Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!zaphod.mps.ohio-state.edu!cis.ohio-state.edu!tut.cis.ohio-state.edu!ucbvax!WATSON.IBM.COM!jbs
From: jbs@WATSON.IBM.COM
Newsgroups: comp.arch
Subject: new instructions
Message-ID: <9105200213.AA05095@ucbvax.Berkeley.EDU>
Date: 20 May 91 01:22:01 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 49


        Herman Rubin writes:
Fixed point arithmetic is little used now because the hardware to support
it reasonably well does not exist.  It is worse than the floating problems
before hardware floating arithmetic, especially if floating is automatically
normalized.  THAT feature of "modern" architectures is, in my opinion, a sheer
horror.

        I think fixed point arithmetic is little used now because the vast
majority of users find it harder to use than floating point with no comp-
ensating advantagres.  Does anyone seriously believe if a few instructions
were added to provide hardware support (btw what is missing?  In what way
has fixed point support deteriorated?) fixed point usage would increase
significantly?  Regarding unnormalized floating point what is this good
for besides simulating multiple precision integer arithmetic?

        Herman Rubin also writes:
In the early FP computers, much function calculation was done in fixed point,
to get increased accuracy at little cost.

        This only makes sense when the floating point fraction length is
less than a full integer.  With 64-bit floating point there is little need
for increased accuracy in any case.

        Herman Rubin also writes:
How do you expect users who do not even know of the existence of the operations
to use them?

        I expect the compiler to generate the instructions for them.  If
the compiler won't generate an instruction this is a strong reason for
not having it in the instruction set.

        Herman Rubin also writes:
There are many more algorithms than are in the philosophy of software, and
especially hardware, designers.

        The argument here seems to be that there are numerous algorithms
which are not used at all today which suddenly would be used all over the
place if only a few changes were made to the instruction set.  I don't
agree.  If two algorithms have similar performance both will be in use as
there will be some problems which are particually suited to one or the
other.  If an algorithm is not used at all on today's machines this means
it is not competive even on those problems for which it is particually
suited.  Making changes to the instruction set is unlikely to drastical-
ly change the relative performance of two algorithms,  hence is unlikely
to drastically change the amount of usage each gets.  I believe it is
perfectly sensible for hardware designers to concentrate on speeding up
the existing mix of applications.
                          James B. Shearer