I was reading about how to build a computer by yourself. The final step is installing the operating system. I was wondering, how does the operating system give instructions to the computer? When I write a program in java, for example, I rely on the operating system as a crutch to pass my instructions along. How do I talk directly to a computer? What if I wanted to build my own operating system? Let's say I wanted to make my computer one glorified calculator. So after I install my OS, every time I turn on my computer, it asks me for two numbers, adds them, displays the answer and asks for two more numbers. Is this hard to do? Thanks!