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The Linux Boot Process: From Power Button to Kernel
- pixelbeat__ 1y agoGRUB is mentioned but not detailed. Here are some details: https://www.pixelbeat.org/docs/disk/ https://www.pixelbeat.org/docs/disk/
- metabagel 1y agoHard to read on my phone due to faded text.
- Brybry 1y agoThe styling is bad on a desktop browser too. If you use Firefox or Firefox Mobile then reader mode is good for cases like this.
- DonHopkins 1y agoThe self deprecatingly downvoted look.
- superkuh 1y agoVideo 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.
- megous 1y agoIt's not a dependency for Linux boot at all. You can do well with serial port alone, as anyone who brought up eg. an ARM SoC in Linux will attest to. Also it's not very interesting either. At simplest, Linux just needs to take a pointer to a beginning of a framebuffer and some metadata, and will write to the framebuffer whenever there's something to update.
- superkuh 1y agoMaybe not linux specifically, but POST requires a video device software (BIOS Option ROM or UEFI GOP Drivers) of some sort does it not? That's been my experience with all PCs for 30 years. But maybe there are cases where it doesn't? edit: Apparently it's a desktop motherboard firmware thing. Ubiquitous but not technically a requirement for POSTing a computer.
- GreenVulpine 1y agoI've found AM4/AM5 boards will still boot Linux without a discrete or integrated GPU, running a GPUless CPU, not an APU.
- p_l 1y agoIt was sorta required by IBM PC compatibles, but is not required by UEFI which supports serial console (or other interfaces). IBM PC assumed existence of graphic output and sometimes Option ROMs did really too crazy things with it, I still have shivers when I hear "intel raid card" because of that one with possibly Win3.x in ROM...
- edoceo 1y agoSoekris (rip) had an x86 network device. Four 10/100s and the disk was a CF. Could only serial console that thing - or SSH once it's running. Best router I ever had. Also, in 2000 when Windows crashed you could get a serial debugger. Wonder if they still do that?
- okanat 1y agoPeople still need to do driver ddvelopement. So you can still set up a Windows PC to expose kernel debug interface over serial port: https://learn.microsoft.com/en-us/windows-hardware/drivers/debugger/setting-up-a-null-modem-cable-connection https://learn.microsoft.com/en-us/windows-hardware/drivers/d...
- okanat 1y agoIf you would like to see an actually "interesting" boot, I recommend checking out how Raspberry Pi's boot. It is a unique monstrosity that boots from the video / GPU core instead of one of the ARM cores. It has an arcane undocumented architecture.
- qingcharles 1y agoIf you can find a copy of this on the high seas, it's a great resource. I wrote my own OS by starting with this and the Linux source in the mid-90s: https://www.amazon.com/-/he/Developing-32-Bit-Operating-System-Cd-Rom/dp/0672306557 https://www.amazon.com/-/he/Developing-32-Bit-Operating-Syst...
- phendrenad2 1y agoIt's because on a brief period of old PC hardware (1994-1998), when Linux was starting off, high-resolution video initialization (SVGA+) was easier to do in 16-bit mode, before the bootloader switched the CPU over to 32-bit mode. Because GRUB handled this transition, GRUB became responsible for video initialization. After 1998 GPU manufacturers added 32-bit support for initialization, but the long tail of users with hardware with that limitation really entrenched this responsibility in GRUB.
- p_l 1y agoOnly on IBM PC because it essentially always assumed presence of either MDA or CGA, and over time evolved to assume presence of VGA. UEFI does not require at all and supports and UI framework to enable work in both graphical and textual (with so-called VT-UTF8) mode.
- pwpwp 1y agoLight gray text on white??
- nightshift1 1y agoThe topic is interesting but it seems to be targeted for my grandmother.
- zahlman 1y ago> When power stabilizes, the CPU resets itself to a tiny, old‑fashioned mode called real mode. Real mode dates back to the original 8086 chip. The rules are simple on purpose. Memory addresses are built from two values the CPU keeps in special fast storage called registers. You combine a segment and an offset like this: physical_address = (segment << 4) + offset Your grandmother sounds unusually proficient with this sort of thing.
- nightshift1 1y agoI dont know, i just don't like the tone. This is a complex subject where the target audience should probably already know what is an hexadecimal number or an interrupt and the explanation of a cpu register ought to be better than: "A register is a tiny slot inside the CPU. It holds a number the CPU is using right now." If the subject interest you, you deserve better.
- typpilol 1y agoAgreed. A lot of these articles leave me with more questions than answers. These blog posts really annoy me because I feel like with 20% more effort you could have something worth reading.
- munchlax 1y agoThe problem is that your 20% isn't the same 20% as anyone elses 20%. If you want a more thorough explanation, go read a book. Many are available for free on sites such as archive.org and programming-motherfucker.com
- IOT_Apprentice 1y agoI wonder if anyone will actually write their own article/blog providing a much better explanation of the subject matter. Or at least provide links to those articles that are alternatives to this.
