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3-332 Vol. 2A FINIT/FNINIT—Initialize Floating-Point Unit
INSTRUCTION SET REFERENCE, A-M
FINIT/FNINIT—Initialize Floating-Point Unit
Description
Sets the FPU control, status, tag, instruction pointer, and data pointer registers to
their default states. The FPU control word is set to 037FH (round to nearest, all
exceptions masked, 64-bit precision). The status word is cleared (no exception flags
set, TOP is set to 0). The data registers in the register stack are left unchanged, but
they are all tagged as empty (11B). Both the instruction and data pointers are
cleared.
The FINIT instruction checks for and handles any pending unmasked floating-point
exceptions before performing the initialization; the FNINIT instruction does not.
The assembler issues two instructions for the FINIT instruction (an FWAIT instruction
followed by an FNINIT instruction), and the processor executes each of these instruc-
tions in 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.
IA-32 Architecture Compatibility
When operating a Pentium or Intel486 processor in MS-DOS compatibility mode, it is
possible (under unusual circumstances) for an FNINIT 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 FNINIT instruction cannot be interrupted in
this way on a Pentium 4, Intel Xeon, or P6 family processor.
In the Intel387 math coprocessor, the FINIT/FNINIT instruction does not clear the
instruction and data pointers.
This instruction affects only the x87 FPU. It does not affect the XMM and MXCSR
registers.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
9B DB E3 FINIT Valid Valid Initialize FPU after checking for pending
unmasked floating-point exceptions.
DB E3 FNINIT
*
Valid Valid Initialize FPU without checking for
pending unmasked floating-point
exceptions.
NOTES:
* See IA-32 Architecture Compatibility section below.