Re: [Oslib-devel] how to...

[Luca A. <la...@mb...>]

Hi Juras,

sorry for the late reply...

>   how to obtain CONTEXT for VM86-task of callBIOS
>   to privide multitasking while a BIOS function is running?
Ok, here is how things work with vm86_callBIOS:
- There is a vm86 task that is used for performing bios calls. This is
  not a real task that can be scheduled together with the other ones (at
  least, I would not schedule it ;), but is considered to be
  nonpreemptable. It is only needed to switch to the vm86 mode (and
  back). It is not created through ll_context_create(), but by directly
  writing the TSS into the GDT (see vm86_init())
- The CONTEXT of a task is just a selector, indicating the position of
  its TSS in the GDT
- There is only a vm86 task (the BIOS is not reentrant), and its CONTEXT
  is X_VM86_TSS, defined in ll/i386/sel.h
- You can know the CONTEXT of the task that is currently running by
  using get_TR(), or context_save(), or ll_context_save() (they are more
  or less equivalent)
- Hence, if you want to see if you are running the vm86 task or not, you
  can do:
	CONTEXT c = get_TR();
	
	if (c == X_VM86_TSS) {
		/* We are in the vm86 task */
	} else {
		/* We are running some other task... */
	}
  See for example the emulate() function in examples/biosdemo.c

Ok, I hope this clarified things a little bit... If your scheduler is
driven by the timer interrupt, I would simply avoid to invoke it if the
interrupted task is the vm86 task.

				Luca

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