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From: andrew@aimmi.UUCP (Andrew Stewart)
Newsgroups: net.micro.68k
Subject: Re: Sun-3 floating point
Message-ID: <691@aimmi.UUCP>
Date: Tue, 10-Dec-85 09:34:01 EST
Article-I.D.: aimmi.691
Posted: Tue Dec 10 09:34:01 1985
Date-Received: Fri, 13-Dec-85 08:46:32 EST
References: <196@gould9.UUCP>
Reply-To: andrew@aimmi.UUCP (Andrew Stewart)
Organization: Heriot-Watt/Strathclyde Alvey MMI Unit
Lines: 25
Xpath: hwcs brahma brahma

In article <196@gould9.UUCP> joel@gould9.UUCP (Joel West) writes:
>I understand two Sun-3 configurations are distinct in their 
>approach to floating point arithmetic:
>	* 68020 + 68881
>	* 68020 + Weitek (?) FP board
>The second is somewhat faster and is expected to be more popular, I'm told.
>
>Are executables for these two variants binary compatible, .....

Yes - as I understand it, the SUN software checks for the FPA board at
boot time. If it's there, it ships stuff over the fast local bus to the
FPA instead of the 68881 chip.
The clever bit is that the FPA doesn't need to emulate the 68881 - in certain
out-of-range cases, the FPA board gives up in disgust, so the CPU passes the
sum to the 68881 which gives the correct-according-to-IEEE answer.
Since this happens in a very small % of cases, the speed loss is trivial.
And the binaries are compatible. Neat.

-- 
-------------------------------------------
Andrew Stewart		 UUCP:	andrew@aimmi.UUCP
			 ARPA:	andrew@aimmi.ac.uk

"Once the subject is clearly understood,
	the explanation will seem sensible."