[coreboot] About Paging, Realmode and what is going on

[Philipp Stanner]

Dear folks and techpriests,

the more I want to contribute and learn about low-level-code the less I 
understand, it seems.

 1. cb switches the CPU immediately to Protected Mode, yet Payloads like
    seaBIOS work in Real Mode. Does coreboot switch the CPU always back
    to RM before jumping to the payload?
 2. When CB switches to PM - who generates and administrates the Page
    Tables and where?
 3. Gustavo Duarte writes
    <http://duartes.org/gustavo/blog/post/how-computers-boot-up/> that
    GRUB switches from protected mode to real mode and vice versa all
    the time to address >1MiB of RAM and also use the BIOS-calls. If
    this is true using GRUB as payload would not work, as GRUB needs to
    call the non-existent BIOS, right?
 4. Once CB is in PM it can't access physical addresses anymore? It
    doesn't need to, too?
 5. PM means RAM-access is only possible through virtual addresses which
    are translated by the MMU using the Page Tables. This question is
    similar to [2.]: If coreboot generates the Page Tables and the
    payload would start in PM as well (is this even possible? At least
    the Linux-Kernel has entry points for RM and PM) this would mean the
    payload needs to use the Page Tables generated by CB. That wouldn't
    be a problem as they're linked in the register CR3 anyways?

And an unimportant bonus question:

  * Why does every modern CPU still start in RM? I do get the
    compatibility problem, but on the other hand: Do you need it for
    anything beside booting MS-DOS on your Ryzen? Is it really
    impossible for AMD and Intel to create a new CPU-generation with the
    x86-instruction set without RM, 16-bit-registers and 20-bit-mode
    registers like CS, SS etc. No modern OS uses bios calls. No CPU is
    ever switched to RM again after booting up. They should get rid of
    this old stuff.

Would be cool if someone could put this in its true light.

Thanks,

Philipp

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