Comments on: Working with bits and bitfields http://www.coranac.com my own little world Mon, 30 Aug 2010 15:42:13 -0400 http://wordpress.org/?v=2.8.2 hourly 1 By: cearn http://www.coranac.com/documents/working-with-bits-and-bitfields/comment-page-1/#comment-2446 cearn Wed, 09 Sep 2009 16:16:46 +0000 http://www.coranac.com/?page_id=50#comment-2446 I don't believe it's possible, no. I'm not sure that a ten-iteration loop is that bad, though. If you must, you can grab the whole set and go through it in a binary search rather than one by one. If you have it, a Count Leading Zero instruction (CLZ) would help out as well. And even if you don't, there are fast algorithms out there that can do this as well. I don’t believe it’s possible, no. I’m not sure that a ten-iteration loop is that bad, though. If you must, you can grab the whole set and go through it in a binary search rather than one by one.

If you have it, a Count Leading Zero instruction (CLZ) would help out as well. And even if you don’t, there are fast algorithms out there that can do this as well.

]]> By: cloud9ine http://www.coranac.com/documents/working-with-bits-and-bitfields/comment-page-1/#comment-2431 cloud9ine Thu, 03 Sep 2009 17:58:15 +0000 http://www.coranac.com/?page_id=50#comment-2431 Is there a way to use prioritized bitfields and directly retrieve the highest priority bit that is set / cleared within it? For example say, I have a 10-bit bitfield that represents whether each of faults 0 through 9 are active now. Say fault 9 is highest priority and fault 0 is lowest. Is there a way I can just use a function and say 'get_highest priority_position_set_in_bitfield(bitfield)' and the function will return, say, 6? I know a loop could but I am looking for better. Is there a way to use prioritized bitfields and directly retrieve the highest priority bit that is set / cleared within it?

For example say, I have a 10-bit bitfield that represents whether each of faults 0 through 9 are active now. Say fault 9 is highest priority and fault 0 is lowest. Is there a way I can just use a function and say ‘get_highest priority_position_set_in_bitfield(bitfield)’ and the function will return, say, 6?

I know a loop could but I am looking for better.

]]> By: cearn http://www.coranac.com/documents/working-with-bits-and-bitfields/comment-page-1/#comment-2131 cearn Tue, 12 May 2009 13:55:53 +0000 http://www.coranac.com/?page_id=50#comment-2131 Yeah, <code>BF_GET()</code> was wrong. As far as I can see, the rest are correct though. Yeah, BF_GET() was wrong. As far as I can see, the rest are correct though.

]]> By: Travis H. http://www.coranac.com/documents/working-with-bits-and-bitfields/comment-page-1/#comment-2130 Travis H. Tue, 12 May 2009 13:09:31 +0000 http://www.coranac.com/?page_id=50#comment-2130 Some of these bit field things are wrong. Here's a tested version: [code lang="cpp"] /* Create a bitmask of length len */ #define BIT_MASK(len) (BIT(len)-1) /* Create a bitfield mask of length len starting at bit start */ #define BF_MASK(start, len) ( BIT_MASK(len)<<(start) ) /* Prepare a bitmask for insertion or combining */ #define BF_PREP(x, start, len) (((x)&BIT_MASK(len))<<(start)) & BIT_MASK(len) ) /* Extract a bitfield of length len starting at bit start from y */ #define BF_GET(y, start, len) ( ((y)>>(start))&BIT_MASK(len) ) /* Insert a new bitfield value x into y */ #define BF_SET(y, x, start, len) \ (y=((y) &~ BF_MASK(start, len)) | BF_PREP(x, start, len)) [/code] <small><b>NOTE: this comment is a reconstruction after a recent DB fail</b> - jv</small> Some of these bit field things are wrong.

Here’s a tested version:

/* Create a bitmask of length len */
#define BIT_MASK(len) (BIT(len)-1)

/* Create a bitfield mask of length len starting at bit start */
#define BF_MASK(start, len) ( BIT_MASK(len)<<(start) )

/* Prepare a bitmask for insertion or combining */
#define BF_PREP(x, start, len) (((x)&BIT_MASK(len))<<(start)) & BIT_MASK(len) )

/* Extract a bitfield of length len starting at bit start from y */
#define BF_GET(y, start, len) ( ((y)>>(start))&BIT_MASK(len) )

/* Insert a new bitfield value x into y */
#define BF_SET(y, x, start, len) \
              (y=((y) &~ BF_MASK(start, len)) | BF_PREP(x, start, len))

NOTE: this comment is a reconstruction after a recent DB fail – jv

]]> By: Ged http://www.coranac.com/documents/working-with-bits-and-bitfields/comment-page-1/#comment-2005 Ged Sun, 12 Oct 2008 18:27:26 +0000 http://www.coranac.com/?page_id=50#comment-2005 Great article! Great article!

]]> By: Kawa http://www.coranac.com/documents/working-with-bits-and-bitfields/comment-page-1/#comment-1822 Kawa Fri, 30 May 2008 17:52:25 +0000 http://www.coranac.com/?page_id=50#comment-1822 I *HATE* bit fiddling! I *HATE* bit fiddling!

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