Streaming SIMD Extensions

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In computing, Streaming SIMD Extensions (SSE) is a SIMD instruction set extension to the x86 architecture, designed by Intel and introduced in 1999 in their Pentium III series processors as a reply to AMD's 3DNow! (which had debuted a year earlier). SSE contains 70 new instructions.

It was originally known as KNI for Katmai New Instructions (Katmai being the code name for the first Pentium III core revision). During the Katmai project Intel was looking to distinguish it from their earlier product line, particularly their flagship Pentium II. It was later renamed ISSE, for Internet Streaming SIMD Extensions, then SSE. AMD eventually added support for SSE instructions, starting with its Athlon XP and Duron (Morgan core) processors.

Intel's first IA-32 SIMD effort, MMX, had two main problems: it re-used existing floating point registers making the CPU unable to work on both floating point and SIMD data at the same time, and it only worked on integers.

Contents

Registers

SSE originally added eight new 128-bit registers known as XMM0 through XMM7. The AMD64 extensions from AMD (originally called x86-64 and later duplicated by Intel) add a further eight registers XMM8 through XMM15. There is also a new 32-bit control/status register, MXCSR. The registers XMM8 through XMM15 are accessible only in 64-bit operating mode.

Each register packs together:

  • four 32-bit single-precision floating point numbers or
  • two 64-bit double-precision floating point numbers or
  • two 64-bit integers or
  • four 32-bit integers or
  • eight 16-bit short integers or
  • sixteen 8-bit bytes or characters.

The Integer operations have instructions for signed and unsigned variants. Integer SIMD operations may still be performed with the eight 64-bit MMX registers.

Because these 128-bit registers are additional program states that the operating system must preserve across task switches, they are disabled by default until the operating system explicitly enables them. This means that the OS must know how to use the FXSAVE and FXRSTOR instructions, which is the extended pair of instructions which can save all x86 and SSE register states all at once. This support was quickly added to all major IA-32 operating systems.

Because SSE adds floating point support, it sees much more use than MMX. The addition of SSE2's integer support makes SSE even more flexible. While MMX is redundant, operations can be operated in parallel with SSE operations offering further performance increases in some situations.

The first CPU to support SSE, the Pentium III, shared execution resources between SSE and the FPU. While a compiled application can interleave FPU and SSE instructions side-by-side, the Pentium III will not issue an FPU and an SSE instruction in the same clock-cycle. This limitation reduces the effectiveness of pipelining, but the separate XMM registers do allow SIMD and scalar floating point operations to be mixed without the performance hit from explicit MMX/floating point mode switching.

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