Help with understanding a very basic main() disassembly in GDB

Question

Heyo,

I have written this very basic main function to experiment with disassembly and also to see and hopefully understand what is going on at the lower level:

Answer 1

The only thing I think that's outstanding from your original questions is why the following statements exist in your code:

0x08048381 :   push   %ecx
0x08048382 :   mov    $0x6,%eax
0x08048387 :   pop    %ecx

The push and pop of %ecx at and don't seem to make much sense - and they don't really do anything in this example, but consider the case where your code invokes function calls.

There's no way for the system to guarantee that the calls to other functions - which will set up their own stack activation frames - won't reset register values. In fact they probably will. The code therefore sets up a saved register section on the stack where any registers used by the code (other than %esp and %ebp which are already saved though the regular stack setup) are stored in the stack before possibly handing control over to function calls in the "meat" of the current code block.

When these potential calls return, the system then pops the values off the stack to restore the pre-call register values. If you were writing assembler directly rather than compiling, you'd be responsible for storing and retrieving these register values, yourself.

In the case of your example code, however, there are no function calls - only a single instruction at where you're setting the return value, but the compiler can't know that, and preserves its registers as usual.

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It would be interesting to see what would happen here if you added C statements which pushed other values onto the stack after . If I'm right about this being a saved register section of the stack, you'd expect the compiler to insert automatic pop statements prior to in order to clear these values.