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Chapter 3 General 64-Bit Optimizations 67
Software Optimization Guide for AMD64 Processors
25112 Rev. 3.06 September 2005
3.3 128-Bit Media Instructions and Floating-Point
Operations
Optimization
Use 128-bit media (SSE and SSE2) instructions instead of x87 or 64-bit media (MMX™ and
3DNow!™ technology) instructions for floating-point operations.
Rationale
In 64-bit mode, the processor provides eight additional XMM registers (XMM8–XMM15) for a total
of 16. These extra registers can substantially reduce register pressure in floating-point code written
using 128-bit media instructions.
Although the processor fully supports the x87 and 64-bit media instructions, there are only eight
registers available to these instructions (ST(0)–ST(7) or MMX0–MMX7, respectively).