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Re: [elysium-developers] User-level IRQ handling added
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From: Kasper V. L. <ve...@da...> - 2000-02-17 14:00:50
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> > We still need to work on PCT (protected control transfer) - anyone?
>
> [snip] I've lost my Insight CD (damm!) but can you explain how the upcalling works? I've only used TSS before.
>
> popl %eax
> popl %eax
> movl %cr3,%eax
> iret
>
> I think that's the code (from memory), it sets up the page tables but does the iret pop stuff off the argument stack? And does that drop the privelage level?
The code is correct. It doesn't set up any page tables, but it does
change the page directory. The assembly code you've included in your
posting is the implementation of a function with the following
prototype:
extern void _process_run(uint32_t cr3, uint32_t eip, uint32_t cs,
uint32_t eflags, uint32_t esp,
uint32_t ss) __attribute__ ((noreturn));
The compiler will push the arguments from right to left leaving the
stack something like this:
[...:ss:esp:eflags:cs:eip:cr3:ret]
^
stack top
The first instruction (popl %eax) just removes the return address,
because we don't need it. The stack is now:
[...:ss:esp:eflags:cs:eip:cr3]
^
stack top
Next we pop the physical address of the page directory we want to switch
to from the stack (popl %eax) and do the context switch. The stack now
looks something like:
[...:ss:esp:eflags:cs:eip]
^
stack top
When we issue the 'iret' instruction the CPU will remove the eip, cs,
and eflags from the stack. The CPU can tell by code segment selector
that we're returning to user-level. This will make the CPU pop the ss
and esp from the stack as well. We're now running in user-space at
address eip with a stack specified by ss:esp.
Hope this helps!
/Kasper
--
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Kasper Verdich Lund, Computer Science Department, Aarhus University
Office: 34P.218 | Phone: (+45) 8942 5680
Email: ve...@da... | WWW: http://www.daimi.au.dk/~verdich
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