Streaming SIMD Extensions 2 (SSE2) SSE2 was first introduced on the Intel Pentium 4, and are also known sometimes as ”Willamette” instructions. These instructions are very similar to the SSE instructions in structure, but allow us considerably more flexibility in crunching numbers. The biggest differences between SSE and SSE2 were the ability to deal with double-precision, or 64bit, floating-point values as well as with 32bit ones, along with the ability to now work on 128bit integer types in XMM registers as well. In total, 144 new instructions were added. One instruction that was introduced with SSE2 is the CLFlush instruction. This instruction has its own bit in the cpuid feature flag which must be checked before using it, even if the CPU supports SSE2. See cpuid for details.