DSimon comments on The Up-Goer Five Game: Explaining hard ideas with simple words - Less Wrong
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So what is Mr. Turing's computer like? It has these parts:
Looking closer, each line in the table has five parts, which are:
Here's a simple table:
Okay, so let's say that we have one of Mr. Turing's computers built with that table. It starts out in the Happy state, and its head is on the first number of a paper like this:
What will the paper look like after the computer is done? Try pretending you are the computer and see what you do! The answer is at the end.
So you can see now that the table is the plan for what the computer should do. But we still have not fixed Mr. Babbage's problem! To make the computer do different things, we have to open it up and change the table. Since the "table" in any real computer will be made of very many parts put together very carefully, this is not a good way to do it!
So here is the amazing part that surprised everyone: you can make a great table that can act like any other table if you give it the right numbers on the paper. Some of the numbers on the paper tell the computer about a table for adding, and the rest of the numbers are to be added. The person who made the great table did not even have to know anything about adding, as long as the person who wrote the first half of the paper does.
Our computers today have tables like this great table, and so almost everything fun or important that they do is given to them long after they are built, and it is easy to change what they do.
By the way, here is how the paper from before will look after a computer with our simple table is done with it: