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From: breuel@h-sc1.UUCP (thomas breuel)
Newsgroups: net.micro.mac
Subject: Re: Processing Speed?
Message-ID: <891@h-sc1.UUCP>
Date: Wed, 29-Jan-86 05:19:09 EST
Article-I.D.: h-sc1.891
Posted: Wed Jan 29 05:19:09 1986
Date-Received: Sat, 1-Feb-86 00:26:20 EST
References: <106@eyelab.UUCP>
Organization: Harvard Univ. Science Center
Lines: 44

|    One of the assumptions I made before purchasing the Mac
|was that the 68000 could whiz through numerical calculations
|and impress my friends with its speed.  Even pardoning
[...]
|    The programs are primarily matrix manipulations using
|double precision arrays.  The PDP runs a representitive
|program in 75 seconds, the Mac takes 4000 seconds (thats over
|an hour!!).  I used Aztec C's 1.00D compiler, purportedly one
|of the faster version of 'C' for the Mac (I'd hate to use a
|slower one).  At least the code was portable, didn't have to
|change a line!
|    Now I realize that the 11/44 has floating point hardware
|and is in an entirely different class, but a 50 fold decrease in
|speed is a little hard to believe.  With the recent revelation

First of all, for certain applications, a PDP-11/44 seems to be
comparable to a VAX 11/750. The problem with the PDP series is not
speed (it seems), but rather the limited address space.

I do not see why you are not willing to accept that FP hardware
can give you a 50 fold performance increase over software floating
point. The easiest way to answer your question about relative performance
might be to use software floating point on your PDP as well, or
to compare the two machines in integer math. They'll probably perform
about equally well. Also note that Aztec 'C' *might* be using 80 bit
FP ('SANE') everywhere (as compared to 32bit floats and 64bit doubles on
the PDP). I don't know; that is something you should check before
comparing the two. Finally, your compiler for the PDP might be vastly
superior to the Aztec compiler. The Aztec compiler is a decent product
from what I hear, but the UN*X PDP-11 'cc' (and probably the DEC 'C' compiler,
judging from the quality of they VAX-11 'C' compiler) is quite good
at optimisation.

Now, since your primary concern is obviously to get your calculations
done and not to make the Mac look bad, there are several possible
solutions: (1) use integer math (yes, I know that it takes more thought
and work, but it runs soooo much faster), (2) get an FP co-processor for
the Mac, (3) get a different machine.

Personally, I try to do any numerical calculations that I have to do (even
DEQ's &c.) with integer math. I care much more about lots of memory than
about FP speed...

						Thomas.