Re: What Bios data is sent to the Bootloader/OS ?

[Rafael Machado <rafaelrodrigues.machado@gmail.com>]

Hi Andrew.
Thanks a lot for the guidance!!

Rafael R. Machado

Em dom, 13 de ago de 2017 às 15:08, Andrew Fish <afish@apple.com> escreveu:

> On Aug 13, 2017, at 6:07 AM, Rafael Machado <
> rafaelrodrigues.machado@gmail.com> wrote:
>
> Thanks a lot Andrew and Rod. Your comments clarified a lot.
>
> Just onde last question.
> In case of int15/e820 and uefi getMemoryMap. Do you know If this
> information os used by the bootloaders?
>
>
> Yes it is, this is how the OS discovers the memory map.
>
> And do you know the format on these calls outputs? (For the  getMemoryMap
> the uefi Spec os clear, but didn't find anything about e820).
>
>
> The modern version of 0xE820 is documented in the ACPI Spec owned by your
> friendly UEFI Forum. ACPI Spec is here:
> http://www.uefi.org/specifications.
>
> There are some other specs from the 1990's that may be useful for legacy
> BIOS. Plug and Play BIOS Specification from 1994, and BIOS Boot
> Specification from 1996. For older stuff most people use Ralf Brown's
> interrupt list. You kind find all this old BIOS stuff searching the
> internwebs.
>
> Thanks,
>
> Andrew Fish
>
> Thanks and Regards
> Rafael Machado
>
> Em sáb, 12 de ago de 2017 00:23, Rod Smith <rodsmith@rodsbooks.com>
> escreveu:
>
> On Aug 11, 2017, at 6:00 AM, Rafael Machado
> <rafaelrodrigues.machado@gmail.com> wrote:
>
>
> Hi everyone
>
> I have a question that probably some guys here can help.
> The scenario I have, is that I need to create a OS image that must be
>
> able
>
> to boot at a UEFI system (with no csm module), and at a legacy bios
>
> system.
>
> My fist thought is that this is not possible.
>
>
> If I understand you correctly, it most definitely IS possible. Most
> major Linux distributions provide installation media that can boot in
> either BIOS/CSM/legacy mode or in EFI/UEFI mode. Replicating what those
> media do might not be the best way to go, though, since they are also
> typically designed to boot when written to optical media or when written
> to USB flash drives. To do this, they use a sort of "Frankenstein's
> Monster" disk format, so unless you need this cross-media compatibility,
> too, using the tools and procedures used to create these installation
> media would be overkill and would create something that's overly
> complex. These media do illustrate the practicality of what you're
> suggesting -- or at least, what I *BELIEVE* you're suggesting. If I've
> misinterpreted, please clarify your needs.
>
> The OS in this case is Linux, and the bootloader is Grub or Syslinux.
>
>
> A single GRUB (or SYSLINUX) binary will not do the job; however, there
> are both BIOS and EFI builds of both GRUB and SYSLINUX. The details of
> what you'd do would depend on the boot medium (hard disk, USB flash
> drive, optical disc, etc.); however, broadly speaking you need to write
> both BIOS-mode and EFI-mode versions of your chosen boot loader to the
> boot medium, with suitable configuration files in appropriate locations.
>
> Both GRUB and SYSLINUX are boot loaders that can load a Linux kernel
> into memory. The Linux kernel, in turn, does not need to be built for
> either BIOS or EFI environments; the same kernel binary will work in
> either environment. (One partial exception is that there's a feature
> known as the EFI stub loader that turns the Linux kernel into its own
> EFI boot loader. If you wanted to use this feature, it would obviously
> need to be compiled into the kernel. GRUB does not require this feature,
> though, and its presence will not interfere with the kernel being booted
> on a BIOS-based computer. Thus, you probably don't need to worry about
> it for your purposes. I mention it simply so you don't think it's an
> issue if you read something about it elsewhere.)
>
> --
> Rod Smith
> rodsmith@rodsbooks.com
> http://www.rodsbooks.com
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