- zahlman 1y agoIn 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?
- hamasho 1y agoFunny 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
- kuekacang 1y agoOh hey, a fellow noticing person!
- nightshift1 1y agoyes, and the bar is not at all at the same level.
- adtac 1y agoweekend hackernews best hackernews
- rramadass 1y agoVery much overdue, i would say! Lately HN was turning into just adware for AI news. Does anybody have pointers to similar low-level details specific to iOS/Android systems? Given that most of us by now have a few of these lying around the house gathering dust, they can make a nice DUT to hack/study/learn a few technical things. I would be quite interested in knowing more about how the RF subsystem works and maybe repurposed as an SDR for example.
- whitehexagon 1y agoAgreed, a very refreshing change. I cant reccomend anything apple or android based, but the PinePhone has some reasonable low-level documentation on itś SoC (Allwinner A64). And there are a great series of tutorials of how Apache Nuttx was ported to the PP. Here's one of the earlier ones: https://lupyuen.org/articles/uboot https://lupyuen.org/articles/uboot I've been working through them myself but still dont understand the boot process, and a long way from radio chip functionality. But I suspect you'll find radio chips quite locked down, since they are licensed for connection to mobile networks. Often there is just a unknown binary blob running the show.
- nickelpro 1y agoUEFI 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.
- MonaroVXR 1y agoWhere do I read more about this?
- heromal 1y agoHere's one source: https://depletionmode.com/uefi-boot.html https://depletionmode.com/uefi-boot.html
- ghostpepper 1y agohttps://www.youtube.com/playlist?list=PLUFkSN0XLZ-ltETI20mpXOCdqC8rdven6 https://www.youtube.com/playlist?list=PLUFkSN0XLZ-ltETI20mpX... The ost2.fyi courses are all high quality
- 7bit 1y agoI'm currently trying to learn more about how the boot process works, specifically with UEFI. Yesterday I watched a video on how to program a UEFI bootloader so I was happy when I saw this article this morning. The article jumps over so many steps and leaves out so many mechanics that I had to stop reading, because it just opened more and more questions without closing any. For example, right at the beginning: > Right after reset the CPU jumps to a special address called the reset vector at 0xFFFFFFF0. Think of it as a permanent bookmark that says “start here.” There is room for almost nothing at that address, so manufacturers put a short jump there that passes control to the firmware on your motherboard. What does that even mean? It jumps there means that it executed whatever is at that address, but it failed to explain how the instruction at that location is even put there in the first place. Reading a little more I truly am confused for who this was even written. It seems you need a lot of prior knowledge to understand what the author talks about and then it also makes me wonder what additional information this article conveys if you already can fill in the gaps.
- panny 1y agoI'm probably going to read this, but who thought putting light grey text on a white background was a good idea?
- kps 1y agoIt's 2025. Respect `:prefers-color-scheme` or don't specify colours at all.
- zzzeek 1y agoI'm going to save this guys blog in one step https://webaim.org/resources/contrastchecker/ https://webaim.org/resources/contrastchecker/ (this is the site: https://webaim.org/resources/contrastchecker/?fcolor=D0D0D0&bcolor=FFFFFF https://webaim.org/resources/contrastchecker/?fcolor=D0D0D0&...)
- neoattikos 1y agoNice 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
- jiveturkey 1y agofascinating 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.
- gmueckl 1y agoThis is old school BIOS boot. EFI bootloaders work very differently.
- okanat 1y agoand with GRUB running under UEFI, it actually uses UEFI load procedures instead of fumbling with 16-bit CS, DS, SS registers
- dgrin91 1y agoIt'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?
- vixen99 1y agoHN is not just read by IT professionals. And they might be a bit curious about Linux startup without necessarily recalling what they know or never knew about hexadecimals.
- 7bit 1y agoIt's fine to explain things to people that are not into that topic. But I also wonder what the target audience is. On one hand, he explains what a hexadecimal ist, but then jumps over essential information needed to understand the boot process. It's like: "Learning to swim: This is water, it is wet, if you jump in you sink and in water you cannot breath. Then you start moving your body parts and then you swim."
- samat 1y agoI think it’s an artifact of AI being used to write it. I spotted a few more down the page and stopped reading. I trust the judgment (and curation) of a person who invested significant time into learning stuff and then took even more time to write it down. I don’t trust people who use AI to generate text. If I want to read AI slop — I can generate one myself. I want some soul in texts. It is a sad day for the internet.
- liqilin1567 1y agoSeems 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 https://www.nan.fyi/database, built with the framework: https://github.com/nandanmen/NotANumber https://github.com/nandanmen/NotANumber
- e-dant 1y agoARM 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
- kgasser88 1y ago[flagged]
- deleted 1y ago[deleted]
- zaxioms 1y agoDoes 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.
