typedef struct _FXSAVE_FORMAT
{
     WORD ControlWord;
     WORD StatusWord;
     WORD TagWord;
     WORD ErrorOpcode;
     ULONG ErrorOffset;
     ULONG ErrorSelector;
     ULONG DataOffset;
     ULONG DataSelector;
     ULONG MXCsr;
     ULONG MXCsrMask;
     UCHAR RegisterArea[128];
     UCHAR Reserved3[128];
     UCHAR Reserved4[224];
     UCHAR Align16Byte[8];
} FXSAVE_FORMAT, *PFXSAVE_FORMAT;

Windows Vista Kernel Structures