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.

						- Bill