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From: stoms@castor.ncgia.ucsb.edu (David Stoms)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Mixing Sane And 881
Message-ID: <4258@hub.UUCP>
Date: 10 Mar 90 21:28:50 GMT
References: <629.25F2006D@blkcat.fidonet.org> <25f44316.711e@polyslo.CalPoly.EDU>
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Reply-To: stoms@castor.ncgia.ucsb.edu (David Stoms)
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In article <25f44316.711e@polyslo.CalPoly.EDU> rcfische@polyslo.CalPoly.EDU (Raymond C. Fischer) writes:
>
>In article <629.25F2006D@blkcat.fidonet.org> Ken.Knight@f421.n109.z1.fidonet.org (Ken Knight) writes:
>>Can anyone tell me how to mix code so that it will be able to take best
>>advantage of the math system available. That is, if an FPU (881/2) is
>>available to use it; otherwise use SANE; or if the user wants to use
>wanted to, you could test for the 68882 and branch to the appropriate
>version of the routine, but this is generally nasty business.  Just
>one of the problems is that 68882 reals are a different format (and 
>different size) than SANE reals.

Actually it isn't that bad. Your compilers can be set to use 80-bit or
96-bit extended numbers which are actually have identical precision.
To convert between the two you have to know the format. The format
works like this:

96-bit: 96- exponent -80- empty    -63- mantissa -0
80-bit:               80- exponent -63- mantissa -0

To convert between the two just move the exponent around. It doesn't
matter wants in the empty space of 96-bit reals. You can write a nice
macro to do this in C. These two work in ThC 4.0:

#define _96to80(ext);	((extended96*)&(ext))->zero = ((extended96*)&(ext))->exp;
#define _80to96(ext);	((extended96*)&(ext))->exp = ((extended96*)&(ext))->zero;

They should work no matter how big the compiler thinks the numbers are
but if you want to pass an 80-bit real to a function and your compiler
is set for 96-bit reals then you have to use a macro like this:

#define passext80(ext)	(*(extended*)&((extended96*)&(ext))->zero)

Another dangerous pitfall is converting an 80-bit real to a 96-bit
real without declaring your varible 96-bits. This can happen if your
compiler is set to 80-bit real and you declare a standard extended.

Disclamer: This doesn't nessesarily apply to compilers other than
Think C 4.0.