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bit field

Hi All,

I have a struct:

   union U
   {
      unsigned char c;
      struct
      {
         unsigned char bit0 : 1;
         unsigned char bit1 : 1;
         unsigned char bit2 : 1;
         .
         .
         .
      }S;
   };
}

If I test multiple bits together I would have to do something like:
if (S.bit0 || S.bit1 || S.bit2)
which might generate insufficient code.

I can group multiple bits together but that would require changing the bitfields all the time as the code changes.

if I used bit mask instead of bit field. It would be something like:
if (var & (BIT_0 | BIT_1 | BIT2))
which generates much better code.

Is there anyway to makeit more efficient testing mutiple bit fields in 1 shot?

Thanks,

Anh

Parents

0 Oleg Sergeev over 20 years ago in reply to Anh Phan

hi,

My question is more of a C language that works for standard data type where byte is the smallest addressing

My suggestion is still the same: do not use union. It may be done with next way:

unsigned char my_bits;
#define BIT0_MASK	(1)
#define BIT1_MASK	(1<<1)
#define BIT2_MASK	(1<<2)


void main(void)
{
	my_bits = 0x00;					// clear variable
	my_bits |= BIT1_MASK;				// set bit 1

	while(1)
	{
		if (my_bits & (BIT1_MASK|BIT2_MASK))	// check either bit 1 or 2 set
		  my_bits &= ~(BIT1_MASK|BIT2_MASK);	// reset both
		else
		  my_bits++;
	}
}

Regards,
Oleg

Reply

0 Oleg Sergeev over 20 years ago in reply to Anh Phan

hi,

My question is more of a C language that works for standard data type where byte is the smallest addressing

My suggestion is still the same: do not use union. It may be done with next way:

unsigned char my_bits;
#define BIT0_MASK	(1)
#define BIT1_MASK	(1<<1)
#define BIT2_MASK	(1<<2)


void main(void)
{
	my_bits = 0x00;					// clear variable
	my_bits |= BIT1_MASK;				// set bit 1

	while(1)
	{
		if (my_bits & (BIT1_MASK|BIT2_MASK))	// check either bit 1 or 2 set
		  my_bits &= ~(BIT1_MASK|BIT2_MASK);	// reset both
		else
		  my_bits++;
	}
}

Regards,
Oleg

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