A simple OS

[Whitebird]

Hello everyone!

I'm developping a simple OS that only it does is execute a single application. So, I'll have only few tasks running: kernel, drivers, and this apllication.

My question is:

Have I need to write a memory manager, implement virtual memory, paging...?

Can I just fixed some region of memory, load application there, and add it to task queue?

Thank you.

[AlfaOmega08]

Have I need to write a memory manager, implement virtual memory, paging...?

A memory manager is required if you plan to use malloc/free or new/delete. If not you can skip this.
Paging will ensure that your application will not destroy the kernel. However this is optional too, just pay attention when writing the app.

Can I just fixed some region of memory, load application there, and add it to task queue?

Yes

[Tosi]

If you pick a fixed area of memory, how do you know it will be guaranteed to be available for use? It might be memory-mapped hardware, or beyond the limits of available RAM. You could get a memory map from the BIOS or a multiboot bootloader, and then parse that. That's what I do. Or, since you only have one task at a time, put it after the "end" of your kernel which hopefully is available RAM.

Some form of memory protection, whether it be through paging or segmentation, is a must. If you're not going to have protection, then your OS is not much more advanced than DOS, other than maybe having more drivers.

I would still recommend writing a memory allocator, whether you have memory protection or not. Even if you only have one application running, if it wants memory, you're going to need something like sbrk or malloc.

[Whitebird]

Thanks for replying me!

I've already implemented kmalloc and free functions, using a heap.
The "fixed regions" are know area of memory, that won't be any BDA, EBDA or something like this. And the application that the OS will run I will develop myself, so I will know that never I will enter in other memory region (eg kernel).

I was just confused if I really have to write something like pagigin for more than just memory protection (I'm sure that I will not need it).

Other confusing thing: if my kernel, my drivers and my application run in ring 0, do I need to set TSS? Is there some other way to do context switch?

Thank you again

[Tosi]

I think you need at least one TSS for switching back to ring 0 from ring 3 on interrupts. However, you can use software task switching for everything else.