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Unformatted text preview: ge fault occurs. If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. Vol. 2 3-389 INSTRUCTION SET REFERENCE, A-M FSTENV/FNSTENV--Store x87 FPU Environment
Opcode 9B D9 /6 Instruction FSTENV m14/28byte 64-Bit Mode Valid Compat/ Leg Mode Valid Description Store FPU environment to m14byte or m28byte after checking for pending unmasked floating-point exceptions. Then mask all floating-point exceptions. Store FPU environment to m14byte or m28byte without checking for pending unmasked floating-point exceptions. Then mask all floating-point exceptions. D9 /6 FNSTENV* m14/28byte Valid Valid NOTES: * See IA-32 Architecture Compatibility section below. Description
Saves the current FPU operating environment at the memory location specified with the destination operand, and then masks all floating-point exceptions. The FPU operating environment consists of the FPU control word, status word, tag word, instruction pointer, data pointer, and last opcode. Figures 8-9 through 8-12 in the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 1, show the layout in memory of the stored environment, depending on the operating mode of the processor (protected or real) and the current operand-size attribute (16-bit or 32-bit). In virtual-8086 mode, the real mode layouts are used. The FSTENV instruction checks for and handles any pending unmasked floating-point exceptions before storing the FPU environment; the FNSTENV instruction does not. The saved image reflects the state of the FPU after all floating-point instructions preceding the FSTENV/FNSTENV instruction in the instruction stream have been executed. These instructions are often used by exception handlers because they provide access to the FPU instruction and data pointers. The environment is typically saved in the stack. Masking all exceptions after saving the environment prevents floating-point exceptions from interrupting the exception handler. The assembler issues two instructions for the FSTENV instruction (an FWAIT instruction followed by an FNSTENV instruction), and the processor executes each of these instructions separately. If an exception is generated for either of these instructions, the save EIP points to the instruction that caused the exception. This instruction's operation is the same in non-64-bit modes and 64-bit mode. 3-390 Vol. 2 INSTRUCTION SET REFERENCE, A-M IA-32 Architecture Compatibility
When operating a Pentium or Intel486 processor in MS-DOS compatibility mode, it is possible (under unusual circumstances) for an FNSTENV instruction to be interrupted prior to being executed to handle a pending FPU exception. See the section titled "No-Wait FPU Instructions Can Get FPU Interrupt in Window" in Appendix D of the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 1, for a description of these circumstances. An FNSTENV instruction cannot be interrupted in this way on a Pentium 4, Intel Xeo...
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- Winter '11