Streaming SIMD Extensions

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In computing, Streaming SIMD Extensions (SSE) is a single instruction, multiple data (SIMD) instruction set extension to the x86 architecture, designed by Intel and introduced in 1999 in their Pentium III series of Central processing units (CPUs) shortly after the appearance of Advanced Micro Devices (AMD's) 3DNow!. SSE contains 70 new instructions, most of which work on single precision floating-point data. SIMD instructions can greatly increase performance when exactly the same operations are to be performed on multiple data objects. Typical applications are digital signal processing and graphics processing.


Intel's first IA-32 SIMD effort was the MMX instruction set. MMX had two main problems: it re-used existing x87 floating-point registers making the CPUs unable to work on both floating-point and SIMD data at the same time, and it only worked on integers. SSE floating-point instructions operate on a new independent register set, the XMM registers, and adds a few integer instructions that work on MMX registers.

SSE was subsequently expanded by Intel to SSE2, SSE3, and SSE4. Because it supports floating-point math, it had wider applications than MMX and became more popular. The addition of integer support in SSE2 made MMX largely redundant, though further performance increases can be attained in some situations[ when? ] by using MMX in parallel with SSE operations.

SSE was originally called Katmai New Instructions (KNI), Katmai being the code name for the first Pentium III core revision. During the Katmai project Intel sought to distinguish it from their earlier product line, particularly their flagship Pentium II. It was later renamed Internet Streaming SIMD Extensions (ISSE [1] ), then SSE. AMD eventually added support for SSE instructions, starting with its Athlon XP and Duron (Morgan core) processors.


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

XMM registers.svg

SSE used only a single data type for XMM registers:

SSE2 would later expand the usage of the XMM registers to include:

Because these 128-bit registers are additional machine 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 that can save all x86 and SSE register states at once. This support was quickly added to all major IA-32 operating systems.

The first CPU to support SSE, the Pentium III, shared execution resources between SSE and the floating-point unit (FPU). [1] 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.

SSE instructions

SSE introduced both scalar and packed floating-point instructions.

Floating-point instructions

Integer instructions

Other instructions


The following simple example demonstrates the advantage of using SSE. Consider an operation like vector addition, which is used very often in computer graphics applications. To add two single precision, four-component vectors together using x86 requires four floating-point addition instructions.


This corresponds to four x86 FADD instructions in the object code. On the other hand, as the following pseudo-code shows, a single 128-bit 'packed-add' instruction can replace the four scalar addition instructions.

movapsxmm0,[v1];xmm0 = v1.w | v1.z | v1.y | v1.x addpsxmm0,[v2];xmm0 = v1.w+v2.w | v1.z+v2.z | v1.y+v2.y | v1.x+v2.xmovaps[vec_res],xmm0;xmm0

Later versions

Software and hardware issues

With all x86 instruction set extensions, it is up to the BIOS, operating system and application programmer to test and detect their existence and proper operation.

User application uptake of the x86 extensions has been slow with even bare minimum baseline MMX and SSE support (in some cases) being non-existent by applications some 10 years after these extensions became commonly available. Distributed computing has accelerated the use of these extensions in the scientific community—and many scientific applications refuse to run unless the CPU supports SSE2 or SSE3.

The use of multiple revisions of an application to cope with the many different sets of extensions available is the simplest way around the x86 extension optimization problem. Software libraries and some applications have begun to support multiple extension types hinting that full use of available x86 instructions may finally become common some 5 to 15 years after the instructions were initially introduced.


The following programs can be used to determine which, if any, versions of SSE are supported on a system

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The VEX prefix and VEX coding scheme are comprising an extension to the x86 and x86-64 instruction set architecture for microprocessors from Intel, AMD and others.

AVX-512 are 512-bit extensions to the 256-bit Advanced Vector Extensions SIMD instructions for x86 instruction set architecture (ISA) proposed by Intel in July 2013, and implemented in Intel's Xeon Phi x200 and Skylake-X CPUs; this includes the Core-X series, as well as the new Xeon Scalable Processor Family and Xeon D-2100 Embedded Series.

The F16C instruction set is an x86 instruction set architecture extension which provides support for converting between half-precision and standard IEEE single-precision floating-point formats.


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