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Unformatted text preview: s mode is provided to improve processor performance for applications such as streaming media processing, where rounding a denormal operand to zero does not appreciably affect the quality of the processed data. The denormals-are-zeros flag is cleared upon a power-up or reset of the processor, disabling the denormals-are-zeros mode. The denormals-are-zeros mode was introduced in the Pentium 4 and Intel Xeon processor with the SSE2 extensions; however, it is fully compatible with the SSE SIMD floating-point instructions (that is, the denormals-are-zeros flag affects the operation of the SSE SIMD floating-point instructions). In earlier IA-32 processors and in some models of the Pentium 4 processor, this flag (bit 6) is reserved. See Vol. 1 10-7 PROGRAMMING WITH STREAMING SIMD EXTENSIONS (SSE) Section 11.6.3, "Checking for the DAZ Flag in the MXCSR Register," for instructions for detecting the availability of this feature. Attempting to set bit 6 of the MXCSR register on processors that do not support the DAZ flag will cause a general-protection exception (#GP). See Section 11.6.6, "Guidelines for Writing to the MXCSR Register," for instructions for preventing such general-protection exceptions by using the MXCSR_MASK value returned by the FXSAVE instruction. 10.2.4 Compatibility of SSE Extensions with SSE2/SSE3/MMX and the x87 FPU The state (XMM registers and MXCSR register) introduced into the IA-32 execution environment with the SSE extensions is shared with SSE2 and SSE3 extensions. SSE/SSE2/SSE3 instructions are fully compatible; they can be executed together in the same instruction stream with no need to save state when switching between instruction sets. XMM registers are independent of the x87 FPU and MMX registers, so SSE/SSE2/SSE3 operations performed on the XMM registers can be performed in parallel with operations on the x87 FPU and MMX registers (see Section 11.6.7, "Interaction of SSE/SSE2 Instructions with x87 FPU and MMX Instructions"). The FXSAVE and FXRSTOR instructions save and restore the SSE/SSE2/SSE3 states along with the x87 FPU and MMX state. 10.3 SSE DATA TYPES SSE extensions introduced one data type, the 128-bit packed single-precision floating-point data type, to the IA-32 architecture (see Figure 10-4). This data type consists of four IEEE 32-bit single-precision floating-point values packed into a double quadword. (See Figure 4-3 for the layout of a single-precision floating-point value; refer to Section 4.2.2, "Floating-Point Data Types," for a detailed description of the single-precision floating-point format.) Contains 4 Single-Precision Floating-Point Values 127 96 95 64 63 32 31 0 Figure 10-4. 128-Bit Packed Single-Precision Floating-Point Data Type
This 128-bit packed single-precision floating-point data type is operated on in the XMM registers or in memory. Conversion instructions are provided to convert two packed single-precision floating-point values into two packed doubleword i...
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- Winter '11