# Details about Intel® Streaming SIMD Extensions Intrinsics

Intel® Streaming SIMD Extensions (Intel® SSE) instructions use the following features:

- Registers – Enable packed data of up to 128 bits in length for optimal SIMD processing

- Data Types – Enable packing of up to 16 elements of data in one register

## Registers

Intel® Streaming SIMD Extensions use eight 128-bit registers (XMM0 to XMM7).

Because each of these registers can hold more than one data element, the processor can process more than one data element simultaneously. This processing capability is also known as single-instruction multiple data processing (SIMD).

For each computational and data manipulation instruction in the new extension sets, there is a corresponding C intrinsic that implements that instruction directly. This frees you from managing registers and assembly programming. Further, the compiler optimizes the instruction scheduling so that your executable runs faster.

### Note

The MM and XMM registers are the SIMD registers used by the systems based on IA-32 architecture to implement MMX™ technology and Intel® SSE or Intel® Streaming SIMD Extensions 2 (Intel® SSE2).

## Data Types

These intrinsic functions use four new C data types as operands, representing the new registers that are used as the operands to these intrinsic functions.

## New Data Types

The following table details for which instructions each of the new data types are available.

New Data Type | Intel® Streaming SIMD Extensions Intrinsics | Intel® Streaming SIMD Extensions 2 Intrinsics | Intel® Streaming SIMD Extensions 3 Intrinsics
--- | --- | --- | ---
`__m64` | Available | Available | Available
`__m128` | Available | Available | Available
`__m128d` | Not available | Available | Available
`__m128i` | Not available | Available | Available

## __m128 Data Types

The `__m128` data type is used to represent the contents of a Intel® SSE register used by Intel® SSE intrinsics. The `__m128` data type can hold four 32-bit floating-point values.

The `__m128d` data type can hold two 64-bit floating-point values.

The `__m128i` data type can hold sixteen 8-bit, eight 16-bit, four 32-bit, or two 64-bit integer values.

The compiler aligns `__m128d` and `__m128i`local and global data to 16-byte boundaries on the stack. To align integer, float, or double arrays, you can use the __declspec(align) statement.

## Data Types Usage Guidelines

These data types are not basic ANSI C data types. You must observe the following usage restrictions:

- Use data types only on either side of an assignment, as a return value, or as a parameter. You cannot use it with other arithmetic expressions (+, -, etc).

- Use data types as objects in aggregates, such as unions, to access the byte elements and structures.

- Use data types only with the respective intrinsics described in this documentation.

## Accessing __m128i Data

To access 8-bit data:

    #define  _mm_extract_epi8(x, imm) \
    ((((imm) & 0x1) == 0) ?   \
     _mm_extract_epi16((x), (imm) >> 1) & 0xff : \
     _mm_extract_epi16( _mm_srli_epi16((x), 8), (imm) >> 1))

For 16-bit data, use the following intrinsic:

    int  _mm_extract_epi16(__m128i a, int imm)

To access 32-bit data:

    #define  _mm_extract_epi32(x, imm) \
     _mm_cvtsi128_si32( _mm_srli_si128((x), 4 * (imm)))

Parent topic: Intrinsics for Intel® Streaming SIMD Extensions (Intel® SSE)

## See Also

__declspec(align) declaration