How does a computer wake up and load its own operating system?
A freshly powered computer has no software in memory, so how does it load the software that loads everything else? The answer is named after a boot.
▶ Start the storyA computer wakes up by running a tiny start-up program that it carries in permanent memory, which then loads a bigger program, which loads the operating system. This is booting, and the name comes from the old phrase about pulling yourself up by your own bootstraps.
The puzzle is real. When you switch a computer on, its processor has no software in main memory at all, so some process must load software before anything can run. But most software is loaded by other software that is already running, so something has to load the very first piece. The bootstrap technique was first described in the 1950s, and the invention of solved the paradox: computers could ship with a start-up program that cannot be erased.
Today it works in stages. On a PC, the start-up program checks and initialises the machine with a , then goes through a list of storage devices until it finds one that is bootable. The first program loaded into memory is called the first-stage , and the program it loads is the second-stage boot loader. That one is not an operating system itself, but it loads one and hands over control. The operating system then initialises itself, possibly loading drivers, and the kernel is one of the first programs to run. Booting is complete when the operating system, and usually some applications, are ready.
Step 1: Power on
The processor has no software in memory yet
Step 2: self-test
Checks and initialises memory and devices
Step 3: Find a boot device
Goes through the list of storage devices
Step 4: Boot loaders
First stage loads second stage
Step 5: Operating system
The kernel starts, drivers load, you are in
This also explains why the old advice to turn it off and on again makes sense: a reboot may be the only way to return a machine from a fault state to a known starting point.
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Recap
Booting is a relay: ROM program, boot loader, kernel, you.
💡 A trick to remember it · Small wakes big: a tiny ROM program wakes the boot loader, which wakes the kernel, like a relay where each runner is bigger than the last.
Surprising fact · Booting is named after pulling oneself up by one's bootstraps.
Connects to
- 🔌 How can a computer use a printer it has never met?
- 🐧 How did a student's hobby kernel end up in servers and phones?
- 🧠 What is the kernel, the one program always in charge?
- 🗄️ Why do programs live in the same memory as the data they work on?
- 💾 Why does a USB drive remember your files with no power at all?
Sources (2)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.