RE: [elysium-developers] PCT

[Kasper V. L. <ve...@da...>]

> Well I've got a quick PCT system working. It ends up w/
>
> _process_run (p->pr_cr3, p->pr_pct, USER_CS, 0x3202, 0, USER_DS);

That looks okay.

> But as soon as the called process tries to push to the stack it
> page faults. I
> didn't spend very long hunting, but anyway:
>
> SS = 0x00100023
> ESP= 0x0
> ESI= 0x0
> EBP= 0x00837000
> DS = 0x23
> CS = 0x1b

Apart from the extra 1 in the SS it seems correct. And as soon as the called
process tries to push something on the stack it SHOULD page fault. The
exception error code should probably indicate that it is an access rights
violation that caused the page fault (as opposed to a non-present page). The
value you try to push on the stack will be written into memory at 0xfffffffc
(which is a part of the read-only mapping of page tables and directories).
If you look at

init/src/crt/crt0.S

you'll see the following lines of code:

	.align	4
_crt0_prolog:
	movl	_crt0_saved_sp, %esp
	popf
	popal
	ret

	.align	4
_crt0_epilog:
	pushal
	pushf
	movl	%esp, _crt0_saved_sp
	int	$0x40

The prolog code is called with the _process_run() function and as you can
see the first thing it does is to set %esp to something sensible.

> Should SS have the extra 1 in it? I've got to order a new Insight
> from Intel,
> so I don't know the selector structure. But I don't think a bad
> selector would
> cause a page fault.

It cannot. AFAIK it should cause a GPF, but who knows what happens.

/Kasper

BTW: I'm working on a specification of the PCT and IRQ stuff - I'm looking
forward to seeing your work under CVS :-)