Help needed w/Mc68881

[Mike "Ford" Ditto]

In article <1143 at csustan.UUCP> robert at csustan.UUCP (Robert Zeff) writes:
>
>  Hi.  I have a Tektronix 4301 (BSD4.2) with GreenHill C compiler
>and need to put the 68881 coprocessor into extended precision mode.
>  The function "setfp ()" is suppossed to be able to change the 68881
>to single (24 bit), double (53 bit) or extended (64 bit) precision
>mode.  I can set it to single or double, but setting it to extended
>still gives me 53 bit precision.  Anyone have a clue?

Assuming that this setfp function sets the "rounding precision" of the
881, this just affects the accuracy with which the numbers will be
stored into floating-point registers.  The calculations are *always*
done in extended precision, and if you are writing in C, the results
are *always* returned in single or double precision.

The purpose of the rounding precision feature of the 881 is to allow
correct execution of software which can not deal with the "extra"
accuracy of the 68881 (IEEE only specifies the "double" precision; the
fact that the 881 internally does calculations with more accuracy is a
"bonus" that actually breaks some programs).

If you are trying to define an extended precision data type in C, you
will have to modify your compiler.


					-=] Ford [=-

"Once there were parking lots,		(In Real Life:  Mike Ditto)
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