Question

This question is in regards to Assembly.
What x86-64 register is changed to allocate local variables? Explain briefly with an example.

Answer 1

What x86-64 bit register is changed or used to allocate the local variables:

Answer: rbp register

Description:

• The base pointer register %rbp is used to keep track of the base value of the current stack frame. The start of the frame can be defined by moving the current stack pointer register to frame pointer.

• Example:

• Consider the following C code which is having variable a and b.

int main()

{

int a = 10, b = 20;

return 0;

}

Stack frame for main function will look like as follows:

• Assembly code to implement above function using %rbp register.

main:

pushq %rbp ; push or save the rbp on stack.

movq %rsp, %rbp ; Move the current stack frame of main function

movl $10, -8(%rbp) ; Assign value 10 to variable a

movl $20, -4(%rbp) ; Assign value 20 to variable b

movl $0, %eax ; return 0 function value

leave ; cleans allocated variables and restore the rbp register.

ret

In x86-32 architecture, register ebp is used to allocate local variables.