Improve this question. Add a comment. Active Oldest Votes. Improve this answer. Alex Alex 1 1 gold badge 6 6 silver badges 15 15 bronze badges. I heard that "CPUs start in 16bit mode. UEFI runs in 32bit or 64bit. But the bootloader is not before the UEFI!!
Grub is a bootloader, it runs after the UEFI. The CPU is in "unreal" mode at this point, with the CS segment base outside of the low 1MiB, but otherwise the same as bit real mode. The system designer makes sure that address is mapped to a flash ROM. That code will eventually switch to bit mode before reading anything from disk. But before that it has to configure the DRAM controllers and stuff like that! It will usually be using cache-as-RAM no fill mode for some of that time Show 2 more comments.
Here's a good answer to this question: Other modern bit machines have new EFI firmwares. Community Bot 1 1 1 silver badge. Buddy Buddy UEFI works in 64 bit long mode! Some platforms may use 32 bit flat mode. The terminology is wrong at first place that's why it confuses a lot of people.
Back in the days everything that was not x86 mode was called protected mode. But that is not entirely correct. You might seem to call that a boot loader of some sort, but the fact is that where it starts in 16bit mode is part of UEFI — barlop. Before that the firmware has to get this specific system's memory controllers configured, and stuff like that, and that part is fully private and isn't governed by any standard. Secure Boot When enabled and fully configured, Secure Boot helps a computer resist attacks and infection from malware.
CS contains 0 remember it's the base that is 0xffff so it will load the first entry. Lewis Kelsey Lewis Kelsey 2, 1 1 gold badge 19 19 silver badges 25 25 bronze badges.
Another source states: "Newer security and management engines shipped with select Intel platforms in and after support a feature called field programmable fuses". Best Portable Monitors. Best Gaming Keyboards. Best Drones. Best 4K TVs.
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Apart from this UI issue, is there anything else that's painfull? Your text gave me the possbily wrong impression that you could do everything with efibootmgr, and with that, bypassing whatever downsides the motherboard UI has. I could imagine this having its own entry in the efi list, and being the default boot entry.
The displayed list would just show the efi entries, selecting an entry would use the nextboot efi feature, a timeout would nextboot a custom default not changing the efi default, which would keen pointing to this grub-like thing. Letting my mind roam freely, this could bypass the problem of motherboards not displaying the list in the same fashion, and OSes being able to rely on it being there and knowing how it presents choice to the user, making it not being every individual OSes headache.
Also gone be the days of OSes overwriting each other's bootmanagers. Any installed OS would just need to add itself to the efi boot list. And add such grub-like thing if there isn't one yet, and make it the default. With a few naming conventions, it could even add temporary boot entries like for one-time modifying kernel params which it would delete again on next occasion.
The one obvious downside would be that you're always rebooting at least once. But I'd accept that for all the advantages I can think of ;. It's mainly the deployment time where people have trouble, and then understanding what actually constitutes their boot config and hence should not be poked with sticks after that.
I mean, it can certainly work; I just see so many people struggling with it and misunderstanding stuff that I get concerned. That section was slightly tongue-in-cheek, but with a serious point: it really is easier if you can just stick to an OS per machine or per disk if you can.
It may just be part of my general inclination after years of fiddling with PCs and trying to help other people fiddle with them: I really, really believe in the 'choice is an excellent way to shoot yourself in the foot' argument, and try to keep my setups as simple as possible. There's enough damn complexity in dealing with computers without you going out and voluntarily adding more on top, IMO : "Your text gave me the possbily wrong impression that you could do everything with efibootmgr, and with that, bypassing whatever downsides the motherboard UI has.
I don't know of anyone who's done it yet, but I don't know everything. Rod if he's reading? People have certainly written things that are meant to sit at the UEFI bootloader level and intermediate between you and all this craziness.
I have to admit that I tend to view them as yet another layer of craziness ; , but some people prefer to take the approach of picking one and making it their primary interface to the whole shebang.
