System call question

[Rajat Sharma]

sys_call_table is the array of system call handlers (function
pointers). The syntax is how you typically access an array in GAS.
%eax i.e. EAX register holder the system call number which inturn is
index into this array. since size of pointer on 32 bit platform is 4
bytes, thats why you give 4 as last argument. So, you multiply 4 with
%eax to reach at exact function pointer and then deference that
pointer with a * (i.e. like value at operator in C) and call the
system call handler function.

Formally it is called indexed memory addressing and syntax is like:

base_address(offset_address, index, size)

The data value retrieved is located at

base_address + offset_address + index * size

here in this example base_address is address of sys_call_table,
offset_address is skipped so it defaults to zero. offset address tells
you where to start your search in array, typically its not used. index
is %eax and size is 4.

I recommend "Professional Assembly Language" book by Richard Blum from
Wrox publication.

Thanks,
Rajat
On Sat, Jun 11, 2011 at 1:05 AM, Naman shekhar Mishra
<namanshekharmishra2 at yahoo.in> wrote:
> Platform: x86 32 bit
> system_call() ,afetr doing some checks, looks up in the sys_call_table and
> finds the correct address of the system call and jumps to it. i.e.:
> call *sys_call_table(,%eax,4)
>
> Can you please explain this syntax to me. I think 'call' is the processor
> instruction which works on a function name.
> * is used in gas syntax to denote that the address following it is to be
> used as the jumping address. Tell me if I m wrong so far.
> () is used to index a memory location(GAS syntax again, I think). Please
> explain to me (,%eax,4). I have trouble understanding this syntax.
>
>
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