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
[off-list ref] 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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