Thanks for the Rodbooks. In short, they seem to mostly leave available features aside and instead focus on what EFI requires an OS to conform to and make use of that. And they're adding some more requirements on top of that like naming schemes, certain min kernel version, or even limitations to certain OSes. Why don't those bootmanagers "simply" manage efi boot entries? If there was such a bootmanager and it was free and opensource ; , each OS would only have to care about adding a working EFI-entry for itself.
Of course the OS-installer could and should also offer to optionally install that bootmanager during installation. I've toyed with the idea of adding such a mode of operation to rEFInd, but I haven't done so because I don't believe it would add anything to what rEFInd already does, and in many ways it would in fact be limiting.
For instance, if you use rEFInd to boot Linux kernels directly, the approach you suggest would require using efibootmgr every time you add a kernel. Another problem with this approach -- and a flaw with EFI generally -- is that placing boot loaders on the hard disk and information about them in NVRAM creates two points of failure in the process of launching boot loaders.
This is a practical real-world problem -- I've both heard of and personally encountered systems that "forget" their boot loader entries on a regular basis. One of my computers does so if a hard disk is temporarily unplugged, for instance. There can also be OS-driven bugs. Some versions of shim can create an ever-expanding boot list, for instance, and this would be a nightmare for a boot loader following your suggested design.
That's not to say that such an approach doesn't have its merits, and might not appeal to some people -- it does have merits, like being configurable from any OS using the standard EFI mechanisms.
I'll consider adding it as an option to rEFInd; in fact, in writing this reply, I'm contemplating ways that the NVRAM-based data might be integrated with the auto-scanned data in ways that might be useful. The poor boot manager menus that some implementations present are an issue, and they represent a good argument for using rEFInd, gummiboot, GRUB, or some other boot manager.
Note that rEFInd and gummiboot are both add-on boot managers only, whereas GRUB does double duty as both a boot manager and a boot loader. The built-in boot managers are usually extremely limited in what they can do. Most notably, most of them require hitting a key which varies from one computer to another, no less!
This sort of design has always struck me as brain-dead, and because of system-to-system differences, documenting it is a nightmare. Fedora's philosophy at one point was to rely on the built-in EFI boot manager for multi-booting, but if I'm not mistaken, the Fedora developers have come to their senses on this one. Thus, when analyzed fully and configured optimally, neither rEFInd nor gummiboot is "another layer of craziness"; they're both alternatives to GRUB, at least functionally. Yes, rEFInd and gummiboot are both boot managers but not boot loaders; but with a boot loader built into 3.
It's true that both rely on the EFI stub loader, which was added with the 3. Not many distributions ship with kernels older than that at this point, though. Thus, this is no longer really a practical concern. The difference is that rEFInd can auto-detect boot loaders including Linux kernels in certain locations and with certain filenames.
Really, my intent in designing rEFInd or expanding rEFIt's OS-scanning features; it's only fair to give credit to Christoph Pfisterer, rEFIt's creator, for designing the basic framework is the same as what you want, at a broad level: To launch any OS's boot loader with little or no extra configuration.
People who use rEFInd tell me that it finds boot loaders and Linux kernels quite reliably; most of the problem reports I see relate to EFI bugs or difficulties installing rEFInd in exotic setups, not to the boot loader scanning features.
Steve Riley wrote on :. I have come to appreciate its superiority over BIOS. I concur with everything Rod writes about how rEFInd simplifies managing multi-boot. Actually, "set up" implies too much work -- the only default I changed was to use text mode boot rather than graphical.
Otherwise, rEFInd is a cinch because its autodetection is so good. Yes, I am gushing. Oh well! Doug wrote on :. They can't be system partitions, because FAT doesn't support permissions. So what and where are they? They're not "system partitions" in the sense of Unix file permissions or something like that.
The sense is more "the stuff here is just uninteresting bits necessary to make the system boot, not anything you're likely to be interested in". The article explains what they're for: put very generically, it's a place where the OS layer can write stuff for the firmware layer to read. The obvious thing that falls into that category is bootloader code, and indeed that is mostly what is put in ESPs.
