Hi all, I have ported the code from netbsd to freebsd . I filed a PR http://www.freebsd.org/cgi/query-pr.cgi?pr=57522 The code is tested on my laptop. I think it needs more test. You can check the following link for the detail information about the algorithm. http://kerneltrap.org/node/view/609 Thank David Laight for his good idea. Thanks. Jun Su