Deep dive into memory

Breaking down the structure of memory

The main() function starts a chain of function calls. When running the program the OS gives us a slice of the memeory in the RAM. It gives us the process memory layout:

STACK -> where all the action takes place
HEAP -> memory allocated at run time/dynamically
.DATA -> initializesd global and static variables
.BSS -> unitialized global and static variables
.TEXT -> machince code

Creating a variable means turning bits on and off somewhere in the RAM.

int main() {
    int nb;
    nb = 42;
}

nb is a local variable that lives in the stack, inside the main() function frame. The memory address provided to the variable is in actuality a virtual address.

Whenever the program reaches a new function, it is then allocated space in the STACK. Each function gets a stack frame.

The stack frame contains the following:

• Return address (where to go back to resume the program after the function call)
• Parameters passed to the function
• Local variables made in that function
• Saved copies of registers (base and frame pointers)

These 2 are stored in CPU registers: stack pointer (rsp/esp) points to the current top of the stack frame. base pointer (rbp/ebp) points to the start of the current stack frame.

The STACK grows downwards, meaning new frames are placed at a lower memory address. It grows towards the HEAP using the free space between the 2 of them.

When a function returns the following happens:

• Its frame is popped off the STACK and the memory is freed up
• stack pointer is adjusted
• base pointer is restored

This continues until even the stack frame for the main() function is removed from the STACK and the program ends.

CPU registers and caches