# The control in the Turing Machine

My question is about the control in the Turing Machine. As far as I know, the control of the Turing Machine is just a set of states. If the Machine needs to record something, it needs to write on the tape.

The following is an excerpt from Michael Sipser's Theory of Computation, the passage is about how a Turing Machine can find the leftmost Cell. What is written is pretty straight forward. But what I don't understand is the part written in bold.

"A trickier method of finding the left-hand end of the tape takes advantage of the way that we defined the Turing machine model. Recall that if the machine tries to move its head beyond the left-hand end of the tape, it stays in the same place. We can use this feature to make a left-hand end detector. To detect whether the head is sitting on the left-hand end, the machine can write a special symbol over the current position while recording the symbol that it replaced in the control. Then it can attempt to move the head to the left. If it is still over the special symbol, the leftward move didn’t succeed, and thus the head must have been at the left-hand end."

What does he mean by this exactly? What does recording a symbol in the control mean? Isn't the only (and infinite) memory of the TM the tape?

• Closely related questions: 1, 2. – Raphael Jun 29 '15 at 20:46

What you can't do is remember an unbounded amount of information in the state. So you can't remember, for example, all the characters you've passed over since you last saw an '$a$', or even the number of characters since then.