happen on the double store to
memory. The double rounding on the x87 architecture is controllable, thus
the x87(387 and above) architecture is compliant to the IEEE 754
specification.


							Jon