Re: [OSDev] How to switch to long mode in x86 CPUs?

"David Brown" <david.brown@hesbynett.no> wrote in message
news:vprtt6$3jah9$1@dont-email.me...

On 27/02/2025 16:57, Ar Rakin wrote:

I am trying to develop my own, simple operating system to learn more
about how kernels work and low level stuff like that.  However, I am
stuck at setting up paging while switching long mode (64-bit protected
mode) in x86 processors.

>
Are you trying to learn about OS kernels, or about the complexities of
low-level x86 stuff?  Those are different things.
>
If you want to learn about how operating systems work, you might
consider starting on microcontrollers.  There are lots of RTOS's
available to study, and it is much easier to understand what is going on
- there is none of the multi-layered disaster of x86 modes to deal with.
>
Once you are happy with threads, context switches, locks, priorities,
and all the rest of that stuff, you can start moving up and adding
features for MMU's, SMP, and more, using bigger target processors.
>
>
On the other hand, if you are interested in learning the intricacies of
the x86 world, you need to look elsewhere for information - as Keith
says, it is not really C related when you are writing assembly.
comp.arch might be a helpful group.
>
>
However, it is possible to use C for almost all code here.  But it will
not be standard C - it is not even standard free-standing C.  You will
make use of compiler extensions, bits of inline assembly, pre-main C
code (so none of the library will be initialised, and program-lifetime
data will not be set up), and other such highly non-portable code.

I'm not disputing anything you have said, I'm just offering a variation.

Do you consider the concept of a BIOS (as seen as the IBM PC),
"legitimate to use" and do you consider MSDOS (which uses that
BIOS) to be an operating system?

If so, then with a suitable BIOS (or pseudo-bios if you want to
quibble what a BIOS is), you can have an operating system
(memory management, file management, program loading,
and an API) written in pure C90, except for the implementation
of setjmp/longjmp which is necessarily assembler (or non-standard
C).

That's what PDOS-generic (at pdos.org) is.

I
remember reading once about LinuxBIOS (now coreboot) having just four
lines of assembly running out of reset, before everything else was in C.

And that sounds like a BIOS - it's in the original name, not
the OS proper.

BFN. Paul.