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MadZarx wrote:So the best idea is to write my own bootloaders (Both for BIOS and UEFI).

MadZarx wrote:But how can BIOS find my bootloader image in an ISO file? I mean that 512 bytes of code inside a FAT image?

Brendan wrote:Hi,

For the best OS, you should write your own boot loaders. However, this is possibly not the best idea for you.

Some people only want to learn about OS development and don't care about how good their OS is; and for these people writing their own boot loaders is probably a bad idea (it takes more time and you don't learn that much).

Some people do want to write the best OS they can. For these people the first step is to gain knowledge/experience; which mostly means writing an initial (bad) OS to learn about OS development, and then using what they've learnt to write a second (good) OS. Basically, for these people it's probably not worth the extra effort to write their own boot loaders for the initial (bad) OS; and by the time these people are actually ready to start writing a good OS they're able to see the advantages/disadvantages and make up their own mind.

Brendan wrote: The "El Torito" specification describes how this all works. Note: If you want a good OS, then you should be using "no emulation El Torito" (where the CD looks like a CD and isn't emulating a floppy disk or hard disk, and you don't have 512-byte sectors); and you should not be using FAT for anything (even on hard disks) because FAT sucks badly.


Cheers,

Brendan

genisoimage -R -b boot/grub/stage2_eltorito -no-emul-boot -boot-load-size 4
            -boot-info-table -o bootable.iso isofiles

MadZarx wrote:Yeah I'm thinking about it. I'm working on a bad OS But I will write my own bootloader to get some more experience. Hopefully the BrokenThorn OS Development series has described everything about bootloaders and it has a tutorial for it. I will follow that but if you have any other good tutorials about bootloaders, I appreciate you if you tell me

MadZarx wrote:

Brendan wrote:The "El Torito" specification describes how this all works. Note: If you want a good OS, then you should be using "no emulation El Torito" (where the CD looks like a CD and isn't emulating a floppy disk or hard disk, and you don't have 512-byte sectors); and you should not be using FAT for anything (even on hard disks) because FAT sucks badly.

The tutorial about the no emulation El Torito with grub legacy has this command for making iso with grub:

Code: Select all

genisoimage -R -b boot/grub/stage2_eltorito -no-emul-boot -boot-load-size 4
            -boot-info-table -o bootable.iso isofiles

Can I replace the stage2_eltorito with my bootloader? Should the bootloader be in 512 bytes or theres no limit for it? Then if I don't use a FAT image file, should I write a CDROM driver in assembly to find and load the kernel I have never done this before so I need a small help and information

Brendan wrote:For "no emulation El Torito" your boot loader can be any size you like (as long as it's less than about 500 KiB) and the BIOS will load the entire thing (not just one sector). This means that (unlike floppy/hard disk) there's no real point bothering with a "stage2" and easier to put the entire boot loader in a single "stage1".

MadZarx wrote:How does BIOS or UEFI knows which file is the bootloader?

Antti wrote:For UEFI, it has to be an actual file. To boot an x86-64 boot loader, the file is "/EFI/BOOT/BOOTX64.EFI" in the ISO 9660 file system. Although it is not hard to use UEFI, I recommend that you put it aside for a while and get back to it later. Your questions are good but it is hard to give step-by-step instructions of how to do all this. Of course, we are happy to answer your questions about details.

MadZarx wrote:Qwen you mean I should put the bootloader inside a FAT image and put that inside the ISO for the UEFI?
Would you give me more information about this please?

• the FAT way, where the common files are in the "FAT UEFI system image". In this case boot loaders that don't use UEFI (e.g. for 80x86 BIOS) have to be able to find the "FAT UEFI system image" (e.g. by including that image as a file in the ISO9660 file system) and also have to understand FAT (which is more bloated, more complex and slower to read from). This is easier for boot loaders designed for UEFI (and worse in every way for boot loaders that aren't designed for UEFI).
• the ISO9660 way, where the files are just normal files in the ISO9660 file system. In this case all boot loaders (those that use UEFI and those that don't) have to understand ISO9660 (which is easier for those that don't use UEFI, and faster for all of them).
• the other FAT way, where nothing uses ISO9660 at all and all files (for all architectures) are included in the "FAT UEFI system image". This is a bad idea because normally you want to have things like documentation and installation instructions that other OSs can read on the same CD (e.g. text or HTML files for Windows, Linux, etc).
• the other other way, where nothing uses ISO9660 at all, only UEFI boot loaders are in the "FAT UEFI system image", and you use any file system you like (e.g. design your own) instead of ISO9660 for almost all of the files. This is also a bad idea (same problem as the "other FAT way") but may have potential advantages maybe (e.g. your own file system might be better than ISO9660 and FAT for unknown reason/s - more efficient, support some feature you want, etc).