fstcw

Store Control Word

FSTCW m2byte

Stores FPU control word to memory.

Pseudocode Operation

[memory] ← FPU_Control_Word;

Encoding

Binary Layout

ModRM

Format Legacy

Opcode 9B D9 /7

Extension x87 FPU

Operands

dest
Memory

Reference

View in Intel® 64 and IA-32 Architectures SDM ↗ Intel® SDM

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	CPUID	Description
9B D9 /7	FSTCW m2byte		Valid Valid		Store FPU control word to m2byte after checking for pending unmasked floating-point exceptions.
D9 /7	FNSTCW1 m2byte		Valid Valid		Store FPU control word to m2byte without checking for pending unmasked floating-point exceptions.

Description

Stores the current value of the FPU control word at the specified destination in memory. The FSTCW instruction checks for and handles pending unmasked floating-point exceptions before storing the control word; the FNSTCW instruction does not. The assembler issues two instructions for the FSTCW instruction (an FWAIT instruction followed by an FNSTCW instruction), and the processor executes each of these instructions 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 FNSTCW 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 FNSTCW instruction cannot be interrupted in this way on later Intel processors, except for the Intel QuarkTM X1000 processor.

Operation

DEST := FPUControlWord;

Exceptions

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. #SS(0) If a memory operand effective address is outside the SS segment limit. #NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used. FSTCW/FNSTCW-Store x87 FPU Control Word Vol. 2A 3-388

Real-Address Mode Exceptions

#GP If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. #SS If a memory operand effective address is outside the SS segment limit. #NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#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. #NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made. #UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0) If a memory address referencing the SS segment is in a non-canonical form. #GP(0) If the memory address is in a non-canonical form. #NM CR0.EM[bit 2] or CR0.TS[bit 3] = 1. #MF If there is a pending x87 FPU exception. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. #UD If the LOCK prefix is used. FSTCW/FNSTCW-Store x87 FPU Control Word Vol. 2A 3-389