FLDENV reloads the FPU environment from the memory area defined by the source operand. This data should have been written by previous FSTENV or FNSTENV instruction.
The FPU environment conists of the FPU control word, status word, tag word, and error pointers (both data and instruction). The environment layout in memory depends on both the operand size and the current operating mode of the processor. The USE attribute of the current code segment determines the operand size: The 14-byte operand applies to a USE16 segment, and the 28-byte operand applies to a USE32 segment. Refer to the Intel documentation for figures of the environment layouts for both operand sizes in both real mode and protected mode. (In virtual-8086 mode, the real mode layout is used.) FLDENV should be run in the same operating mode as the corresponding FSTENV or FNSTENV.