Moving on from last weeks operating system post, today we’ll look at how a computer boots up and loads an operating system.

Let’s start with why booting is a question at all. When a computer turns on, what happens? What we’re using to seeing is that the disk drive turns on and starts spinning, and the computer loads something from the disk.

The question is how does the computer know how to turn on the disk? As I said in the OS post, the CPU only really knows how work with memory. To talk to a disk drive, it needs to do some very specific things – write to certain memory locations, wait for things to happen. Basically, in order to turn on that disk drive and load the operating system, it needs to run a program. But how does it know what program to run?

I’m going to focus on how modern PCs work. Other computers have used/do use a similar process. The details vary, but the basic idea is the same.

A quick overview of the process:

1. CPU startup.
2. Run BIOS initialization
3. Load bootloader
4. Run bootloader
5. Load and run OS.

As that list suggests, it’s not a particularly simple process. We think of it as one step: turn on the computer, and it runs the OS. In fact, it’s a complicated dance of many steps.

On the lowest level, it’s all hardware. When you turn on a computer, some current gets sent to a clock. The clock is basically a quartz crystal; when you apply current to the crystal, it vibrates and produces a regular electrical pulse. That pulse is what drives the CPU. (When you talk about your computer’s speed, you generally describe it in terms of the frequency of the clock pulse. For example, in the laptop that I’m using to write this post, I’ve got a 2.4 GHz processor: that means that the clock chip pulses 2.4 billion times per second.)

When the CPU gets a clock pulse, it executes an instruction from memory. It knows what instruction to execute because it’s got a register (a special piece of memory built-in to the CPU) that tells it what instruction to execute. When the computer is turned on, that register is set to point at a specific location. Depending on the CPU, that might be 0, or it might be some other magic location; it doesn’t matter: what matters is that the CPU is built so that when it’s first turned on and it receives a clock pulse that starts it running, that register will always point at the same place.

The software part of the boot process starts there: the computer puts a chunk of read-only memory there – so when the computer turns on, there’s a program sitting at that location, which the computer can run. On PCs, that program is called the BIOS (Basic Input/Output System).

The BIOS knows how to tell the hardware that operates your display to show text on the screen, and it knows how to read stuff on your disk drives. It doesn’t know much beyond that. What it knows is extremely primitive. It doesn’t understand things like filesystems – the filesystem is set up and controlled by the operating system, and different operating systems will set up filesystems in different ways. The BIOS can’t do anything with a filesystem: it doesn’t include any programming to tell it how to read a filesystem, and it can’t ask the operating system to do it, because the OS hasn’t loaded yet!

What the BIOS does is something similar to what the CPU did when it started up. The CPU knew to look in a special location in memory to find a program to run. The BIOS knows to look at a special section on a disk drive to find a program to run. Every disk has a special chunk of data on it called the master boot record (MBR). The MBR contains another program, called a boot loader. So the BIOS loads the boot loader, and then uses it to actually load the operating system.

This probably seems a bit weird. The computer starts up by looking in a specific location for a program to run (the BIOS), which loads something (the bootloader). The thing it loads (the bootloader) also just looks in a specific location for a program to run (the OS). Why the two layers?

Different operating systems are build differently, and the specific steps to actually load and run the OS are different. For example, on my laptop, I’ve can run two operating systems: MacOS, and Linux. On MacOS (aka Darwin), there’s something called a microkernel that gets loaded. The microkernel is stored in a file named “mach_kernel” in the root directory of a type of filesystem called HFS. But in my installation of linux, the OS is stored in a file named “vmlinuz” in the root directory of a type of filesystem called EXT4. The BIOS doesn’t know what operating system it’s loading, and it doesn’t know what filesystem the OS uses – and that means that it knows neither the name of the file to load, nor how to find that file.

The bootloader was set up by the operating system. It’s specific to the operating system – you can think of it as part of the OS. So it knows what kind of filesystem it’s going to look at, and how to find the OS in that filesystem.

So once the bootloader gets started, it knows how to load and run the operating system, and once it does that, your computer is up and running, and ready for you to use!

Of course, all of this is a simplified version of how it works. But for understanding the process, it’s a reasonable approximation.

(To reply to commenters: I’ll try to do a post like this about compilers when I have some time to write it up.)