Using external ASM function in C++

I want to know the advantages of using external ASM functions, in a C++ program. If anyone has an example of how this is done would be nice. Also, in what situations and how often should this be used? Do I still get the same flexibility with ASM functions as I do with C++ function in my program? Thanks! :D

pros: speed, closer hardware interaction
cons: not portable outside the architecture. Not as easy to interact with the rest of the C/C++ program.
uses: optimization, device driver programming, OS programming
example:

Code:

---

extern "asm"
{
    mov ax, bx
}

---

Code:

---

extern "C" void Foo(int iParams)

---

Parameters begin at ebp+8. Don't forget this:

Code:

---

push ebp
mov ebp,esp

...
...

pop ebp

---

Or your code will crash.