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Date: Thu, 28 Mar 2013 13:02:17 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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Subject: Re: [Qemu-devel] [RFC PATCH 1/3] target-i386: Add 486sx, old486,
 and old486sx CPU models
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To: Aurelien Jarno <aurelien@aurel32.net>
Cc: qemu-devel@nongnu.org

On 03/28/2013 12:15 PM, Aurelien Jarno wrote:
> 
> This really looks like Linux kernel specific. I haven't been able to
> test on a real machine, but the documentation I have found suggest that
> without and x87 FPU, the FPU instructions are simply ignored. The common
> way to detect an FPU is therefore to initialize registers to a given
> value, run fnstsw and fnstcw instructions with the register in arguments
> and see if they have been modified.
> 
> The Linux kernel indeed set the initial value of these registers to
> 0xffff, but I am not sure all codes are doing the same.
> 
> For me it looks like better to skip such instructions directly in
> translate.c. As a bonus it seems easy to do that for all FPU
> instructions.
> 

At least *some* real-life CPUs returned 0xffff, at the very least the
machines with external buses did (e.g. 386 without 387).  I don't have
access to a live 486SX so I can test if 486SX behaved differenly.

	-hpa