A Thought on the Discussion of DEC Alphas

Dave Love d.love at dl.ac.uk
Mon Mar 14 13:10:37 EST 1994


>>>>> "Phil" == Phil Bourne <system at CUHHCA.HHMI.COLUMBIA.EDU> writes:

 Phil> In response to an article I wrote some time ago on items to consider
 Phil> in optimizing code for use on Alpha's I received this note re the
 Phil> use of IEEE arithmetic. Given the current discussion I thought I 
 Phil> would pass it on.. if you would like code to test this problem
 Phil> please let me know.. 
 Phil>          /Phil Bourne

 Phil> ============ Note from Layne ==================
 Phil> 	A real service to the DEC user community would be to point out
 Phil> that the AXP does NOT support fully the IEEE 754 Standard for
 Phil> floating point arithmetic.  In particular, programs using IEEE
 Phil> arithmetic on a DECstation, when ported to an Alpha (either via
 Phil> mx or via recompiling), do NOT produce the same results as on
 Phil> the DECstation!  If you want, I can send you a program which
 Phil> illustrates the different results on the MIPS RISC and the AXP.

If this is a worry, it's worth pointing out that Un*x boxes sometimes
(often?) don't implement IEEE 754 properly by default.  One reason is
that they may have to implement `gradual rounding' by traps
(i.e. slowly).  Before worrying about it, it would be interesting to
know what documentation claims in each case.




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