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Hi everyone, I am a 13-year-old student from Bangladesh. I have built a 32-bit x86 kernel from scratch. It currently supports PMM, Paging, Kernel Heap, Multitasking, and a basic Ramdisk File System.

Right now, I am trying to implement User Space (Ring 3) and Syscalls. I have read the OSDev wiki articles on GDT/TSS and Syscalls, but I am confused about the exact steps for setting up the TSS for Ring 3.

Could anyone point me to the correct order of steps or a good resource for this specific transition?

GitHub: https://github.com/porag2025-glitch/PoragOS

Thank you!

You need to set up TSS.ESP0, so that interrupt handlers can load a new stack when an interrupt occurs while in ring 3. You of course need a code and data segment for ring 3 in the GDT (both with DPL 3). For the transition, the easiest is to craft your own fake interrupt stack frame and iret. Be aware that iret only loads CS and SS, you need to load DS, ES, FS, and GS yourself in the transition code. Although I don't quite know what significance the data segment descriptors have in 32-bit mode, but all the OS code I've seen loads them.

For system calls, honestly you should be looking into the fast system call mechanisms SYSENTER and SYSCALL. Those make demands on the layout of your GDT, but that can be accommodated by just switching a few segments around. And they are easier to set up than system call interrupts.

But if there must be system call interrupts, you need a new entry in your IDT that has its DPL set to 3, but references the kernel CS.

Thank you!