In February 1999, Intel introduced the Pentium III processor and included in that processor an update to MMX called Streaming SIMD Extensions (SSE). These were also called Katmai New Instructions (KNI) up until their debut because they were originally included on the Katmai processor, which was the codename for the Pentium III. The Celeron 533A and faster Celeron processors based on the Pentium III core also support SSE instructions. The earlier Pentium II and Celeron 533 and lower (based on the Pentium II core) do not support SSE.

SSE includes 70 new instructions for graphics and sound processing over what MMX provided. SSE is similar to MMX; in fact, besides being called KNI, SSE was also called MMX-2 by some before it was released. In addition to adding more MMX style instructions, the SSE instructions allow for floating-point calculations and now use a separate unit within the processor instead of sharing the standard floating-point unit as MMX did.

SSE2 was introduced in November 2000, along with the Pentium 4 processor, and adds 144 additional SIMD instructions. SSE2 also includes all the previous MMX and SSE instructions.

SSE3 was introduced in February 2004, along with the Pentium 4 Prescott processor, and adds 13 new SIMD instructions to improve complex math, graphics, video encoding, and thread synchronization. SSE3 also includes all the previous MMX, SSE, and SSE2 instructions.

The Streaming SIMD Extensions consist of new instructions, including SIMD floating-point, additional SIMD integer, and cacheability control instructions. Some of the technologies that benefit from the Streaming SIMD Extensions include advanced imaging, 3D video, streaming audio and video (DVD playback), and speech-recognition applications. The benefits of SSE include the following:

• Higher resolution and higher quality image viewing and manipulation for graphics software

• High-quality audio, MPEG2 video, and simultaneous MPEG2 encoding and decoding for multimedia applications

• Reduced CPU utilization for speech recognition, as well as higher accuracy and faster response times when running speech-recognition software

The SSEx instructions are particularly useful with MPEG2 decoding, which is the standard scheme used on DVD video discs. SSE-equipped processors should therefore be more capable of performing MPEG2 decoding in software at full speed without requiring an additional hardware MPEG2 decoder card. SSE-equipped processors are much better and faster than previous processors when it comes to speech recognition, as well.

One of the main benefits of SSE over plain MMX is that it supports single-precision floating-point SIMD operations, which have posed a bottleneck in the 3D graphics processing. Just as with plain MMX, SIMD enables multiple operations to be performed per processor instruction. Specifically, SSE supports up to four floating-point operations per cycle; that is, a single instruction can operate on four pieces of data simultaneously. SSE floating-point instructions can be mixed with MMX instructions with no performance penalties. SSE also supports data prefetching, which is a mechanism for reading data into the cache before it is actually called for.

Note that for any of the SSE instructions to be beneficial, they must be encoded in the software you are using, so SSE-aware applications must be used to see the benefits. Most software companies writing graphics-and sound-related software today have updated those applications to be SSE aware and use the features of SSE. For example, high-powered graphics applications such as Adobe Photoshop support SSE instructions for higher performance on processors equipped with SSE. Microsoft includes support for SSE in its DirectX 6.1 and later video and sound drivers, which are included with Windows 98 Second Edition, Windows Me, Windows NT 4.0 (with service pack 5 or later), Windows 2000, and Windows XP.

SSE is an extension to MMX; SSE2 is an extension to SSE; and SSE3 is an extension to SSE2. Therefore, processors that support SSE3 also support the SSE2 instructions, processors that support SSE2 also support SSE, and processors that support SSE also support the original MMX instructions. This means that standard MMX-enabled applications run as they did on MMX-only processors.

The first AMD processors to support SSE3 are the 0.09-micron versions of the Athlon 64 and all versions of the dual-core Athlon 64 X2.