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From: "Paul Edwards" <mutazilah@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: [OSDev] How to switch to long mode in x86 CPUs?
Date: Sat, 1 Mar 2025 15:50:09 +1100
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"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.