Timing of PDP-11 Instructions
hilpert at cs.ubc.ca
Sun Nov 27 01:55:59 CST 2005
The other way of parsing Jerome's message is that he wants to *use* the
reciprocal *to calculate* the log,
log(x) = F( 1/x )
as opposed to taking the log of the reciprocal ( log(1/x) ).
F() is a rather basic anti-derivative function in calculus, but I've
forgotton too much calculus to remember how to calculate it.
Jim Battle wrote:
> Jerome H. Fine wrote:
> > Hi All,
> > Background: I want to calculate the reciprocal (inverse) of a
> > number between 1 and 65535 to an accuracy of 256 bits after
> > the binary (or decimal) point. This result will then be used to
> > calculate the logarithm of that value which in turn will be used
> > to calculate li(x) for values of x up to 10**38. The code is in
> > FORTRAN 77 (could also be in FORTRAN IV) which calls
> > MACRO-11 code to do the really low level stuff such as
> > repeated addition of many words with MANY Adc instructions
> > following each addition (99.9% of the time the Adc instructions
> > are never used).
> Jerome, I didn't read all of this, but I have to ask: why do you need to compute
> the reciprocal at all?
> log(x^y) = y*log(x)
> in this case, y=-1, so
> log(1/x) = -log(x)
> accuracy is limited to your log computation, rather than being limited by your
> reciprocal and your log approximations.
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