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

Everything in programming is built by layers that depend on the previous one, so that one operation available in one layer is “taught” how to do it in the previous one.

At the most basic layer, the computer’s calculator (the ALU part of the CPU) knows how to do these because it has special circuits that make the calculation as in “if there is a zero here and a one there, the result is that”.

Having zeros here or there actually means letting current pass through a transistor, but I’m not a hardware guy.