Dedicating a patent is the same as putting it into the public domain. It means that anyone can use the invention described in the patent without paying for its use, it is donated to the world's knowledge base free of charge prior to the time the patent expires. Indeed, that PATENT expired in 1996.
Computer code is usually protected by copyright law, which lasts much longer than the approximate 17 years that patent protection provided at that time.
Note that Tanenbaum left the AT&T code out of his project. That is because it is/was owned by AT&T for at least 70 years and to use it he would need permission or could be sued for copyright infringement.
Note that with software, the usual protection of copyright can also be put into the public domain for free use with some controls -- the GNU general public license -- such as allowing code to be used but not further restricted in the derivative work (the new program that uses the older code).
The prime example would be Linux. You can use the code but cannot then copyright your program and must freely share your code among other restrictions that are meant to keep it all free to the public.
You can read more about the GNU General Public License details HERE.