[Random Dude]

Originally Posted by Joe Johnson

Hey, can anyone give me an idea about the relatative effeciency of the underlying assembly language code for using the shifting macros vs. using the union/structures method?

I suppose that most times the code ends up more or less identical but I wonder on this particular CPU with this particular compiler, if there is a major advantage one way or the other.

Your comments welcome...

Joe J.

Well, lets throw a sample together and see what we get:

First, the union method:

typedef struct bits {
	unsigned char b0:1;
	unsigned char b1:1;
	unsigned char b2:1;
	unsigned char b3:1;
	unsigned char b4:1;
	unsigned char b5:1;
	unsigned char b6:1;
	unsigned char b7:1;
	} BITS;

typedef union bit_char {
	unsigned char byte;
	BITS b;
	} BIT_CHAR;

main ()
{
	BIT_CHAR test1;
	test1.byte = 5;
	test1.b.b2 = 1;
}

And the corresponding assembly:

000802   cfd9     MOVFF     0xfd9,0xfe6                                                 
000806   cfe1     MOVFF     0xfe1,0xfd9                                                   
00080a   52e6     MOVF      0xe6,0x1,0x0           
00080c   0e05     MOVLW     0x5
00080e   6edf     MOVWF     0xdf,0x0    
000810   84df     BSF       0xdf,0x2,0x0
000812   52e5     MOVF      0xe5,0x1,0x0  
000814   52e5     MOVF      0xe5,0x1,0x0
000816   cfe7     MOVFF     0xfe7,0xfd9                                                  
00081a   0012     RETURN    0x0

And the macro version (using bit-shifting)

//get a bit from a variable
#define GETBIT(var, bit)(((var) >> (bit)) & 1)

//set a bit to 1
#define SETBIT(var, bit)var |= (1 << (bit))

//set a bit to 0
#define CLRBIT(var, bit)var &= (~(1 << (bit)))
main ()
{
	char test1;
	test1 = 5;
	SETBIT(test1,2);
}

ASM:

000802   cfd9     MOVFF     0xfd9,0xfe6    
000806   cfe1     MOVFF     0xfe1,0xfd9 
00080a   52e6     MOVF      0xe6,0x1,0x0
00080c   0e05     MOVLW     0x5
00080e   6edf     MOVWF     0xdf,0x0  
000810   84df     BSF       0xdf,0x2,0x0
000812   52e5     MOVF      0xe5,0x1,0x0
000814   52e5     MOVF      0xe5,0x1,0x0  
000816   cfe7     MOVFF     0xfe7,0xfd9   
00081a   0012     RETURN    0x0

So it appears that the compiler optimizes both methods to the same ASM code. Therefore I'd say that which to use is a matter of personal preference.