Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!mailrus!ncar!hao.ucar.edu!moses
From: moses@hao.ucar.edu (Julie Moses)
Newsgroups: comp.sys.atari.st.tech
Subject: Floating Pt. Math with a 68881 not always faster
Message-ID: <8379@ncar.ucar.edu>
Date: 1 Sep 90 23:00:09 GMT
Sender: news@ncar.ucar.edu
Reply-To: moses@hao.ucar.edu (Julie Moses)
Organization: High Altitude Observatory/NCAR, Boulder CO
Lines: 23


     I have recently installed a math copressor into my Mega system
and have now linked my code with libraries supporting a 68881 chip 
in a ST. 
     While more complex math functions, such as transcendentals and 
square root really show a speed increase, adding or multiplying two
floating point numbers do not show any speed increase over just
doing it with the 68000 chip. Matter of fact some of my functions
that do repetitive mulitply and additions with floats (without more
complex functions) are significantly slower when using the math 
copressor. 

Q- Can someone explain why?

     My best guess is that the time the processor takes to transfer
the floating point numbers and also the time waiting for the math
copressor to be ready to receive makes the simpler math operations
slower than just doing them inside the 68000 chip. I am using the
Prospero math libraries that look once for the math chip so that one
must link for either a 68K chip with or without the math chip, 
exclusively.

Julie Moses