There are other potential uses for it, but the main thing is bootloaders. The spec explicitly states that you can have as many EFI system partitions as you like on whatever disks you like in whatever locations you like on those disks. At least some EFIs give the option to read firmware updates from the ESP, so that's another example of what can go there. In principle, though, you could put a driver for a plug-in Ethernet card, a video card, or whatever on the ESP so as to give the firmware the ability to use that hardware, even if the hardware lacks EFI-compatible firmware itself.
A few such drivers do exist, but they're pretty rare. The freedesktop. SteveSi wrote on :. Hi Very nice article. I am unclear about 'Removable devices' and 'Removable Media'. That's an interesting question, and I don't have the answer on hand. Looking through the spec, I don't believe it ever defines what should be considered 'removable' or 'non-removable'. The sensible thing for a firmware to do would of course be to respect the RMB. My opinions on how commonly firmwares do sensible things, are, I believe, on the public record ; It does explicitly list "Hard Drive" in section Of course, the distinction isn't actually incredibly important; as I read it, what it basically boils down to is that when everything in the boot list is invalid and the firmware's doing fallback path processing it is required to check removable media before fixed media, and removable media are required in the spec to have only a single EFI system partition.
I suspect the BIOS interrogates devices on external interfaces first. Where the spec. It should be simple enough to test, but of course, it will depend on how different BIOS vendors interpret the spec.! Yeah, as the spec doesn't clearly state it, you can't assume all firmwares will do the same thing. And you don't mean "the BIOS". It is not a BIOS. John King wrote on :.
There is so much garbage on the web on UEFI. Your article is both articulate, accurate, and a joy to read. I'm going to use this in my lecutures if you don't and if you do. I love you. You say that "In the BIOS world, absolutely all forms of multi-booting are handled above the firmware layer".
Mikhail Ranish actually wrote a multi-boot loader that fit entirely within the MBR code section Cylinder 0, Head 0, Sector 1 in the late s. I'm not referring to the XOSL implementation and other similar ones that booted code in the partition boot sector, this work or art fit entirely into the MBR. Paul wrote on :. Currently reading the huge web page. Very impressive and delightfully educational. Pale GREY coloured http links should be in a more easily readable colour. Paragraph numbers should be added - to assist referencing and to assist readers returning to the web page to resume reading.
One thing that's worth mentioning re: UEFI vs. UEFI, however, is running in protected mode or long mode , so you're not working with your hands tied behind your back. Otherwise, good and thorough information, thanks! Hello Adam Williamson. This is really a great essay! May I re-distribute this essay in my mother language?
I like it very much, and it is a good essay for us to learn. If it is OK, I may combine your essay with some of the comments. Yes, absolutely, I give you permission, thanks! Please just drop a comment here with a link to the translation when you're done :. I refered to your source link at the beginning of my post. Anyway, the translation work takes much more time than I expected lol.
I think I might be in love with you. This was totally awesome. I think I might print it out and frame it. But that came at the cost of blood, sweat, tears, and massive headaches. This condensed it into a short, well-written, easy to understand post.
I learned a lot, and what I already knew was greatly solidified. Thank you, thank you, thank you! I am deeply offended that you would suggest this post is "short" ; Seriously, thanks very much, and I'm glad to help! Ha ha! Okay, it's not short. It's extremely long. But I read it all in one sitting without alcohol or caffeine, and I finished with a much better understanding of the subject matter. Which is more than I can say for any other document on the boot process I've ever looked at.
They're either lacking in information, plain wrong, or stunningly dull, long, and difficult to understand. Sergio Belkin wrote on :. Hi Adam, I love your job since Mandriva days. Thanks for explaining this complex topic in such a friendly way. However I have a doubt.
We here the vboxdrv kernel module, it's not signed so only works with Secure Boot disabled. What I cannot understand if I should recompile and sign my own kernel, of if can I sign only the module vboxdrv Thanks in advance! David wrote on :. I'd really love to get my hands on a series of books about computer hardware history from as far as the first PC onward to cover as much as posible as clear and deep as your article. Thank you very much.
Green wrote on :. I'm an IT professional with over 35 years of hardware and software experience. I've been working with hard drives and disk booting since we had to low level format each drive and scan for sector errors before shipping a PC.
