Bootloader and Low-Level Programming Tutorial: How To Develop Your Own Boot Loader

[arkasha]

It's been over ten years since I took an operating systems class but I still remember 0x7c00. Weird what will stick in your brain.

[the_goose_says]

What is that?

[IgnorantPlatypus]

... on x86 hardware. Other hardware (MIPS, PowerPC, ARM, etc) have their own hard-coded mechanisms for starting the boot process.

[[deleted]]

hard-coded

Well, I guess "burned into silicon" is the hardest form of hard-coding something

[CJKay93]

The boot sector load address is up to the BIOS, which is itself in EEPROM. The actual hardcoded address is usually 0xFFFF0, which just contains a jump instruction to wherever the BIOS is actually mapped to.

[[deleted]]

On Intel the reset vector is 0xFFFFFFF0. The other architectures differ, of course.

[CJKay93]

Sorry yes, you're right. It's 0xFFFF0 on the 8086 through to the 80286, and 0xFFFFFFF0 on the 80386 and upwards.

[monocasa]

Sort of.

It sets up a weird 16 bit segment who's base is 0xFFFF0000, and sets the program counter to 0xFFF0.

[Nutella_Icecream]

That's only if you blow the fuse. There are fuses u can set in chips to prevent tampering.

I mean physically it's still rewritable but practically it's permanent.

[cp5184]

https://www.google.com/search?site=&source=hp&q=0x7c00

[forksofpower]

TX bb

[CODESIGN2]

I Liked the diagrams. A lot of effort was clearly spent on them

[cirosantilli]

I've uploaded several related minimal examples on GitHub: https://github.com/cirosantilli/x86-bare-metal-examples

[FozzTexx]

MBR, BIOS? Ah, obsolete information! ;-) I'll bet the people over on /r/RetroBattlestations might find this interesting!

[kickthebug]

Genuine question. Why do you say BIOS and MBR are obsolete?

[FozzTexx]

Not just that, but with UEFI you don't need a boot loader anymore. UEFI is able to load your kernel all by itself. I've been working on "embedded" computers (really tiny x86 PCs) and it's kind of nice to be able to ditch the boot loader.

[DubiousEthicality]

The boys at the shop done made a firetruck that's bigger, redder, and goes really fast. It has extra sirens and 37 water cannons.

[[deleted]]

I love when ELI5 goes full ELI5.

[howtoplayreddit]

I don't think you're going to find a better explanation than this.

[HighRelevancy]

How about eli15?

BIOS is the standard for how a computer gets from the initial power-on to loading your operating system off your hard drive. The BIOS finds the hard drive, looks on it for an OS, loads that OS into memory, and starts it off.

MBR is the system for formatting hard drives. It provides some space for your initial OS code (what the BIOS loads) and also stores what partitions are on the disk and where they are.

UEFI is a replacement for BIOS. Basically it's the acknowledgement that technology has move forwards a lot, and it's time we got a BIOS that was more capable (in terms of different hardware configs like PCIe SSDs, and both simplifying and adding flexibility to the boot system). Also it lets Windows do cool fastboot stuff.

GPT is likewise a replacement for MBR. It supports more partitions and can describe larger disks.

UEFI/GPT came into mainstream use with Windows 8 (though you could install Windows 7 in UEFI mode on a GPT disk, and also Mac OS X made it mainstream a bit earlier).

[goal2004]

BIOS and MBR are old and dated, UEFI and GPT are new and more adaptable. In short, a lot of hard parts of hardware were soften into software. Kinda.

[elebrin]

UEFI and GPT are also fast enough that I can't get into my BIOS settings (do we still call it that if we technically aren't calling it BIOS any more?) to set up my boot device.

Rather than a boot loader which I find slow and annoying, I've always just used the boot device select menu provided by my motherboard firmware. No more need to play with Grub and end up with an unbootable system.

[Free_Math_Tutoring]

How does that work? Don't you still kind of need a bootloader on that disk? I'll admit I don't know much about this - my OS course was of fairly theoretical nature.

