Making an OS (x86) Chapter 6 - Entering Protected mode, GDT 

There are still a couple of videos left to it, then I'd like to move to other topics, while posting an update about this project from time to time

@@DaedalusCommunity It does not owrk for me (the code.) I copied the same from github to make sure but it does not work.

@@xulum1299 remove the first jmp $, it gets stuck

i second this lol

In this case we are in text mode, so a couple of bytes in video memory represents a coloured character. There are several other modes, among which various graphics modes, that let you set coloured pixels in a similar way. Check out the links in the description to know more :)

Thanks

Yeah post it, I'm a big fan of accidental animations :))

Hitfilm express, it's a free video editor whit a fairly rich compositing tool. I'd love to write some code to animate some of my stuff faster though, like 3b1b did with Manim.

@@DaedalusCommunity thanks a lot!

start_protected_mode is a label, "located" in the code segment. Once the gdt is loaded, the new code segment is set, and in order to jump to the absolute address of start_protected_mode you need to perform a far jump, by specifying the segment.

@@DaedalusCommunity I am having trouble understanding how CODE_SEG is a real segment? Generally in a far jump we specify the start address (segment address) where we want to calculate the offset from. But CODE_SEG is just a difference between the gdt_start and code_descriptor. How is that number supposed to be the start of any segment? seems very arbitrary. Thanks

I defined CODESEG at 8:42 We extend with CODESEG because we need to jump to the new code segment, offset by the start_protected_mode label (we want to execute code in real mode)

@@DaedalusCommunity i know im late but didint you define CODE_SEG and now you wanna use CODESEG?

@@VaporGame404 I think he made a mistake there. You should use CODE_SEG:start_protected_mode You are right that he never defined CODESEG

Uhm, check out the code on the OS-Reference github repo, maybe there's some bug in your code

@@DaedalusCommunity I've been messing with your code on GitHub. Yours works great but mine wasn't working. I think the issue was that I put all this code in the next sector. If I jump to 32 bit mode while staying in the boot sector it seems to work just fine. I can then jump to the next sector/s. I have to say, thank you for being active on a 2 year old video, so many tutorials are abandoned or don't maintain support. I'm curious if you plan on continuing the series in video format? (I've read the text version of part 8 and it's pretty good

because GDT size starts at 0

Xor that bit from cr0 Set "bits 16"

Could you share your code?

That happened to me too, i checked the os reference and could fix it (The code was after the GDT_Descriptor, it worked when i put it before)

У меня Bochs перезагружается вообще

start_protected_mode: mov ax, DATA_SEG mov ds, ax mov ss, ax mov es, ax mov fs, ax mov gs, ax

When writing a program, one should think in terms of general approaches. Yes, if you're sure each and every time you're going to change the last bit it will be a zero, which is indeed the case here, you can simply increment the register. But I'd say, unless the performance is dramatically worsened by the general approach, it is not a bad idea to keep the code general and intuitive to the others reading it :)

@@DaedalusCommunity Thanks :D

Sure! It wouldn't be that hard to code in C

You're right, it could be done in real mode too, by setting the register bl. My bad, I'll add it to the corrections

You can use those registers even when in 16-bit mode. E.g. if you move a value to to eax in 16 bit mode, in practice it's the same as moving to ax. Sometimes it's necessary to use registers such as eax in 16 bit mode, due to constraints of the machine language, as for certain instructions using only half of the register is not allowed.

It's a label at the end of the GDT, it's used to compute its length

ok everything fine . my bad

@@user-ng8rl3jb1i glad you sorted it out, if you have any questions let me know :)

@@DaedalusCommunity sure , thanks :)

Uhm, weird.. does it shut down or restart? If it restarts it might be some sort of overflow

@@DaedalusCommunity Sorry, yes it restarting

Why is this happening?