LOL Anyway, I've been trying to figure out for months if I can enable secure boot and still swap drives in my laptop. There was so much conflicting info on UEFI out there that I was concerned that turning SB on would tie the laptop to just a single bootable drive and that I wouldn't be able to boot from the old drive once the OS was installed on the new one.
Now after reading this I understand that as long as both drives have signed operating systems on them I can swap them at will.
Now I can finally format a new hybrid drive with Win8. Again, thank you! Mariano S. A tear rolled in my eye when I find that "someone else" has gotten to appreciate the magic of this program. I'm using it to this date, safely booting 7 OS from a single drive. Actually I use the beta-beta-beta and I've been using it for so many years that I want to find the coder to tell him he can remove that "beta" tag from it :. Kalashnikov wrote on :. You do have some explanation here, but it appears that you have failed to exclude the garbage on the other sites too.
Here is how your article appears to readers: Garbage about other sites something about UEFI garbage about other misconceptions something about UEFI garbage and something about uefi here funny talk something about UEFI here garbage Try keeping the nonsense entirely out. People are actually spending their precious time reading stuff on the internet. Make it count.
This is my personal blog. I'll include as much garbage as I want. You are free, at any point, not to read it. Ralph wrote on :. I will build a Hackintosh within the next days for the first time — after beeing pissed off by Apple more and more , and your elucidation will surely be helpful. I will dual boot with Linux — but as I have learned now, I shall use separate disks, what I will observe, though using rEFInd many thanks to you, Rod, too!
Dear greetings from Germany Ralph. Thanks for this information on UEFI. Which one will be used when booting? Andy wrote on :. Thanks for the article.
The option to boot windows failed seems obvious now but I then lost the ability to view the Windows UEFI boot manager. Lu wrote on :. I'll be happy if you to get you're help.
Mertsch wrote on :. Also thanks for clearing up the "Secure Boot hate" which I only heard on the side and never really knew what it was about. Dean wrote on :. Hi Adam, I have a questuion. Thank you. But it's not something we think has been very widely used in The Real World and I haven't tested it for real myself.
Honiix wrote on :. On Windows, Diskpart crashes if you try to do so. I haven't tested with Linux yet. Why is that? My guess is, ESP partition is optional on removable media!? That is in the spec. Max wrote on :.
Hello Adam, I've done a few things of the previous lines, and my computer is behaving kind of weird. Also, I have the gdisk tool in a Hiren's CD.
Thanks in advanced for reading and your possible help. Stinger wrote on :. Most of what I have seen on fora's and websites are, as you so nicely put it, half baked truths, propaganda or downright lies.
As I see it, this can be caused by two things? The distro has poor support for UEFI firmware. The pc has a poorly designed UEFI firmware. Hypothetical, to get as many as possible to enjoy their first encounter with a distro, you would have to have good support for UEFI in general and beyond that, you would have to do your best to deal with poorly designed UEFI firmware. As I see it, the biggest problem is that the installation routine of many distros doesn't deal gracefully with installing on a UEFI pc, leaving the user to try and deal with it themselves afterwards even worse if its a multiboot scenario.
Your opinion would be highly appreciated :. SomeName wrote on :. Really nice article! I like your sarcastic style of writig ;. Rajan K Srivastava wrote on :. Yash Pal wrote on :. It does give a good overall introduction to UEFI. Kan wrote on :. Waste of my time! Nate wrote on :. Simon Trangmar wrote on :. Found it very helpful although I had to read it copious times and with copious amounts of gin - as recommended! Thanks for all the time and effort you have put into this article - I really appreciate it.
Joe Pesco wrote on :. Humphrey Bogart was regular everyday folk and George Clooney is regular everyday folk at least in their movie roles. Anon wrote on :. This seems to imply that UEFI is doing a pretty damn bad job, if something as simple as having a dual boot has to be a massive pain and recommanded-against.
