Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!apple!vsi1!wyse!mips!mark
From: mark@mips.COM (Mark G. Johnson)
Newsgroups: comp.arch
Subject: SPARC supports 80- & 128-bit floats.  So does MIPS.
Summary: Telephone story below is actually a fib; it's untrue.
Keywords: multiple-precision fp arithmetic is infrequent
Message-ID: <10452@obiwan.mips.COM>
Date: 31 Dec 88 16:59:41 GMT
References: <8561@alice.UUCP> <3688@s.cc.purdue.edu> <285@loligo.fsu.edu> <6132@ecsvax.uncecs.edu> <1080@l.cc.purdue.edu> <2339@garth.UUCP>
Reply-To: mark@mips.COM (Mark G. Johnson)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 30


The SPARC instruction set (hence the "architecture", not a particular
"implementation") includes opcodes for manipulating IEEE-standard
80-bit and 128-bit floating point numbers.  As I recall, the IEEE
standard calls them double-extended (80b) and quad (128b).

The MIPS instruction set includes them too.

However, in the current 16.7 MHz implementations of SPARC and MIPS,
these longer-precision instructions are not executed directly in hardware.
They trap, and the requested operation (128-bit divide etc) is done by
software.

We did a weird thing :-) at MIPS with these instructions.  Whenever one
of these instructions is encountered, the trap handler emulates the
instruction, AND it does something else.  :-)   It instructs the console
maintenance processor to activate its remote-diagnostics modem, which
dials a special phone number back at the factory, here at MIPS.  In fact
it dials a phone which sits on my desk :-).  The maintenance processor
logs the date and time, the serial number of the machine, and the type of
longer-precision operation that was requested :-).

So far, in over 2 years, the phone has never rung.  :-)  Thus it is
tempting to conclude that user programs do not suffer a serious
performance degradation from emulating longer-precision (80 and 128 bit)
floating point operations in software :-).
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	...!decwrl!mips!mark	(408) 991-0208