Local variables (iii) function to the assembler language from C

To sum function as an example

int sum(int a,int b)
{
	return a+b;
}

Its assembly code

.text
.global sum
sum:
	pushl %ebp
	movl %esp,%ebp
	movl 8(%ebp),%eax
	addl 12(%ebp),%eax
	leave
	ret

Then the main function

int main(void)
{
	int a;
	int b=12;
	int c=24;
	a=sum(b,c);
	return 0;
}

Assembler code for

.text
.global main
main:
	pushl %ebp
	movl %esp,%ebp
	
	subl $12,%esp
	movl $12,-8(%ebp)
	movl $24,-12(%ebp)
	pushl -12(%ebp)
	pushl -8(%ebp)
	call sum
	addl $8,%esp
	movl %eax,-4(%ebp)
	movl $0,%eax
	leave
	ret

Local variables are used to apply subl $ 12,% esp on the application stack space to hold the 12 bytes a, b, c. a is -4 (% ebp), b is -8 (% ebp), and so on.

Register usage conventions
accordance with established practice, the register% eax,% edx,% ecx register is divided saved caller. When the procedure calls p q, q can override these registers, without disrupting any data desired p. On the other hand, the register% ebx,% esi,% edi caller is divided into a save register. This means q must be covered before these registers, save them first into the stack and restore them before returning, should these values may be required in the future for computing p.