Why he is using 32-bit esp/ebp registers in 16-bit environment? There are 16-bit sp/bp registers. And there is 0xFFFF limit on segment descriptors in real mode anyway.
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Writing a Tiny x86 Bootloader
21–30 of 55 posts
Re: Writing a Tiny x86 Bootloader
#22Earlier quoted context omitted.
What would be the equivalent of this in UEFI? I mean is there a one page tutorial on booting with UEFI by writing a few lines of assembly?
I actually looked this up after reading this comment. Best I could find after a quick google search was this: http://x86asm.net/articles/uefi-programming-first-steps/ which is certainly not an easy tutorial to follow. Seems like more robustness tends towards a greater barrier to entry.
Once you outgrow your boot sector space, writing a BIOS based bare metal project gets a lot harder as you need to start loading stuff from disk with more BIOS calls.
It's a lot easier and nicer to use either UEFI or a Multiboot-protocol bootloader (GRUB, or qemu/bochs built-in bootloader). That also means you will be starting in long mode or protected mode.
In order to work with bare metal efficiently, you need to get a debuggable and bootable ELF image up and running as soon as possible. Once you can attach a remote GDB debugger to Qemu, you get things done much faster than using the Qemu/Bochs monitor/debugger.
Anything more complex than a hello world example is easier with UEFI or multiboot than BIOS boot sectors. You will need a linker script and a build script but things will be much easier after the first steps.
I made this pull request for someone else's bare metal project to add debuggable ELF images. It's a small but practical example:
Re: Writing a Tiny x86 Bootloader
#23Does anyone know of any resources for doing stuff like this on ARM? I have a board with u-boot, but I can't find any information about how to do even a hello world type program.
Re: Writing a Tiny x86 Bootloader
#24This looks like it would go together well with the article yesterday of the little OS handbook, which used GRUB for its bootloader to focus on the OS part.
Re: Writing a Tiny x86 Bootloader
#25Not saying he is wrong or anything,but he seems to have missed how modern machines ship with UEFI which tries to solve this problem on firmware level. That said: if you're curious and want to learn, I have no objections to digging into stuff, even "obsolete" stuff like BIOS boot :)
Thanks for giving it a look! You're certainly right about UEFI. I figured it would be easier and informative enough in the long run to just write a simple MBR bootloader instead of an EFI-format one. Either way, it was definitely fun.
It surely was fun, and I personally liked the way you detailed and commented the code, but that is not a bootloader, it is a bootload, i.e. something that is loaded at boot, executes some instructions and doesn't actually load anything.
In the good ol' days there were tens of similar bootsectors (to be used on non bootable floppies) that could display mesages or draw ASCII art.
If anyone wants to play with actual MBR bootloaders, a basic MBR is by Peter Anvin (and Andrea Mazzoleni) in makebootfat:
http://www.advancemame.it/doc-makebootfat
and for an example of how much stuff you can actually put inside some 440 bytes, the base reference is mbldr:
Re: Writing a Tiny x86 Bootloader
#26Does anyone know of any resources for doing stuff like this on ARM? I have a board with u-boot, but I can't find any information about how to do even a hello world type program.
32-bit ARM is a bit awkward because traditionally there is no equivalent of the x86 BIOS APIs for doing things like printing output, and the code started by a bootloader is expected to talk directly to the hardware (serial port etc), which is different on every board. If your board uses UEFI then you can use those APIs, or if your grub was built with CONFIG_API that might provide something useful. Otherwise even 'hello world' will require talking to the serial port and adjusting any tutorial code you find to the specifics of your board.
Re: Writing a Tiny x86 Bootloader
#27It's sad that most of these "bootloader" guides don't take you into actualy boot-loading something. No searching a file system. No loading another binary. No setting up 32 mode. Nothing, just printing text. That's what most of these are on.
Re: Writing a Tiny x86 Bootloader
#28Earlier quoted context omitted.
The "findings" you are describing are characteristic of security-conscious users suffering from paranoid schizophrenia. This is not an isolated incident. The "badBIOS" incident reported by Dragos Ruiu a few years ago ([1]) had many of the same characteristics as what you are reporting: • Vague symptoms, affecting all computers (and, often, even non-computer devices) owned by the individual • Inability to isolate the…
Except I have isolated the code and can reproduce it on demand, and have the photo's & video's as evidence so nice try. However having just read this https://www.facebook.com/dragosr/posts/10151655183445588 I can see he has explained much of what I was witnessing on some systems as well. Cant rule out a modern day version of one of these https://en.wikipedia.org/wiki/Phoebus_cartel considering the Windows MSR partiti…
With all due respect, everything you said sounds like some kind of conspiracy theory. If you have isolated this case, write a blog post of how to reproduce it, post said photos & videos, so people might at least take their time to read it. I'm sure with all the experts here on HN somebody would provide an explanation. There are hell lots of processes happening at modern system boot time, if it does something you don't understand, it doesn't necessarily mean malware.
" The OS's seem to actively hide the malware if you use a hex editor to scan the drive or infected files, so over time, some of that open source code has become compromised"
This part, in particular, I find hard to believe. You can inspect open source system and compile everything yourself. Did you try to reproduce this behavior on minimal systems (i.e. BeagleBone) or QEMU?
Re: Writing a Tiny x86 Bootloader
#29It's sad that most of these "bootloader" guides don't take you into actualy boot-loading something. No searching a file system. No loading another binary. No setting up 32 mode. Nothing, just printing text. That's what most of these are on.
He uses bios calls , thats what you need to learn to be able to write a bootloader, the rest is just reading up some documentation about what you are going to load. You can't fit a filesystem parser into 1 sector and have anything other sensible in there... from bootloader you load sectors from disk, using bios generally (on x86 atleast). and you can understand how to do it from this tutorial.
Re: Writing a Tiny x86 Bootloader
#30If you want to save some bytes in the bootloader, use bios interrupt to change video mode to clear screen ^^ it's very few instructions to do it.