union{
  int Word;
  struct 
  {
    char Byte1:8;
    char Byte2:8;
  }structBytes;
}myVar;

myVar.Word  = 0xFFFF;           // myVar = 1111111111111111
myVar.structBytes.Byte1 = 0xF0; // myVar = 0000000011110000
myVar.structBytes.Byte2 = 0xF0; // myVar = 1111000011110000