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Vol. 2A 3-301
INSTRUCTION SET REFERENCE, A-M
FCLEX/FNCLEX—Clear Exceptions
FCLEX/FNCLEX—Clear Exceptions
Description
Clears the floating-point exception flags (PE, UE, OE, ZE, DE, and IE), the exception
summary status flag (ES), the stack fault flag (SF), and the busy flag (B) in the FPU
status word. The FCLEX instruction checks for and handles any pending unmasked
floating-point exceptions before clearing the exception flags; the FNCLEX instruction
does not.
The assembler issues two instructions for the FCLEX instruction (an FWAIT instruc-
tion followed by an FNCLEX 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.
IA-32 Architecture Compatibility
When operating a Pentium or Intel486 processor in MS-DOS* compatibility mode, it
is possible (under unusual circumstances) for an FNCLEX instruction to be inter-
rupted 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 FNCLEX instruction cannot be interrupted in
this way on a Pentium 4, Intel Xeon, or P6 family processor.
This instruction affects only the x87 FPU floating-point exception flags. It does not
affect the SIMD floating-point exception flags in the MXCRS register.
This instruction’s operation is the same in non-64-bit modes and 64-bit mode.
Operation
FPUStatusWord[0:7] ← 0;
FPUStatusWord[15] ← 0;
Opcode* Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
9B DB E2 FCLEX Valid Valid Clear floating-point exception flags after
checking for pending unmasked floating-
point exceptions.
DB E2 FNCLEX
*
Valid Valid Clear floating-point exception flags
without checking for pending unmasked
floating-point exceptions.
NOTES:
* See IA-32 Architecture Compatibility section below.