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@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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