GCC Optimization?

[asmfreshmeat@gmx.de]

Hello,

where is one question I didnt found a good answer. 
When I'm disassembling the main function with gdb or objdump
I see always code like this:

0x08048354 <main+0>:    push   %ebp
0x08048355 <main+1>:    mov    %esp,%ebp
0x08048357 <main+3>:    sub    $0x8,%esp
0x0804835a <main+6>:    and    $0xfffffff0,%esp
0x0804835d <main+9>:    mov    $0x0,%eax
0x08048362 <main+14>:   sub    %eax,%esp

I understand what the first 3 lines are good for. Pushing the 
Basepointer, moving the Basepointer to the Stackpointer,
reserving 8 Bytes on the stack for variables.

But why do they use and .... I think and does only even %esp
so that the last 4 Bits are set to Zero. 
But if %esp was 0xcccccccc before it would now be 0xccccccc0
what happens to the 4 Bits from 0xccccccc0 to 0xcccccccc?
And what is this good for?

Second question:
What does mov $0x0,%eax? Why dont they use xor %eax and what is it good for?

Third question: 
What is sub %eax,%esp good for? We subtract 0 from esp... mmmhh... for what?

Best regards

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