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C51: Bit Field Unions Don't Work as Expected

Information in this article applies to:

  • C51 Version 5.50 and later


I have created a union of a bit field structure and an unsigned integer. When I set bit 0 of the unsigned int, the corresponding bit field is not set.

struct bit_st
  unsigned char bit_0 : 1;
  unsigned char bit_1 : 1;
  unsigned char bit_2 : 1;
  unsigned char bit_3 : 1;
  unsigned char bit_4 : 1;
  unsigned char bit_5 : 1;
  unsigned char bit_6 : 1;
  unsigned char bit_7 : 1;
  unsigned char bit_8 : 1;
  unsigned char bit_9 : 1;

union bit_un
  unsigned int number;
  struct bit_st bits;

void main (void)
  union bit_un x;

  x.number = 1;


The 8051 is a big endian architecture (opposite of all other Intel chips). This means that the MSB comes first and the LSB comes last. Your assignment sets bit 0 in the second byte of the union. The bit field bit_0 is located at bit 0 in the first byte of that union.


In your example, to set the bit field bit_0, you would need to change your assignment from:

  x.number = 1;


  x.number = 0x0100;


Last Reviewed: Thursday, February 25, 2021

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