Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 1462

FRSTOR—Restore FPU State
Opcode
DD /4
Description
Loads the FPU state (operating environment and register stack) from the memory area
specified with the source operand. This state data is typically written to the specified
memory location by a previous FSAVE/FNSAVE instruction.
The FPU operating environment consists of the FPU control word, status word, tag
word, instruction pointer, data pointer, and last opcode. See the Intel
Architectures Software Developer's Manual for the layout in memory of the stored
environment, depending on the operating mode of the processor (protected or real)
and the size of the current address attribute (16-bit or 32-bit). In virtual-8086 mode,
the real mode layouts are used. The contents of the FPU register stack are stored in the
80 bytes immediately follow the operating environment image.
The FRSTOR instruction should be executed in the same operating mode as the
corresponding FSAVE/FNSAVE instruction.
If one or more unmasked exception bits are set in the new FPU status word, a
floating-point exception will be generated. To avoid raising exceptions when loading a
new operating environment, clear all the exception flags in the FPU status word that is
being loaded.
Operation
FPUControlWord  SRC(FPUControlWord);
FPUStatusWord  SRC(FPUStatusWord);
FPUTagWord  SRC(FPUTagWord);
FPUDataPointer  SRC(FPUDataPointer);
FPUInstructionPointer  SRC(FPUInstructionPointer);
FPULastInstructionOpcode  SRC(FPULastInstructionOpcode);
ST(0)  SRC(ST(0));
ST(1)  SRC(ST(1));
ST(2)  SRC(ST(2));
ST(3)  SRC(ST(3));
ST(4)  SRC(ST(4));
ST(5)  SRC(ST(5));
ST(6)  SRC(ST(6));
ST(7)  SRC(ST(7));
FPU Flags Affected
The C0, C1, C2, C3 flags are loaded.
Floating-point Exceptions
None; however, this operation might unmask an existing exception that has been
detected but not generated, because it was masked. Here, the exception is generated
at the completion of the instruction.
4:160
Instruction Description
FRSTOR m94/108byte Load FPU state from m94byte or m108byte .
®
64 and IA-32
Volume 4: Base IA-32 Instruction Reference