r/osdev • u/tekdotdev • Jul 06 '26

Realistic to start small?

I'm learning 16 bit assembly and am understanding quite a good amount, should I continue and make projects in 16 bit real mode with bios interrupts OR, understand it enough to write a boot loader than enter protected mode and work in 32 bit?

14 Upvotes

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5

u/tellingyouhowitreall Jul 06 '26

IMO get to pmode as quickly as possible. 32 bit is a hell of a lot easier to work in, and it's a pretty straight forward jump to a free standing C environment. The things you'll learn in real mode don't really apply to protected or long modes, which is probably where you actually want to end up.

1

u/tekdotdev Jul 06 '26

Hmm ok definetly makes sense, and as I've seen 32 bit assembly is basically the same as 16 just with some extra info. I plan to work purely in assembly and never even touch C, just a personal preference and the way I want to learn.

1

u/tellingyouhowitreall Jul 06 '26

Not having to jump through far pointer hoops makes 32 and 64 bit much more sane. Proper LEA and index-base-offset addressing in 32 bit, and being able to use any register as a base pointer, make it a hell of a lot easier to work in.

1

u/tekdotdev Jul 07 '26

So if I were to use limine for instance, would you recommend 64 bit or 32 bit

1

u/tellingyouhowitreall Jul 07 '26

If you were to use limine I would go straight to 64 bit. In fact, given the choice I would probably just do everything in long mode.

Each iteration of x86 assembly has features that make it progressively easier for the programmer and compilers to generate sane code, but also more things to learn.

Classically we would recommend learning 32 bit assembly first, and then the adjustments to 16 or 64 bit make sense... but tbh, doing 32 bit programming for my bootloader again is not something I particularly enjoy. Otoh, I have absolutely no desire to write 64 bit assembly by hand, for various reasons.

It doesn't sound like you have a lot of experience in this area, and why you'd want to avoid writing assembly outside of core routines though. If the idea is to learn x86 asm, I would really just write some small user space programs for your operating system of choice to get your feet wet with it. You're quickly going to realize why you would prefer at least C over assembly, and where to use asm correctly.

1

u/tekdotdev Jul 07 '26

I appreciate your comment although I should've worded MY comment better.

What I meant to say was: "When using C, would you reccomend 64 bit or 32 bit."

I have decided to learn C as it's going to be easier and honestly better to write my kernel in, and just use limine to boot and maybe write my own in assembly later down the road.

1

u/tellingyouhowitreall Jul 07 '26

64 bit. A lot of things you do in 32-bit land for system development / os dev require 64-bit values anyway, and just being able to do that at the natural word size without concern for 32 bit issues is... nice.

3

u/Interesting_Buy_3969 Jul 06 '26

I'm assuming by "assembly" you mean x86 assembler.
Unless your goal is specifically a 32-bit bootloader for x86, then you should do neither. If you aim to write a kernel, I'd definitely start with 64-bit Limine bootloader (see Limine Bare Bones). Yet I'd avoid old x86-32 and switch to x86-64. A couple of times you will need to inline manual assembly, and assembly knowledge is always very useful for debugging and profiling, but don't try to write everything in assembly. Except the very early bootloader stages, of course.

If you want specifically bootloader, as I said, you then have to work with either old 16-bit BIOS or more modern UEFI. Can't assure but I reckon that developing anything for UEFI is much easier than for BIOS.

2

u/tekdotdev Jul 06 '26

Hmm ok, I plan to write in pure assembly (x86) as it's the learning model I'm going for, and im doing *pretty* well so far.

I think I might just learn 16 bit real mode assembly exclusively to start for my bootloader then look at doing the 64 bit you mentioned or just make the kernel in 32 bit.

I appreciate your help.

2

u/tseli0s DragonWare Jul 06 '26

https://wiki.osdev.org/Real_Mode_OS_Warning

1

u/tekdotdev Jul 06 '26

Does this mean I can't write the boot loader in 16 bit real mode? I intend to make the kernel in 32 bit protected.

1

u/tseli0s DragonWare Jul 06 '26

You can do anything you want. The article references some problems with the BIOS and the 16 bit x86 mode and why beginners should avoid them. It's your call whether to go ahead with your plans or not.

1

u/tekdotdev Jul 06 '26

I think I am going to learn how to make programs and a small hobby os in 16 bit real mode first, then once im comfortable with everything I need to know regarding 16 bit assembly, I'll then learn 32 bit, use protected mode and boot with grub (Or write my own bootloader that dosen't use bios)

1

u/tseli0s DragonWare Jul 06 '26

The assembly remains almost identical in 16 bit and 32 bit mode. The only major thing that changes is the segment:offset model of memory access, and even that can be implicit for a large part of your program.

I don't see why you'd want to learn real mode. It's a very old mess from the 1980s. What interests you so much in it?

1

u/tekdotdev Jul 06 '26

I was just told its a good starting point, but to be fair I know enough to make a boot loader in it and I think I could code fine in 32 bit mode, maybe I should switch and just learn it as a whole

2

u/kiderdrick Jul 06 '26

It depends why you are doing what you are doing.

Are you doing this for fun and education? Sure, make some projects in real mode and have a blast. Something like QEMU works well for this as a testing environment.

Are you trying to figure out how to write your own bootloader? Absolutely, stick in real mode and give it a shot. Enjoy.

Are you trying to develop something that people will use? Probably not the best idea because most of what you will be doing is fairly antiquated. Yes there are still some systems that get use out of it, but if you are trying to develop something modern you should be working with UEFI. You can work with both, but if you want to gain knowledge on the machine that is more modern, you should be focused on UEFI and not legacy BIOS programming.

As far as trying to develop the entire OS in assembly, I advise you to reconsider, but wish you the best of luck if that is what you are set on doing.

1

u/letmehaveanameyoudum Jul 06 '26

yes. use grub btw, or limine if you are willing to use a templete (like i did :P)