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

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On 28 Feb 2025 09:59, David Brown <david.brown@hesbynett.no> wrote:

> On 27/02/2025 16:57, Ar Rakin wrote:
>> Hello there,
>> 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.

I am just trying to learn about OS kernels, by building one.

> 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.

I actually wanted to build a simple kernel that would run specifically
on x86_64 systems.  I understand why you are recommending me to learn
about microcontrollers first, but at the moment I am almost done with
what I wanted to achieve - I just need to fix the paging issue I
described earlier.

> 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.

Looks like comp.arch might be the group I needed to find! Thanks.

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

That is true, I could have just used C.  Even articles on the osdev.org
website heavily use C, for stuff like setting up the Global Descriptor
Table (GDT), which I prefer to do in assembly because you would still
need to use a special x86 assembly instruction to load the GDT (lgdt).

I would much rather prefer having separate assembly files rather than
inlining them in the C source files whenever possible.  It's probably
just my preference.

Also just to clarify, from what I know, since the x86 processors start
in 16-bit real mode for the sake of compatibility, you'd have to start
writing 16-bit code first, then switch to 32-bit protected mode, then
finally switch to 64-bit long mode.  However if you use an existing
bootloader such as GRUB, then you usually would not need to worry about
16-bit code since GRUB drops you in 32-bit protected mode.  But still,
if you wish to switch to 64-bit long mode, you *will* have to mix 64-bit
and 32-bit code together.
Either I don't know how to, or I just don't feel comfortable mixing
32-bit and 64-bit code together *when in C* because you will have a hard
time linking those together.  Please correct me if I am wrong.

--
Ar Rakin

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Thread

[OSDev] How to switch to long mode in x86 CPUs? Ar Rakin <rakinar2@onesoftnet.eu.org> - 2025-02-27 21:57 +0600
  Re: [OSDev] How to switch to long mode in x86 CPUs? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2025-02-27 14:02 -0800
    Re: [OSDev] How to switch to long mode in x86 CPUs? Ar Rakin <rakinar2@onesoftnet.eu.org> - 2025-02-28 22:18 +0600
      Re: [OSDev] How to switch to long mode in x86 CPUs? antispam@fricas.org (Waldek Hebisch) - 2025-02-28 18:25 +0000
      OT: (Was: How to switch to long mode in x86 CPUs?) Salvador Mirzo <smirzo@example.com> - 2025-02-28 19:56 -0300
        Re: OT: USENET Salvador Mirzo <smirzo@example.com> - 2025-02-28 19:57 -0300
          Re: OT: USENET Richard Harnden <richard.nospam@gmail.invalid> - 2025-02-28 23:38 +0000
            Re: OT: USENET Ar Rakin <rakinar2@onesoftnet.eu.org> - 2025-03-01 15:09 +0600
              Re: OT: USENET candycanearter07 <candycanearter07@candycanearter07.nomail.afraid> - 2025-03-05 19:30 +0000
            Re: OT: USENET Salvador Mirzo <smirzo@example.com> - 2025-03-01 23:34 -0300
              Re: OT: USENET Ar Rakin <rakinar2@onesoftnet.eu.org> - 2025-03-02 16:48 +0600
                Re: OT: USENET yeti <yeti@tilde.institute> - 2025-03-02 17:27 +0042
                Re: OT: USENET Salvador Mirzo <smirzo@example.com> - 2025-03-08 20:06 -0300
          Re: OT: USENET Ar Rakin <rakinar2@onesoftnet.eu.org> - 2025-03-01 15:08 +0600
        Re: OT: Ar Rakin <rakinar2@onesoftnet.eu.org> - 2025-03-01 15:07 +0600
  Re: [OSDev] How to switch to long mode in x86 CPUs? David Brown <david.brown@hesbynett.no> - 2025-02-28 09:59 +0100
    Re: [OSDev] How to switch to long mode in x86 CPUs? Ar Rakin <rakinar2@onesoftnet.eu.org> - 2025-02-28 22:30 +0600
      Re: [OSDev] How to switch to long mode in x86 CPUs? cross@spitfire.i.gajendra.net (Dan Cross) - 2025-03-01 13:34 +0000
    Re: [OSDev] How to switch to long mode in x86 CPUs? "Paul Edwards" <mutazilah@gmail.com> - 2025-03-01 15:50 +1100
      Re: [OSDev] How to switch to long mode in x86 CPUs? scott@slp53.sl.home (Scott Lurndal) - 2025-03-01 14:55 +0000
        Re: [OSDev] How to switch to long mode in x86 CPUs? Salvador Mirzo <smirzo@example.com> - 2025-03-01 23:37 -0300
          Re: PC BIOS (was [OSDev] How to switch to long mode in x86 CPUs?) cross@spitfire.i.gajendra.net (Dan Cross) - 2025-03-02 16:01 +0000
          Re: [OSDev] How to switch to long mode in x86 CPUs? scott@slp53.sl.home (Scott Lurndal) - 2025-03-02 16:42 +0000
            Re: [OSDev] How to switch to long mode in x86 CPUs? cross@spitfire.i.gajendra.net (Dan Cross) - 2025-03-02 20:26 +0000
        Re: [OSDev] How to switch to long mode in x86 CPUs? "Paul Edwards" <mutazilah@gmail.com> - 2025-03-02 19:22 +1100
          Re: [OSDev] How to switch to long mode in x86 CPUs? bart <bc@freeuk.com> - 2025-03-02 16:16 +0000
            Re: [OSDev] How to switch to long mode in x86 CPUs? "Paul Edwards" <mutazilah@gmail.com> - 2025-03-05 05:31 +1100
              Re: [OSDev] How to switch to long mode in x86 CPUs? bart <bc@freeuk.com> - 2025-03-04 20:27 +0000
                Re: [OSDev] How to switch to long mode in x86 CPUs? "Paul Edwards" <mutazilah@gmail.com> - 2025-03-05 16:38 +1100
  Re: [OSDev] How to switch to long mode in x86 CPUs? cross@spitfire.i.gajendra.net (Dan Cross) - 2025-03-01 13:15 +0000
  Re: [OSDev] How to switch to long mode in x86 CPUs? Andy Valencia <vandys@vsta.org> - 2025-03-01 20:02 -0800

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