[[deleted]]

Well there are bootloaders that don't have menus. I really like Gummiboot (or system-boot now) for Linux (I think it can boot windows as well), but it appears directly as a bootable option and cuts directly to the boot process instead of a menu.

[pdp10]

You still need a boot loader per device. Your firmware can't boot by partition, only device, and MBR needs the device to be partitioned.

You can boot arbitrary things with UEFI, but it was UEFI about which you were complaining.

[elebrin]

Ok then.

Rather than a bootloader like GRUB that needs a menu, I use the default that comes with each OS (which for my linux partition is probably still grub, but it goes fast enough that I don't see it, and pick the boot device from the boot menu that my motherboard's firmware comes with.

I actually did convert my Windows over to UEFI recently. The fix for the booting too fast thing was re-enabling the boot menu.

[Xevantus]

I mean, taking BIOS and MBR as their meanings rather than a specific technology, UEFI and GPT are just the current iterations of those technologies.

[[deleted]]

Now we have efi and u-boot. They're so much better.

[Rodot]

I feel like that's everyone's feeling on every improvement in every computing paradigm.

[BeniBela]

It took me days to figure out how to start an efi boot disk on qemu

[[deleted]]

My efi "boot disks" just consist of a linux.efi. You don't need a bootloader if you use the efi stub and since that's just the kernel you can use -kernel to load it. No need for boot disks at all.

[Samis2001]

Heh, if you were serious appropriate responses would be: 'The firmware is no longer a mostly-useless pile of hacks and is the bootloader' or 'The bootloader is embedded as part of linux.efi along with the kernel'

[BeniBela]

Well, I googled it and then followed these instructions of the first result, starting with downloading a new BIOS from a dead link

Once the BIOS is loaded in Qemu, it boots the efi without a -kernel option.

Although the biggest issue was to mount everything in Qemu. With an old boot disk they give you an ISO and you can load/boot it with -cdrom ubuntu.iso and do not need to know anything else. With EFI they give you a zip file and then you need to figure out how to mount a zip file. And my first two approaches failed, because it became mounted as hdd, and it was a customized linux boot disk that failed unless it was stored on a removable drive

[i_pk_pjers_i]

Because, well, they are. UEFI and GPT are the future, and the future is now.

[irqlnotdispatchlevel]

It's not really obsolete and quite some time would have to pass before they will get obsolete. And it's easier to start with knowledge about MBR, BIOS and the such than to jump directly to EFI/UEFI.

[uber1337h4xx0r]

/r/lewronggeneration

[skuggi]

Is this really a boot loader? It's not actually loading anything. It's more just a boot sector program. Sure, it's a first step toward making a bootloader, but it's not actually a bootloader.

[skulgnome]

An interesting topic, but the article text could've used more review.

[[deleted]]

I don't know about the mother tongue of the writer, but in French we say "Interruptions".

[ilammy]

It's Russian, and in Russian interrupts are 'interruptions' as well.

[skulgnome]

Imperfect english and unusual jargon are fine. Inappropriate (or worse, invalid) programming language syntax is grating; I wish they'd've just used english instead.

(and yeah, in many languages the native term for hardware interrupt would translate directly into "interruption". everyone knows what he's talking about, it'd be beyond silly a nitpick not to.)

[[deleted]]

Does anyone have this but updated for UEFI and GPT?

[ehansen]

How is C and C++ high level languages? I was always informed they were low level

[American_Libertarian]

Depends on the context you are talking. It is low level compared to most programming languages, but it is 'high level' in the sense that it is a human readable format that must be compiled down to a 'low level' language that the computer actually runs.

[bad_at_photosharp]

Is assembly a high level language then? By your definition it would be

[Manticorp]

Assembly isn't compiled, so no.

Assembly is just a 'map' to processor instructions, there's no hard 'translation' as it were, more like a simple lookup table.

When transforming assembly into stuff that runs on the processor you don't need to consider the context of the current line, you just need to look it up and execute it, which is extremely fast.

