1. An infinitely long tape TAPE. The tape is divided into small grids one after the other, each grid containing a symbol from a finite alphabet, and a special symbol in the alphabet indicates a blank. The grid on the tape is numbered 0, 1, 2, from left to right. .. , the right end of the tape can be extended infinitely.
2. A read/write head HEAD. The head can be moved left and right on the tape, it can read the symbols on the currently pointed grid and can change the symbols on the current grid.
3. A set of control rules TABLE. It determines the next action of the read/write head based on the current state of the machine and the symbol on the grid pointed by the current read/write head, and changes the value of the status register to bring the machine into a new state.
4. A status register. It is used to save the current state of the Turing machine. The number of all possible states of the Turing machine is limited and there is a special state called the shutdown state. See the shutdown issue.
Although the Turing machine model introduction is somewhat boring, please keep reading it. I will reinterpret it in the form of better understanding. Here you only need to know the outline of it. A Turing machine is a device like the one below:
This device consists of the following parts: an infinitely long paper strip and a read/write head. (The big box in the middle), the internal state (blocks on the box, such as A, B, E, H), in addition, there is a program to control the box. This device is to read, write, and move the tape according to the command of the program and its internal state. It works like this: read a square of information from the head on the tape, and start looking up the program according to its current internal state, and then get an output action, that is, whether to go to the paper Take the information, or move the head to the next square. The program will also tell it which state the internal state will be transferred to at the next moment.
The specific program is a list, also called a rule table, like this:
Current internal state s Input value i Output action o Internal state s' at the next moment
B 1 forward C
A 0 Write 1 B to the tape
C 0 back A
... ... ... ...
Therefore, the Turing machine can determine the internal state and output action of the next moment according to the information read by the head and the current internal state at each moment.
The Turing machine is as simple as that! Unbelievable? And as long as you change its program (that is, the rules table above), then it may do whatever work the computer can do for you. So it can be said that the Turing machine is the simplest computer model!
Perhaps, you will think that the Turing machine model is too simple, how can you complete the complex tasks of the computer? The key to the problem is how to understand this model.
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