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Vol. 2A 3-395
INSTRUCTION SET REFERENCE, A-M
FSTSW/FNSTSW—Store x87 FPU Status Word
FSTSW/FNSTSW—Store x87 FPU Status Word
Description
Stores the current value of the x87 FPU status word in the destination location. The
destination operand can be either a two-byte memory location or the AX register. The
FSTSW instruction checks for and handles pending unmasked floating-point excep-
tions before storing the status word; the FNSTSW instruction does not.
The FNSTSW AX form of the instruction is used primarily in conditional branching (for
instance, after an FPU comparison instruction or an FPREM, FPREM1, or FXAM
instruction), where the direction of the branch depends on the state of the FPU condi-
tion code flags. (See the section titled “Branching and Conditional Moves on FPU
Condition Codes” in Chapter 8 of the Intel® 64 and IA-32 Architectures Software
Developer’s Manual, Volume 1.) This instruction can also be used to invoke exception
handlers (by examining the exception flags) in environments that do not use inter-
rupts. When the FNSTSW AX instruction is executed, the AX register is updated
before the processor executes any further instructions. The status stored in the AX
register is thus guaranteed to be from the completion of the prior FPU instruction.
The assembler issues two instructions for the FSTSW instruction (an FWAIT instruc-
tion followed by an FNSTSW 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.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
9B DD /7 FSTSW m2byte Valid Valid Store FPU status word at
m2byte after checking for
pending unmasked floating-
point exceptions.
9B DF E0 FSTSW AX Valid Valid Store FPU status word in AX
register after checking for
pending unmasked floating-
point exceptions.
DD /7 FNSTSW
*
m2byte Valid Valid Store FPU status word at
m2byte without checking for
pending unmasked floating-
point exceptions.
DF E0 FNSTSW
*
AX Valid Valid Store FPU status word in AX
register without checking for
pending unmasked floating-
point exceptions.
NOTES:
* See IA-32 Architecture Compatibility section below.