C51: BIT FIELD UNIONS DON'T WORK AS EXPECTED

Information in this article applies to:

• C51 Version 5.50
• C51 Version 6.00 Beta
• C51 Version 6.00

SYMPTOMS

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;
  }

CAUSE

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.

RESOLUTION

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

  x.number = 1;

to:

  x.number = 0x0100;

SEE ALSO

• C51: BIT FIELD UNIONS GIVE STRANGE RESULTS
• C51: ORDER OF BITFIELDS
• C51: SIGNED BIT FIELD NOT SUPPORTED

FORUM THREADS

The following Discussion Forum threads may provide information related to this topic.

• constant bit-fields in struct
• Address bits in XDATA
• union, bit field and bdata
• how many mem does union cost?
• Union
• CRC-32 or CRC-16 code

Last Reviewed: Monday, July 25, 2005