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From: guy@sun.uucp (Guy Harris)
Newsgroups: comp.arch
Subject: Re: How does compiled code use the floating point unit?
Message-ID: <10249@sun.uucp>
Date: Tue, 9-Dec-86 12:19:31 EST
Article-I.D.: sun.10249
Posted: Tue Dec  9 12:19:31 1986
Date-Received: Wed, 10-Dec-86 04:18:49 EST
References: <394@houxs.UUCP> <164@hqda-ai.UUCP>
Organization: Sun Microsystems, Inc.
Lines: 33
Summary: No, Sun didn't do anything like that

> Anyone (particularly old Sun engineers) care to correct my memory?

Yes.  The Sun-2 did, in fact, have an FPU option, namely a Sky floating
point board.  However, unless we did so in release 1.x, Sun NEVER generated
code to trap to the OS.  In 2.0, I think the compiler generated subroutine
calls for floating-point operations.  By default, the subroutines either
jumped to software floating-point routines or to Sky floating-point
routines, depending on whether the program detected that there was a Sky
board on the machine when it started (the C startup code did this check).
An option could tell the compiler to generate direct calls to the Sky
routines.

In 3.0 and later releases, the same scheme was used to support the 3
possibilities for Sun-3 floating point support: no hardware, MC68881, and
FPA.  There were now options to tell the compiler to generate code to call
the "switched" floating-point routines (that used the information on what
hardware was present to choose which routines to jump to), to call the
software routines directly, to call the Sky routines, to use the 68881
instructions, or to use the FPA "instructions" ("move"s, etc. to the FPA
registers).

Since it took *several* 68010 instructions to make the Sky board perform a
floating-point operation (unless you're talking about the 68881 or, in some
cases, the FPA, there aren't any single-instruction floating-point
operations on Suns; there certainly weren't any on the Sun-2), you couldn't
just replace a TRAP with the operation in question.  And, since many Sun-2s
didn't have a Sky board, the overhead of doing floating point in the OS
would have been prohibitive in most cases, so the OS certainly wouldn't have
done floating-point computations if there wasn't a Sky board.
-- 
	Guy Harris
	{ihnp4, decvax, seismo, decwrl, ...}!sun!guy
	guy@sun.com (or guy@sun.arpa)