Why Tilck runs only on x86 (ia-32)?

[vvaltchev]

(Question moved from the main README.md file.)

[vvaltchev]

Actually Tilck runs only on x86 for the moment. The kernel was born as a purely
educational project and the x86 architecture was already very friendly to me at
the time. Moving from x86 usermode assembly to "kernel" mode (real-mode and the
transitions back and forth to protected mode for the bootloader) required quite an
effort, but it still was, in my opinion, easier than "jumping" directly into a
completely unknown (for me) architecture, like ARM. I've also considered writing
from the beginning a x86_64 kernel running completely in long mode but I
decided to stick initially with the i686 architecture for the following reasons:

• The long mode is, roughly, another "layer" added on the top of 32-bit
  protected mode: in order to have a full understanding of its complexity, I
  thought it was better to start first with its legacy.

• The long mode does not have a full support for segmentation, while I wanted
  to get confident with this technology as well.

• The long mode has a 4-level paging system, which is more complex to use that
  the classic 2-level paging supported by ia-32 (it was better to start with
  something simpler).

• I never considered the idea of writing a kernel for desktop or server-class
  machines where supporting a huge amount of memory is a must. We already have
  Linux for that.

• It seemed to me at the time, that more online "starters" documentation existed
  (like the articles on https://wiki.osdev.org/) for i686 compared to any other
  architecture.

Said that, likely I'll make Tilck to support also x86_64 and being able to run
in long mode at some point but, given the long-term plans for it as a tiny kernel
for embedded systems, making it to run on ARM machines has priority over supporting
x86_64. Anyway, at this stage, making the kernel (mostly arch-independent code)
powerful enough has absolute priority over the support for any specific architecture.
x86 was just a pragmatic choice for its first architecture.