From owner-freebsd-stable  Fri Jul  2  2:22:45 1999
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Date: Fri, 02 Jul 1999 10:22:07 +0100
From: Roger Hardiman <roger@cs.strath.ac.uk>
Organization: Strathclyde University
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To: Randall Hopper <aa8vb@ipass.net>
Cc: Frode Vatvedt Fjeld <frodef@acm.org>,
	"Daniel J. O'Connor" <darius@dons.net.au>,
	Geff Hanoian <boing@kusanagi.boing.com>, freebsd-stable@FreeBSD.ORG,
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Subject: Re: MMX
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> Frode Vatvedt Fjeld:
>  |What is the preferred way for my application to determine if the CPU
>  |is MMX-capable?
> 
> In leiu of that, I believe I recall reading that the SIGILL approach works
> on FreeBSD.  Seems like Roger Hardiman uses/used MMX on FreeBSD for some of
> his signal processing work.  You might chat with him.

Indeed, I do write MMX code.

The proper way, if you want your code to be portable to other OSs
(like NetBSD/OpenBSD/Linux/ etc), is to ask the CPU what it can do.

Here is the almost proper way to determine if a CPU has MMX support.
I say _alomost_ because it uses the CPU-ID instruction.
This instruction is not on 386 machines or some early 486 machines.
There is another test you should do first to determine if there was a
CPU-ID
instruction. I'll dig that up shortly.


// mmx.c
// Detect MMX Instruction set
// Roger Hardiman
// University of Strathclyde
// August 1997


// Test for the Presence of an MMX Instruction Set
// Returns TRUE if MMX CPU, FALSE if plain CPU.
int test_mmx() {
   int result;

        __asm __volatile(
        "movl $1,%%eax\n"
        "cpuid\n"
        "andl  $0x800000,%%edx\n"
        "shrl  $23,%%edx\n"
         : "=d" (result) :: "ax", "dx");

   if (result) printf("MMX detected\n");
   else printf("No MMX.\n");

   return result;
} 
                       

My program is then full of

if (mmx_detected) {
  MMX code
else
  old C code


Bye
Roger
--
Roger Hardiman
Strathclyde Uni Telepresence Research Group, Glasgow, Scotland.
http://telepresence.dmem.strath.ac.uk      0141 548 2897
roger@cs.strath.ac.uk


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