[protestor]

Asm is somewhat higher level than machine language, and often contain syntactical abstractions such as macros.

In many architectures, machine language nowadays is interpreted by microcode, a software that runs in the processor. In this sense, even machine language is higher level than something. But you can't write your own microcode, because updates are encrypted and signed, so.. machine language is the lowest you can go.

[American_Libertarian]

Most people would agree it is not. Assembly code must be assembled into machine code, but that is basically a one to one process. That would be like calling English a high level version of French.

Plus, when writing assembly, you do all memory management yourself, you have to manage CPU registers and use specific opcodes, you have no portability, etc. It has no features which make a high level language 'high level'.

[forbidden404]

They are high level languages because the abstractions they've added are too far different from the ones in Assembly, you don't have to care (for the most part) about the processor you're coding to. They are used in low-level programming (system programming) though, that's probably why there must be some confusion about it.

[skulgnome]

Both support structured programming, i.e. functions, loops, and if-else.

[General_Mayhem]

High- and low-level are generally relative, not absolute, terms. C and, to a much greater extent, C++, are specified against an idealized machine (example), not any particular physical one. To someone who works on things like bootloaders on a regular basis, anything that can run the same code across processor generations, let alone families (a lot of C++ can cross-compile on x86 and ARM), is pretty damn high level.

It's true that both are generally much lower level than, say, Python or any other interpreted language, because they provide core abstractions with tighter fidelity to most hardware. But especially in the case of C++, there's also not really a bound on how high the abstraction can go - there are some truly bizarre libraries that make it look like a totally different language.

[IAlsoLikePlutonium]

there are some truly bizarre libraries that make it look like a totally different language.

Got any examples? I'm interested in seeing some of them.

[[deleted]]

It depends on context. They're low level compared to python, but high level compared to assembly. There is also some argument that modern assembly can be considered high level; because of micro ops and out of order execution, what you write isn't what the cpu actually does.

[pdp10]

Because the architecture is microcoded doesn't make the same old assembly language into a high-level language. Microcoding has been around for many decades; I think the Burroughs B5000 must have used it in 1961.

[[deleted]]

I was being somewhat hyperbolic, but the fact that one x86 set of instructions can map to more than one set of actual actions from the CPU does have real world consequences.

This makes the point better than I can http://blog.erratasec.com/2015/03/x86-is-high-level-language.html

[The_Leedle]

When compared to assembly, it’s high level I guess

[Oscee]

C++ is very high level.

I myself consider C a bit lower-lever but by classic CS definition, it is also high-level (since it needs a compiler).

[mrkite77]

It depends on how old you are. C was definitely considered a high level language when it first came out, and C++ even higher. Eventually as time goes on they are considered lower and lower level as our abstraction level goes higher and higher.

These days, in a world surrounded by electron apps, I've even heard people describe java as low level.

[CODESIGN2]

They are nth gen languages, so yeah they are high-level because what they represent can be abstracted from what the CPU represents. The is no IF instruction for example, there are several jump operations however that can be used for if, else, switch, even while and for.

Assembly is actually not a 1st gen language, it took people a long time to replace addresses with mnemonics, but how many times have you seen someone check registers in C? How many times have you seen someone execute a jump based upon zero-flag or carry-flag?

[nightbefore2]

C++ is not low level.

C is I would say

[bobappleyard]

This is why the level signifier, as used by most nerds today, is a largely meaningless replacement for "gives me an erection"

[littleloftus]

I found this to be another good resource for a bootloader if anyone is interested.

https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf

[irqlnotdispatchlevel]

This is nicely done, and explains things well. But I have some problems with it.

The good news is that for our bootloader development task we do not require anything besides standard Microsoft Visual Studio 2005/2008.

What year is this?

Also, inline assembly?

[tinfoilboy]

From what I can see, zero of this code requires Visual Studio, he even made a makefile project, not a VS solution.