Chris Chua wrote on :. Dave Thompson wrote on :. Richard wrote on :. Very well Written. And Clear. And after reading, I can safely say It fits on a kb floppy disk. Back then it was was EMBR 1. And all this fit on 1 floppy disk which could have been made in to a firmware BIOS. Im sticking with EMBR, it works without complication, and its a hell of a lot easier to understand. I neglected to add, EMBR also blows away the drive space limits i.
Joe wrote on :. There is nothing here that sells me on using UEFI on my computer. I am not changing the way I describe computers, to recognize UEFI as some kind of important development, at least until I know why it is a good idea. A BIOS is a firmware that starts a computer. Why is it better for the UEFI bootloader code to be located on the motherboard instead of on the hard drive anyway? It just scatters the data in the computer. I don't feel a better sense of security over my data by having Microsoft offload its operating system onto my computer's motherboard instead of keeping it contained on the hard drive.
I know that Microsoft did not invent the part of the Windows which has been offloaded onto the motherboard. But Microsoft insists that this non-Microsoft code be incorporated into the design, so it is part of the Windows operating system.
Otherwise, Windows could work without it. UEFI gets pushed in the way of other operating systems which are forced to work around the intrusion. I am annoyed with Microsoft about my lack of good access to the firmware boot manager. My hardware vendor may have been able to mitigate my inconvenience by designing better support, but I dwell more on the fact that Microsoft pressured my vendor to implement UEFI in the first place, and I do not want UEFI.
Also, Microsoft has been refusing to license the sale of Windows on ARM devices that allow for legacy boot or Insecureboot. If it's not Secureboot, it must be Insecureboot, right? The blog post here observes that it is not a good idea to mix legacy boot with UEFI native boot. Then, it preferentially boots one of those drives, even if I have legacy boot turned on. It only boots legacy-style if a drive that is natively bootable by UEFI cannot be found.
You could now blame all this on the UEFI-spec being too vague - i. Instead they just copped out by saying 'we will not define the CSM compatibility functions! Big mistake! I would have preferred having this UEFI in some "arbitrary" pre-partition area instead of jammed into my motherboard. Then old things could automatically still work on my new computer. My computer came with Windows 8. I went through all the trouble to determine how to make the system bootable in legacy mode. It is lots and lots of trouble, and I eventually got it working.
My computer now seems to boot every bit as efficiently as it did before the switch. How do I know that I succeeded? I got Diskcryptor full disk encryption to work properly on my computer. It needs the disk to be BIOS bootable.
That company that Richard mentioned 2 comments behind my first one, Terabyte, has excellent instructions on how to reconfigure Windows 8 from native UEFI boot to legacy boot. The solution to that problem is to create a standard where partition information does not have to be stuck within 64 bytes in sector 0. Just use the next sector for partitions. BIOS was not causing the 64 byte limit on partition tables.
The lack of operating system support for partitions outside of sector 0 was enforcing the limits. Instead of a few bytes of assembly code for loading the operating system, each installed OS should have its own bootloader e.
This bootloader will have enough logic to either display some sort of boot menu or start loading an operating system. Basically, UEFI is its own mini-operating system. Using GPT eliminates the limitation on the number of partitions and also ensures support for larger partitions of up to 9ZB.
A zettabyte is 10 2 1 bytes. The specification allows for nearly an unlimited number of partitions, but specific implementations might impose more practical limits. Windows, for instance, actually limits the number of partitions to The general structure of the EFI filesystem means that each operating system or vendor has its own directory. This directory can contain any and all necessary files for loading the operating system:.
Some directories have a fairly flat hierarchy, whereas others e. The bootloader programs by default have the extension. Taking this one step further, though, a bootloader is just a program that performs the operation of loading the operating system. What if this program were a shell? A UEFI-capable shell would allow the user to interact with the UEFI system to modify boot parameters, launch bootloaders, and obtain information on the firmware environment.
The Tianocore shell includes several options that I currently have no use for but that might come in handy in the future. With this shell, it is possible to select a new boot-loader manually, do directory listings, edit text files, or remove files. When starting your shell, you need a starting point. Oddly enough, when running my shell, the current working directory is not actually on any of the filesystems, which causes an error for each command.
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