I'm confused about GNU Assembler starting and ending section in a function declaration

I have been recently learning x86 assembly language via GNU Assembler on Ubuntu by th开发者_StackOverflowe book Programming Ground UP at somewhere on the internet.

There are always 2 sections that's a "Must-Have" when creating a function:

At the beginning, it's considered to save old %ebp and set new frame pointer

pushl %ebp

movl %esp,%ebp

At the end, it's used to restore old %ebp and pop out return address

movl %ebp, %esp

popl %ebp

ret

please help me know what's really happen and what's that used for. - Why they must copy the bottom-most stack pointer to %ebp to set a new frame pointer? - And why the must copy back the %ebp to %esp when done ? - When copy without ( ) is that just an address ?

Thanks.

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This is a function perilogue. The Frequently Given Answer giving the gen on function perilogues explains what's happening, stack frames, and the frame pointer register.