The TLB should be extremely fast, faster than main memory and as fast as registers. To achieve the necessary speeds, TLBs are associative memories, which means that items are looked up by their content rather than their address. We humans are very good at associative memory since this seems to be how our minds work. For example, you might vainly be trying to remember the name of the famous actress who starred as the evil sister in the movie "Whatever Happened to Baby Jane?" You try all day to remember it but can't. All of a sudden, you get a call from your friend Betty and it pops into your head that the evil sister was played by Bette Davis. The rest of the name, Davis, is associated with "Bette" so that when you hear part of her name, your mind automatically fills in the rest. (Bette Davis deliberately spelled her name "Bette" rather than "Bettie" or "Betty.")