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From: carlton@aldebaran (Mike Carlton)
Newsgroups: comp.sys.next
Subject: Re: Bug in the 68040 chip
Message-ID: <11654@pasteur.Berkeley.EDU>
Date: 4 Mar 91 19:12:12 GMT
References: <1991Mar4.042515.4740@math.ucla.edu>
Sender: news@pasteur.Berkeley.EDU
Reply-To: carlton@aldebaran (Mike Carlton)
Organization: University of California at Berkeley
Lines: 47

In article <1991Mar4.042515.4740@math.ucla.edu> barry@pico.math.ucla.edu (Barry Merriman) writes:
>Don't be alarmed---but there appears to be a bug in the 68040
>chip. It does not compute the hyperbolic tangent function
>correctly. 
>
>To see this for yourself, run the following bit of C code
>(donated by Steve Wolfram) on a 68040 Next:
>
>printf("%g %g\n", tanh(-1.73287), tanh(-1.73288));
>
>and you will see a clear discontinuity in the values.
>
>At best this would indicate the C math library for the 68040
>has a bad tanh(x). But I think (not sure though) that the
>68040 itself computes the tanh(x)---in which case the bug
>is at a deeper level. 
...
>Barry Merriman
>UCLA Dept. of Math
>UCLA Inst. for Fusion and Plasma Research
>barry@math.ucla.edu (Internet)

I believe that tanh is handled in software, not hardware, so at
least it is possible to fix this.  The following came across
comp.sys.next a while ago:

--The 040 floating point hardware instructions are
--        FADD, FSUB, FMUL, FDIV, FSQRT, FMOVE (most addressing modes), 
--        FABS, FNEG, FMOVEM, FCMP, FSAVE, FScc, FDBcc, FBcc, FRESTORE
--
--The 040 floating point software instructions are
--        FACOS, FASIN, FATAN, FATANH, FCOS, FCOSH, FETOX, FETOXL, FGETEXP,
--        FGETMAN, FINT, FINTRZ, FLOG10, FLOG2, FLOGN, FLOGNP1, FMOD, FMOVECR,
--        FREM, FSCALE, FSIN, FSINCOS, FSINH, FTAN, FTANH, FTENTOX, FTWOTOX
--
--Hope this helps.
--
--                --morris
--
--Morris Meyer          NeXT, Inc.
--Software Engineer     900 Chesapeake Drive
--NeXT OS Group         Redwood City, CA   94063
--mmeyer@next.com

Cheers,
--mike
Mike Carlton	carlton@cs.berkeley.edu