Thanks Eric.
But dint get much of it.
>1st part is the vector_irq routine offset relative to the start of
>__stubs_start, __vectors_start is used to compensate for the
>assembler to generate relative branch instruction.
How exactly the flow will go?
>= (vector_irq - __stubs_start) + 0x200 + __vectors_start
Here it looks like that we are going to jump after __vectors_start since we are adding some value to it. 'vector_irq - __stubs_start' is in a sense distance between the vector_irq function and the location where stubs start.
Could you please elaborate a bit? Where do we jump after executing 'b vector_irq + stubs_offset'?
Thanks
On 4/25/08, eric miao <eric.y.miao@xxxxxxxxx> wrote:
On Fri, Apr 25, 2008 at 5:01 PM, sahlot arvind <asahlot@xxxxxxxxx> wrote:
> Guys,
>
> I am trying to understand the flow of control when an interrupt comes.
> I am reading linux-2.6.24 src code and looking at
> arch/arm/kernel/entry-armv.S.
> At the bottom of this file I see the vector table as below -
>
> __vectors_start:
> swi SYS_ERROR0
> b vector_und + stubs_offset
> ldr pc, .LCvswi + stubs_offset
> b vector_pabt + stubs_offset
> b vector_dabt + stubs_offset
> b vector_addrexcptn + stubs_offset
> b vector_irq + stubs_offset
> b vector_fiq + stubs_offset
>
> .globl __vectors_end
>
>
> Here is not 'stubs_offset' a constant? and after seeing an IRQ where are we
> branching by doing ' b vector_irq + stubs_offset' and what is the flow of
> control???
> Thanks in advance.
>
stubs_offset is a constant decided by the assembler, note the .equ
above. So actually, the vector_irq + stubs_offset (for example)
= vector_irq + __vectors_start + 0x200 - __stubs_start
= (vector_irq - __stubs_start) + 0x200 + __vectors_start
And at run-time there will be dedicated page allocated for the
vectors and the stubs code, and the stubs code will be copied
at a offset of 0x200.
see? 1st part is the vector_irq routine offset relative to the start of
__stubs_start, __vectors_start is used to compensate for the
assembler to generate relative branch instruction.
> - A Sahlot
> --
> http://linuxexplained.blogspot.com
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Cheers
- eric
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