x86 Instruction Set Reference

FSTSW/FNSTSW - Store x87 FPU Status Word

Opcode Mnemonic Description
9B DD /7 FSTSW m2byte Store FPU status word at m2byte after checking for pending unmasked floating-point exceptions.
9B DF E0 FSTSW AX Store FPU status word in AX register after checking for pending unmasked floating-point exceptions.
DD /7 FNSTSW* m2byte Store FPU status word at m2byte without checking for pending unmasked floating-point exceptions.
DF E0 FNSTSW* AX Store FPU status word in AX register without checking for pending unmasked floating-point exceptions.
Description

NOTE: * See IA-32 Architecture Compatibility section below.

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 exceptions before storing the status word; the FNSTSW instruction does not.

FSTSW/FNSTSW: Store x87 FPU Status Word (x86 Instruction Set Reference)

The assembler issues two instructions for the FSTSW instruction (an FWAIT instruction 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.
Operation 
Destination = FPUStatusWord;
FPU flags affected

The C0, C1, C2, and C3 are undefined.

IA-32 Architecture Compatibility

When operating a Pentium or Intel486 processor in MS-DOS compatibility mode, it is possible (under unusual circumstances) for an FNSTSW 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 IA-32 Intel Architecture Software Developer's Manual, Volume 1, for a description of these circumstances. An FNSTSW instruction cannot be interrupted in this way on a Pentium 4, Intel Xeon, or P6 family processor.

Floating-Point Exceptions
None.
Protected Mode Exceptions
#GP(0)If the destination is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register is used to access memory and it contains a null segment selector.
#GP(0)If the destination is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register is used to access memory and it contains a null segment selector.
#SS(0)If a memory operand effective address is outside the SS segment limit.
#NMEM or TS in CR0 is set.
#PF(fault-code)If a page fault occurs.
Real-Address Mode Exceptions
#GPIf a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#GPIf a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SSIf a memory operand effective address is outside the SS segment limit.
Virtual-8086 Mode Exceptions
#GP(0)If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#GP(0)If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS(0)If a memory operand effective address is outside the SS segment limit.
#NMEM or TS in CR0 is set.
#PF(fault-code)If a page fault occurs.