BIT_BYTE
size_t byte_offset = BIT_BYTE(nr);
unsigned int first_byte = start + BIT_BYTE(pos);
unsigned int last_byte = start + BIT_BYTE(pos + len - 1);
unsigned int first_byte = start + BIT_BYTE(pos);
unsigned int last_byte = start + BIT_BYTE(pos + len - 1);
u8 *p = ((u8 *)map) + BIT_BYTE(start);