A processor(CPU) has a set of instructions for performing its functions (move, add, compare, jump, etc,etc). Since the first CPU used by IBM Pc (8088), every new Cpu used by Pc had some new instructions added (80286, 80386, 80486, Pentium, etc), many of them were very usefull and others had no use. I don't remember the year, Intel added to the Pentium Cpu the MMX instructions, so you had Pentium with MMX and Pentium without MMX. The goal of these instructions was helping multimidia or graphic tasks, for a word processor as Microsoft Word are completely useless. With them you can multiply 4 items at the same time. But very few software takes the advantage of them. Since Pentium II all Intel, Amd and other Cpus have them. And here ended all compatibility. With the K6-3 AMD added a new instruction set : 3dNow, that are similar to MMX, but deals with floating point, so AMD was in front of Intel. Some time later Intel included with their Pentium III the SSE instructions, that are similar to AMD's 3dNow, but not compatible. SSE2 instructions are an expansion of SSE and were included in the Pentium IV. I don't know if AMD added some expansion of their 3dNow instructions with the Athlon processor. But the lack of compatibility with AMD and Intel, prevents most the software makers of using them. If you have some technical knowledge about Cpu's hardware and software, you can access Intel's and Amd's site and download the documentation about them.

Stupidity also evolves!