Eli5: Computers can calculate based on instructions. But how do you teach computers what does it mean to add something, multiply, divide, or perform any other operation?

Question

Edit: Most of the answers here are wonderful and spot on.

For those who interpreted it differently due to my incorrect and brief phrasing, by ‘teaching’ I meant how does the computer get to know what it has to do when we want it to perform arithmetic operations (upon seeing the operators)?

And how does it do it? Like how does it ‘add’ stuff the same way humans do and give results which make sense to us mathematically? What exactly is going on inside?

Thanks for all the helpful explanations on programming, switches, circuits, logic gates, and the links!

In: 583

Answer 1

How do you teach a pulley system to raise the load when you pull the rope?

You don’t teach it, you build it that way

The code for “add” loads specific values into the input of the adder which is an arrangement of transistors that add the inputs together. There’s no need for it to understand addition, just that if you push one lever down then the output lever goes down too, if you push two down then output is up and the carry lever is down.

We built specific structures in CPUs then set up the code to use these structures to give us the desired result. The rock doesn’t think