BITS(3) 						   BSD Library Functions Manual 						   BITS(3)

NAME

     __BIT, __BITS, __SHIFTIN, __SHIFTOUT, __SHIFTOUT_MASK -- macros for preparing bitmasks and operating on bit fields

SYNOPSIS

     #include <sys/param.h>
     #include <sys/cdefs.h>

     uintmax_t
     __BIT(n);

     uintmax_t
     __BITS(m, n);

     __SHIFTIN(v, mask);

     __SHIFTOUT(v, mask);

     __SHIFTOUT_MASK(mask);

DESCRIPTION

     These macros prepare bitmasks, extract bitfields from words, and insert bitfields into words.  A ``bitfield'' is a span of consecutive bits
     defined by a bitmask, where 1s select the bits in the bitfield.

     Use __BIT() and __BITS() to define bitmasks:

	   __BIT(n)
		   Return a bitmask with bit n set, where the least significant bit is bit 0.

	   __BITS(m, n)
		   Return a bitmask with bits m through n, inclusive, set.  It does not matter whether m > n or m <= n.  The least significant bit
		   is bit 0.

     __SHIFTIN(), __SHIFTOUT(), and __SHIFTOUT_MASK() help read and write bitfields from words:

	   __SHIFTIN(v, mask)
			    Left-shift bits v into the bitfield defined by mask, and return them.  No side-effects.

	   __SHIFTOUT(v, mask)
			    Extract and return the bitfield selected by mask from v, right-shifting the bits so that the rightmost selected bit is
			    at bit 0.  No side-effects.

	   __SHIFTOUT_MASK(mask)
			    Right-shift the bits in mask so that the rightmost non-zero bit is at bit 0.  This is useful for finding the greatest
			    unsigned value that a bitfield can hold.  No side-effects.	Note that __SHIFTOUT_MASK(m) = __SHIFTOUT(m, m).

EXAMPLES

     The following example demonstrates basic usage of the bits macros:

	   uint32_t bits, mask, val;

	   bits = __BITS(2, 3); 		   /* 00001100 */
	   mask = __BIT(2) | __BIT(3);		   /* 00001100 */

	   val = __SHIFTIN(0x03, mask); 	   /* 00001100 */
	   val = __SHIFTOUT(0xf, mask); 	   /* 00000011 */

SEE ALSO

     bitops(3), cdefs(3)

HISTORY

     The bits macros first appeared in atw(4), with different names and implementation.  In their current form these macros appeared in
     NetBSD 4.0.

AUTHORS

     The bits macros were written by David Young <dyoung@NetBSD.org>.
     Matt Thomas <matt@NetBSD.org> suggested important improvements to the implementation, and contributed the macro names SHIFTIN() and
     SHIFTOUT().

BUGS

     __BIT() and __BITS() can only express 32-bit bitmasks.

BSD
								 October 17, 2012							       BSD