software task switching in only ring 0

daxhuiberts · Post by **daxhuiberts** » Wed Apr 09, 2008 4:27 pm

Hello,

It's been a long time ago since I was last active with os development. Got as far as setting up paging and extremely basic malloc and getting the PIT to run. The next step is to get familiar with task switching.

All my code runs in ring 0 in a single address space and I was wondering if it is possible to do software task switching with only this environment, or do I need a ring 0 and a ring 3 and a TSS to get minimal software task switching working?

Because as far as I can tell, when I'm in ring 0 and a interrupt happens, the interrupt handler runs in the same stack as the code before the interrupt and changing the the currently in use stack for task switching is very impractical if not impossible.

Any thoughts on this one?

AJ · Post by AJ » Wed Apr 09, 2008 4:34 pm

Hi,

You only need the TSS when changing from ring 3->0, so no, if you just use ring 0, you do not need a TSS. You just switch stack halfway through the int handler, which means setting up the stack of a new task before it runs for the first time.

Cheers,
Adam

daxhuiberts · Post by **daxhuiberts** » Wed Apr 09, 2008 4:47 pm

Yes, of course. But I do have to make sure I'm not dependent on some local interrupt handler data on the current stack while I switch to a different task's stack, right?

Combuster · Post by **Combuster** » Wed Apr 09, 2008 5:00 pm

True.

You can have your scheduler function return the pointer to the next stack to be used, and then change it at the end of the function, when you don't need the locals anymore. That saves you from having to worry about what happens within the scheduling code with a bit of assembly.

Code: Select all

push registers
mov eax, esp
push eax
call scheduler
mov esp, eax
pop registers
iret

AJ · Post by AJ » Wed Apr 09, 2008 5:05 pm

Hi,

The easiest way is to have an asm stub. Before passing control to your task scheduler, save the value of ESP. The scheduler then returns the desired new ESP back to the end of the ASM stub which loads the stack pointer and does an IRET.

As your scheduler is 'stack neutral', you should find that everything aligns nicely. If you do end up adding ring 3 support, just ensure you write the correct stack value to the TSS at ESP0.

Have a look at the ASM snippet > here < for an example about half way down the page (ignore the rest of the tutorial, which isn't that well written IMHO!).

Cheers,
Adam

daxhuiberts · Post by **daxhuiberts** » Thu Apr 10, 2008 3:46 am

Thank you both for the info,

I eventually came to the same sollution. Receiving the esp in eax
of the desired task by the scheduler and loading that in esp and doing an iret.
For the time being, this sure is a good enough sollution, till I decide to start working with ring 3 support.

Dax (The one with the 'real' DaxOS)