3DNow!: Расширение набора инструкций x86 от AMD

Extension to the x86 instruction set by AMD
3DNow! is a deprecated extension to the x86 instruction set developed by Advanced Micro Devices (AMD). It adds single instruction multiple data (SIMD) instructions to the base x86 instruction set, enabling it to perform vector processing of floating point vector operations using vector registers. This improvement enhances the performance of many graphics intensive applications. The first microprocessor to implement 3DNow! was the AMD K6 2, introduced in 1998. In appropriate applications, this enhancement raised the speed by about 2–4 times. However, the instruction set never gained much popularity, and AMD announced in August 2010 that support for 3DNow! would be dropped in future AMD processors, except for two instructions, PREFETCH and PREFETCHW. These two instructions are also available in Bay Trail Intel processors.

3DNow! was developed at a time when 3D graphics were becoming mainstream in PC multimedia and games. Realtime display of 3D graphics depended heavily on the host CPU's floating point unit (FPU) to perform floating point calculations, a task in which AMD's K6 processor was easily outperformed by its competitor, the Intel Pentium II. As an enhancement to the MMX instruction set, the 3DNow! instruction set augmented the MMX SIMD registers to support common arithmetic operations (add/subtract/multiply) on single precision (32 bit) floating point data. Software written to use AMD's 3DNow! instead of the slower x87 FPU could execute up to four times faster, depending on the instruction mix.

3DNow! Professional is a trade name used to indicate processors that combine 3DNow! technology with a complete SSE instructions set (such as SSE, SSE2 or SSE3). The Athlon XP was the first processor to carry the 3DNow! Professional trade name, and was the first product in the Athlon family to support the complete SSE instruction set (for the total of: 21 original 3DNow! instructions; five 3DNow! extension DSP instructions; 19 MMX extension instructions; and 52 additional SSE instructions for complete SSE compatibility).

One advantage of 3DNow! is that it is possible to add or multiply the two numbers that are stored in the same register. With SSE, each number can only be combined with a number in the same position in another register. This capability, known as horizontal in Intel terminology, was the major addition to the SSE3 instruction set. A disadvantage with 3DNow! is that 3DNow! instructions and MMX instructions share the same register file, whereas SSE adds 8 new independent registers (XMM0–XMM7). Because MMX/3DNow! registers are shared by the standard x87 FPU, 3DNow! instructions and x87 instructions cannot be executed simultaneously. However, because it is aliased to the x87 FPU, the 3DNow! and MMX register states can be saved and restored by the traditional x87 F(N)SAVE and F(N)RSTOR instructions. This arrangement allowed operating systems to support 3DNow! with no explicit modifications, whereas SSE registers required explicit operating system support to properly save and restore the new XMM registers (via the added FXSAVE and FXRSTOR instructions.) The FX* instructions from SSE provide a functional superset of the older x87 save and restore instructions. They can save not only SSE register states but also the x87 register states (hence are applicable also for MMX and 3DNow! operations where supported). On AMD Athlon XP and K8 based cores (i. e. Athlon 64), assembly programmers have noted that it is possible to combine 3DNow! and SSE instructions to reduce register pressure, but in practice it is difficult to improve performance due to the instructions executing on shared functional units.