Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!usc!elroy.jpl.nasa.gov!ames!uhccux!virtue!hamish
From: hamish@waikato.ac.nz
Newsgroups: comp.sys.amiga.tech
Subject: Re: MC68881/2 Support (hello, Dave Haynie)
Message-ID: <664.266d2e99@waikato.ac.nz>
Date: 6 Jun 90 04:26:00 GMT
References: <1181@metaphor.Metaphor.COM> <11996@cbmvax.commodore.com>
Organization: University of Waikato, Hamilton, New Zealand
Lines: 59

In article <11996@cbmvax.commodore.com>, valentin@cbmvax.commodore.com (Valentin Pepelea) writes:
> In article <1181@metaphor.Metaphor.COM> djh@dragon.metaphor.com (Dallas J.
> Hodgson) writes:
>>
>> Since the FFP instructions trap out thru the FLINE vector anyway (if there's
>> no coprocessor present) why don't we EMULATE a 6888x when the traps occur?
> 
> Too much time would be spent decoding and emulating the coprocessor
> instructions. And what if someone plugs in a Weitek math coprocessor? The
> math coprocessors execute in 30 cycles what would take 3000 cycles otherwise.
> Emulating the instruction in software would take more than that. And what if
> the user has merely a 68000? Only the 68010 and higher processors have
> instruction suspension-completion mechanisms.

So what? We weren't talking about instruct re-start, but rather exception
processing. Any 68000 can emulate an FPU by doing fline processing for
you. It's the "Oh my god, there's a page fault, quick grab it from virtual
memory half way through an instruction" that the 68000 can't complete.
Different story altogether. 

> 
> The correct solution is to provide a shared library of math functions, and
> that's what we do. Those functions automatically take advantage what ever
> hardware the user has, thus the programmer does not have to worry about
> the configuration of the platform on which his software is about to run.
> 
> The 68040 does not have a floating point coprocessor, nor the coprocessor
> interface of the 68020 & 68030. But it implements the most used instructions
> in hardware, and lets a software emulate the remaining less used instructions.
> The 25MHz 68040 will therefore achieve a superior performance than a 33MHz
> 68030/68882. Now that makes sense.
> 

The 68040 DOES have a floating point coprocessor. The fact that its a subset
of the 68882, and shares the same piece of silicon, doesn't matter. It still
has 11 80 bit registers, 3 control registers and still looks like an ordinary
68881/2 to the user (except for the trig stuff which is emulated via a trap)

Compare this to the 68030. Are you going to say that it doesn't have an MMU,
even though its in the same boat? ie subset of 68851, on same piece of silicon.

> Valentin
> -- 
> The Goddess of democracy? "The tyrants     Name:    Valentin Pepelea
> may distroy a statue,  but they cannot     Phone:   (215) 431-9327
> kill a god."                               UseNet:  cbmvax!valentin@uunet.uu.net
>              - Ancient Chinese Proverb     Claimer: I not Commodore spokesman be

-- 
==============================================================================
|  Hamish Marson                        |  Internet  hamish@waikato.ac.nz    |
|  Computer Support Person              |  Phone  (071)562889 xt 8181        |
|  Computer Science Department          |  Amiga 3000 for ME!                |
|  University of Waikato                |                                    |
==============================================================================
|Disclaimer:  Anything said in this message is the personal opinion of the   |
|             finger hitting the keyboard & doesn't represent my employers   |
|             opinion in any way. (ie we probably don't agree)               |
==============================================================================