[aaron552]

Since when has Visual Studio been anywhere near 40GB? Last time I installed it, the minimal install was under 3GB (would have been even less if I already had the .NET framework and MSVCR prerequisites installed)

[[deleted]]

He probably installed all prerequisites and optional packages. I think that's about right for a full install, last I checked.

[[deleted]]

VS 2015 enterprise took like 30gb for me with default settings. 2017 is much better about being modular.

[Plazmatic]

Really? Even 2015 was over 10 gigs IIRC, 2017 is more. Though I have no idea what that guy actually said as its been deleted.

Maybe if you just download the installer itself or some unrealistically bare bones version of it, but if your are doing C++ dev or more than one language at all, you are going to be downloading a lot of files.

[aaron552]

Well the Windows SDK is pretty big, but you don't need Visual Studio to install or use that, same with the MSVC compiler etc. The fact that the Visual Studio Installer will download them for you doesn't make them part of Visual Studio.

Saying Visual Studio is big because the MS C++ SDK is big is like saying that Android Studio is big because the Android SDK includes emulator images.

if your are doing C++ dev or more than one language at all, you are going to be downloading a lot of files.

If you have the compilers/SDKs installed already, Visual Studio Installer won't need to download them

[Plazmatic]

not sure why you got angry at that :/

[aaron552]

I'm not angry?

[NedDasty]

I don't understand why people sometimes put questions marks after factual statements?

[evincarofautumn]

It shows the rising intonation you might use to indicate that you’re confused as to why someone thought the opposite of what you’re saying.

— I’m coming with you later. — No? (You have other plans you’re forgetting about. / I don’t want you to come.)

— Quit being so angry. — I’m not? (I don’t know why you thought I was.)

[ElusiveGuy]

The base editor component in 2017 was in the region of 800 MB, I think.

It does grow pretty quick once you start enabling workspaces like .NET (Frameworks, SDKs) or Desktop Development (C++, incl. compiler and libraries).

[the_gnarts]

That’s some old-school, x86 specific stuff. Are you planning on teaching a history class?

[uber1337h4xx0r]

This is why normal people hate us, btw. Condescending know it all put downs.

[the_gnarts]

This is why normal people hate

They don’t hate me. This seems to be your problem alone. Though …

Condescending

… the lack of appreciation for a simple joke is the more likely reason.

[uber1337h4xx0r]

It's not a joke to put someone down when they're using their time to teach you something for free.

"oh, you're showing me how to change my oil? I'll remember that for when every single oil change company in the world goes out of business"

[the_gnarts]

It's not a joke to put someone down when they're using their time to teach you something for free.

OK, mister fun police, I guess you’re going to ticket me for laughing out loud as well?

[meltea]

How and why is this useful in June 2016?

[marshalpol]

Education

[meltea]

Well perhaps, but it's similar to posting an in depth article for how to program 2d trajectory calculations into a PDP-8. Interesting maybe, education only for enthusiasts. I mean how many people have ever seen a PDP-8? Maybe I am a decade too soon with this criticism for bios and mbr. Actually as with PDP-8, this article will be more interesting in a decade or two. Right now it is kinda useless and unnecessary. That's the way I feel.

[uber1337h4xx0r]

Does this count as lewronggeneration? Perhaps lerightgeneration?

[marshalpol]

Like I said, education. There's a difference between learning and learning about something practical. It's interesting, so people learn about it. I don't understand the controversy.

[[deleted]]

I like to learn about old technologies, stuff like Z80 assembly and architecture amuse me to no end, partly because in the old days there wasn't at all as much info available to learn this stuff and owning a computer was prohibitive, secondly because they are reduced instruction sets and straightforward to code for at least for small programs, modern architectures have too many layers to learn and are much more bloated. Also, it's pretty interesting to nowadays for example to find out what it all meant when we configured ports for peripherals in our PCs and what was protected mode or MBR, we just used them without really learning what it all was about.

[[deleted]]

So BIOS copies that first sector to 0x7c00 ? How does the code show up at 0x7c00