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Date:      Wed, 24 Jan 1996 16:01:07 +0530
From:      A JOSEPH KOSHY <koshy@india.hp.com>
To:        asami@cs.berkeley.edu (Satoshi Asami)
Cc:        hackers@freebsd.org
Subject:   Re: Pentium bcopy 
Message-ID:  <199601241031.AA281999468@fakir.india.hp.com>
In-Reply-To: Your message of "Wed, 24 Jan 1996 01:19:34 PST." <199601240919.BAA28229@silvia.HIP.Berkeley.EDU> 

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>>> sa == Satoshi Asami said:

sa> Does anyone know if there are any `gotchas' concerning the use of fp
sa> regs in the kernel?

Are the FPU registers saved and restored as part of interrupt handling?
I'm not sure where to look but /usr/src/sys/i386/i386 doesn't seem to
have code to do this.  Am I looking at the wrong place?

You would need to ensure this if you are using your FP-enabled bcopy from 
any interrupt routine.

Blindly saving all FP registers when 'bcopy' is invoked has its own
cost, so you probably need to use the FP registers method only if the
amount of data to be copied is large.  You may need to experiment
and determine the best size to switch from regular bcopy to the FP version.

Also you need to be sure that the FP registers are accessible.  A machine
with a 486SX or a plain 386 cannot use this technique.  Indeed on a 387
the technique could even be slower than a rep movsl.

Lots of tradeoffs here :).

Koshy



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