- tremon 1y agoAt a very high level, the processor initialization procedure is similar to most microcontrollers: - processor is powered up with RESET pin asserted, making sure that all circuits are in a known-safe state. - when RESET is deasserted, all autonomous circuits (e.g. core control units, cache controllers, ALUs) go through a hardware setup phase. - the processor accesses the boot firmware according to the platform design. There's many options here, but the three most common are 1) the boot ROM is permanently mapped to a dedicated hardware address, and the processor uses its memory controller to access that memory; 2) the boot ROM is directly connected to the processor via dedicated I/O pins; 3) the initial boot code is located on the processor itself (factory-programmed EEPROM). - if the boot ROM is not mapped to a memory address, the data is loaded into CPU cache (external RAM needs to be initialized first, and that task is usually performed by the firmware). - the primary cpu core starts executing boot code (UEFI/BIOS/etc) from said predefined hardware address. Other cores (and processors) are usually left uninitialized throughout the startup process, and it is left to the operating system to bring up the remaining parts of the system. Specifically for Intel, see https://binarydebt.wordpress.com/2018/10/06/how-does-an-x86-processor-boot/ https://binarydebt.wordpress.com/2018/10/06/how-does-an-x86-... . Or see https://electronics.stackexchange.com/a/726918 https://electronics.stackexchange.com/a/726918 and its comments to get an idea of the differences between platforms.
- aborsy 1y agoRelated to this topic, what is the best way to replace the code involved in the entire boot process? This is useful when sanitizing a system received from a provider that may not be trustworthy, as malware could be hidden at low levels. 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.
- gldrk 1y agoAs 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.
- messe 1y agoThis isn't quite right either. That's not what unreal mode is. Unreal mode has 32-bit data segments limit, not code segment limits. In this case the code segment still has a 16 bit limit, it's just that its base address in the descriptor cache is outside the bounds typically associated with real mode.
- ramanvarma 1y agoReading this just reinforces how much of the x86/Linux boot chain is fossilized ceremony glued together for backward compatibility.
- phendrenad2 1y agoKinda 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.
- rollcat 1y agoThis. I haven't seen or touched GRUB or BIOS in about a decade.
- deivid 1y agoReally 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/ https://blog.davidv.dev/posts/booting-x86-64/
- Imustaskforhelp 1y agoOh wow! Combined with the explaination of iso files which can be a single file which can have things like fat and bzimage and rootfs etc. with the bootloader and combined with software like dd / gnome-disks and your article and the HN thread article, this finally makes me understand how Linux starts and how we can install things from https / internet and use software to fundamentally change the operating system Its magic. I was recently trying to build my own linux operating system and I felt so frustrated as to how to build an iso image from bzImage / linux kernel + initramfs but I was able to withstand its pain somehow and it took me a week to actually learn a lot more about the fundamentals of linux which are so fascinating. Now, I was trying to do some mix and matching (imagine tinycorelinux initramfs and buildroot bzImage and merging them) and doing some other shenanigans. Sometimes my computer kernel didn't work or the userspace didn't work in the vm and I guess now WHEN I AM WRITING THIS POST, I realized that it was because of the difference in 32 bit and 64 bit, I think I had 64 bit kernel and 32 bit userspace which had some issues There were some different things as well, which had caused some issues which made me want to scratch my remaining hair but it was so worth it (maybe i used some llm assistance in the end as well), its all just makes sense now, I always wanted to learn how computers work from scratch, now I feel like I have a very basic idea on how things work and they are complex indeed :sob: I was looking for a post like yours yesterday and so I am still glad I found it when the topic is still fresh. I faced so many issues trying to build my own iso but I don't know if it was my skill issue which streched it to a week. But the whole process of building your own iso seems less daunting to me now knowing what it really is I may have skill issues indeed but I fixed those, I feel like that there is a lot to learn from those errors that I missed but maybe that's also because they got too overwhelming. But I now know a path which works and some explainations of why they work. I feel more confident in my ability to make my own custom distro even, but my appreciation for linux grows so much. Its just such a massive rabbit hole and most of linux is literally just "it just works", linux is amazing.
- gblargg 1y agoI 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.
- panny 1y agoI think that is the job of the embedded controller, which is before the bootloader bringing up CPU. On Chromebooks, that part is open source along with the coreboot bootloader. https://chromium.googlesource.com/chromiumos/platform/ec/+/HEAD/docs/zephyr/README.md https://chromium.googlesource.com/chromiumos/platform/ec/+/H...
- darkwater 1y agoThis 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.
- a022311 1y agoThanks for the detailed write-up! I'd love to see something like this for ARM too!
- ra1231963 1y agoBased 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.
- Dwedit 1y agoThis doesn't cover the things that the CPU secretly does before it finally enters Real Mode, such as loading and validating the microcode.
- monegator 1y agoArticles 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.
- gosub100 1y agoI'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 https://www.youtube.com/watch?v=ZFHnbozz7b4