Included in Intel's Streaming SIMD Extensions were a number of new instructions that extended the functionality of MMX. AMD incorporated this subset of extended MMX instructions into the Athlon microarchitecture. These instructions are commonly known as "Extended MMX instructions".
These instructions debuted in May 1999, in the Intel Pentium III Processor, as part of the SSE instruction set. Next month, in late June 1999, AMD's Athlon processor was released which featured the extended MMX instructions, but not SSE.
Today, these extended MMX instructions are notable as being the common subset of MMX extensions that work across both AMD Athlon and SSE-capable Intel processors.[1]