Subject: x87 floating-point overlap with MMX/SSE
To: None <,>
From: Thor Lancelot Simon <>
List: current-users
Date: 07/20/2001 17:30:50
On Fri, Jul 20, 2001 at 04:40:32PM -0400, Bill Sommerfeld wrote:
> The MMX registers (introduced in pentium mmx and pentium II) are
> *architecturally defined* to overlap with the x87 floating point
> stack.
> With the Pentium III, Intel introduced *another* extension (SSE,
> sometimes referred to as MMX2 or XMM), with new registers which do not
> overlap; NetBSD does not yet save these registers.  This extension is
> not MMX.
> Intel subsequently extended SSE to SSE2 in the pentium 4.

Interestingly, SSE2 includes register-to-register scalar floating point
operations -- in other words, with SSE2, you can ditch the hoary old
stack-based x87 floating point architecture alltogether.  SSE2 is really
a whole new FPU architecture for the x86.  With kernel and compiler support,
you can achieve significantly better floating-point performance with SSE2
than with anyone's implementation of x87 stack floating point; this is
one of the major wins of the much-misunderstood Pentium 4.

Thor Lancelot Simon	                            
    And now he couldn't remember when this passion had flown, leaving him so
  foolish and bewildered and astray: can any man?
						   William Styron