размер bitfields на I_A_R for ARM Опции

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Пример:
typedef struct
{
unsigned M1 : 1;
unsigned M2 : 1;
unsigned M3 : 1;
unsigned M4 : 1;
unsigned M5 : 1;
unsigned M6 : 1;
unsigned M7 : 1;
unsigned M8 : 1;
unsigned M9 : 1;
unsigned M10 : 1;
unsigned M11 : 1;
unsigned M12 : 1;
unsigned M13 : 1;
unsigned M14 : 1;
unsigned M15 : 1;
unsigned M16 : 1;
} T_BitFields;
typedef unsigned short u16;
typedef union
{
T_BitFields bR;
u16 R;
} T_BitReg;
T_BitReg BitReg;
Как сделать, чтобы в И_А_Р_е под BitReg выделялось два байта, а не 4.