What is the use of “push

Question

What effect these two instructions cause in the assembly code generated by gcc for x86 machines:

push %ebp
movl %esp, %ebp

Answer 1

The piece of code sets up the stack for your program.

In x86 stack information is held by two registers.

    Base pointer (bp): Holds starting address of the stack
    Stack pointer (sp): Holds the address in which next value will be stored

These registers have different names in different modes:

                            Base pointer           Stack pointer
    16 bit real mode:       bp                     sp
    32 bit protected mode:  ebp(%ebp)              esp(%esp)
    64 bit mode:            rbp                    rsp

When you set up a stack, stack pointer and base pointer gets the same address at the beginning.

Now to explain your code,

    push %ebp

This code pushes the current stack address into the stack so that the function can "exit" or "return" properly.

    movl %esp, %ebp

This code sets up the stack for your function.

For more info, refer to this question.

Hope this help!