do {} 13% CPU usage ?!?!?!?

my PC uses 13% of CPU

why what does PC while this???

Code:

---

#include <iostream>

using namespace std;

int main ()
{
    bool game_main_menu = true;
    do
    {}
    while ( game_main_menu == true );
    int z;
    cin >> z;
    return 0;
}

---

I don't get it

when I do that he is out of loop it goes to 0% CPU

Code:

---

#include <iostream>

using namespace std;

int main ()
{
    bool game_main_menu = true;
    do
    {}
    while ( game_main_menu == false );
    int z;
    cin >> z;
    return 0;
}

---

why I don't get it

I think this is question for experts

PC should do nothing if there is do {}

Quote:

---

Originally Posted by military genius

my PC uses 13% of CPU
PC should do nothing if there is do {}

---

It IS "doing nothing". As fast as possible.

if it would do nothing it wouldn't use neighter 1% of CPU


I wont to know what exactly it does

You don't think game_main_menu == true is doing something?

It's comparing your variable to true, and then returning to the earlier part of the code. Over and over and over again.

Quote:

---

Originally Posted by military genius

if it would do nothing it wouldn't use neighter 1% of CPU


I wont to know what exactly it does

---

Here's what it does:

Code:

---

mov    ax, 1
do_loop:
cmp    ax, 0
jne    do_loop

---

It just executes the instructions cmp and jne over and over until the application is killed. Of course the actual instructions depends on the compiler, what optimizations are set, and the CPU the application is running on, but you should get the picture.

of corse

yea thanks

Since the cpu does not need for any memory or disk access, it is pretty much just storming through the instructions, using up 100% of its resources executing the code.
Which is exactly why using loops for these purposes is Not a Good Idea™.

Quote:

---

Originally Posted by military genius

if it would do nothing it wouldn't use neighter 1% of CPU


I wont to know what exactly it does

---

It's not doing nothing, it is checking if the condition is true or false, repeatedly, as fast as possible.

It looks to me as if that code would suck up 100% of at least 1 core depending on how the instructions were shared among the cores.