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Re: NON FPU cpus - way to go

To: Florian Lohoff <>
Subject: Re: NON FPU cpus - way to go
From: "Maciej W. Rozycki" <>
Date: Wed, 7 Feb 2001 19:19:45 +0100 (MET)
In-reply-to: <>
Organization: Technical University of Gdansk
On Wed, 7 Feb 2001, Florian Lohoff wrote:

> The problem with not compiling in the FPU Emulator at all means some
> of your FPU instructions (even on FPU hardware) will fail as on some
> specific operators the hardware decides to handle it in software. So
> usually you would need an FPU Emulator even on FPU enabled CPUs.

 I mean a full emulator.  I know that for simplicity certain actions
required by the IEEE spec are handled in software (Alpha does it as well). 
These bits have to be always included, of course.  I would like to save
wasted bits for hardware that always has an FPU, though.

> This isnt true if you decide to compile your complete userland with
> fpu emulation.

 I'm not sure if that approach has any advantages when using an operating
system such as Linux.  It might certainly be beneficial for firmware or
similar dedicated software.

> I dont know if this is a generic way to go - I saw complete "full-stops"
> on an R3912 using the ctc/cfc instructions - I'll try the autodection
> when i come home.

 We might work around pathological cases as usual -- such a behaviour
should count as a bug (I hope IDT did have a clue here -- is there any
original MIPS statement on how to handle FPU presence detection?).  You
might use the i386 setup code for a reference as a large mine of bug


+  Maciej W. Rozycki, Technical University of Gdansk, Poland   +
+        e-mail:, PGP key available        +

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