HN Reader

The Linux Boot Process: From Power Button to Kernel

482

UEFI is an interface implemented by firmware (literally, Unified Extensible Firmware Interface), it's not the firmware itself. Saying "it starts the machine" is a bit of a nomenclature faux pas. The firmware starts the machine, you talk to the firmware via UEFI.

This post skips all the interesting things in the modern firmware dance. Not the least of which is when you call ExitBootServices() you're already in long mode. There's no need for the journey through real and protected.

3 months agoby nickelpro

In the page source:

  <body>
      <!-- Femboy Mode Button - Hidden on Mobile -->
      <button class="rave-button" id="raveButton" onclick="toggleRaveMode()" title="Femboy Mode" style="display: none;">
          <span class="button-text">uwu</span>
      </button>

OwO what's this?

3 months agoby zahlman

Really nice post, some months ago I also wrote about linux boot, but a bit more focused on the IO side (what's on disk, how is it loaded), here: https://blog.davidv.dev/posts/booting-x86-64/

3 months agoby deivid

It's a weird article for me. On one side it is an interesting topic. On the other hand why are we explaining what a hex number is? Who is interested in this level of detail but doesn't know hex? Maybe I'm overanalyzing.

At the same time this doesn't address my biggest open question on the topic - how do we get from the physical push to the reset vector? Somehow that magic works in HW, physics and electronics - how?

3 months agoby dgrin91

Articles like this make me wish i had enough points to downvode them into oblivion.

What a waste of time. I'm with the users that argue that if you need a "pretty please" explanation of what HEX values are, you shouldn't even begin to worry about the linux boot process.

3 months agoby monegator

Does anyone have a similar article with more detail? I don't quite want to read the datasheet of your favorite microprocessor, but I would like a decent amount more detail than what's provided. Especially before UEFI/BIOS.

3 months agoby zaxioms

GRUB is mentioned but not detailed.

Here are some details: https://www.pixelbeat.org/docs/disk/

3 months agoby pixelbeat__

The topic is interesting but it seems to be targeted for my grandmother.

3 months agoby nightshift1

Hard to read on my phone due to faded text.

3 months agoby metabagel

fascinating how it's all over the place wrt level of detail. and absolutely unreadable. luckily the layout is simple and reader mode works.

> Hex is base 16

i would argue that someone that understand bases (in the first place), understands what the << operator does (context where base 16 is explained), but doesn't understand what base 16 is, doesn't exist. this is the kind of haphazard approach of this article i'm talking about. even the author's name, 0xkato, is an example of this.

as to the content, i wish it had touched on TPM, PCRs, UEFI secure boot, and ME pre-boot.

i'm forgiving all the actual errors since it is a pretty broad overview.

i'm guessing first-year uni student.

rather amazed a post like this can make it to the #1 spot.

3 months agoby jiveturkey

This is old school BIOS boot. EFI bootloaders work very differently.

3 months agoby gmueckl

Video device initialization is intimately intertwined and a dependency for all this early boot stuff. I was hoping to learn more but it's not even mentioned. Still, neat.

3 months agoby superkuh

Seems like there are many useful suggestions for the author. Here is mine: maybe an interactive style would work much better for educational content.

There is a well praised post on HN: https://www.nan.fyi/database, built with the framework: https://github.com/nandanmen/NotANumber

3 months agoby liqilin1567

As should be clear from the reset vector, the 80286 and its successors actually boot in unreal mode. On the 80386, the base address of the code segment is 0xffff0000, which cannot be obtained by shifting the 16-bit CS register by 4. The descriptor cache simply gets loaded with the correct value at reset. Writing to CS in real mode overwrites the cached value with CS * 16.

3 months agoby gldrk

Reading this just reinforces how much of the x86/Linux boot chain is fossilized ceremony glued together for backward compatibility.

3 months agoby ramanvarma

Kinda missing the point of UEFI if you don't mention that it skips the first 3 sections and allows your kernel to boot directly from 64-bit mode ("long mode") with paging, a framebuffer, and a function to read files from your boot drive, already set up for you. Removes the need for GRUB entirely.

3 months agoby phendrenad2

I wonder whether the power button really directly starts the CPU. I wouldn't be surprised if the Intel Management Engine (or AMD equivalent), a CPU that's running all the time, is what gets the power button signal and starts the CPU.

3 months agoby gblargg

Light gray text on white??

3 months agoby pwpwp

The disk could be wiped from the BIOS. One could also run “fwupdmgr update” from a live USB to update the motherboard firmware and then reinstall the operating system. However, I’m not sure if this would completely clear the system.

3 months agoby aborsy

I'm a bit late to comment on this, but people might find it interesting. The title is "programming in assembly without an operating system", but it gives really good insight to how the early boot process works, and how to begin interacting with it.

Thank you to the author:

https://www.youtube.com/watch?v=ZFHnbozz7b4

3 months agoby gosub100

This doesn't cover the things that the CPU secretly does before it finally enters Real Mode, such as loading and validating the microcode.

3 months agoby Dwedit

Based on all the negative comments, and being a non-expert, I had an LLM generate an article for that same title. I suspect it was more approachable and more technically accurate than this. In fact I asked it to compare its article with this one and the negative comments.

The author of any technical blog post should be using an LLM to vet things before posting to find gaps or inconsistencies.

3 months agoby ra1231963

ARM and lots of non-x86 architectures often use a series of bootloaders to kick up ram, wake up parts of the hardware, blah blah, and read devicetree blobs to know what the hardware looks like

3 months agoby e-dant

I'm probably going to read this, but who thought putting light grey text on a white background was a good idea?

3 months agoby panny

I'm going to save this guys blog in one step

https://webaim.org/resources/contrastchecker/

(this is the site: https://webaim.org/resources/contrastchecker/?fcolor=D0D0D0&...)

3 months agoby zzzeek

Thanks for the detailed write-up! I'd love to see something like this for ARM too!

3 months agoby a022311

This post made me remember the technical interview by phone I had with Facebook circa 2010 for a Production Engineer role (but maybe it wasn't called yet that way back then) where they asked me exactly this. Well, they actually asked "explain me the boot process of a Linux server", with no more hints and "go into the details you feel are important" as the only help.

Anyway I should have moved to Dublin and something something surveillance capitalism so the grape wasn't ripe enough anyway.

3 months agoby darkwater

Nice to see the good old hacker energy & independent blogs explaining things showing up on top of hacker news. Welcome change from insufferable agent this and vibe that

3 months agoby neoattikos

Funny how those three posts are in hacker news top 5 now. I guess today is the low level appreciation day.

  * The Linux Boot Process: From Power Button to Kernel
  * The Journey Before main()
  * How programs get run: ELF binaries (2015) (lwn.net)

edit: format

3 months agoby hamasho