Let’s say you just got a new program for your computer. You have already installed it to your machine and are ready to use it when you get prompted restart your device. Why do you have to restart your device if the program is already on your computer?

Most modern programs work by tying themselves to the operating system. They may need to install new drivers or APIs that the program depends on, therefore requires changes to the system files that the device is currently using. If your operating system is currently using that file, it cannot automatically change it.  Think of it like trying to change the tire on your car while driving it.

Another reason you may need to restart your machine is that your computer may require a Dynamic Link Library. DLL files that contain bits of code or files that are shared among many programs installed on the device. Rebooting the system allows the program to ensure that nothing else is using the DLL before making changes to it. The program will also prompt a reboot to ensure that the registry is not going to be messed up for the operating system or other programs use it.

Now, you may have tried updating other programs and it just requires the program to be restarted.  The reasons are the same, you can not change things that you are currently using; but these programs might not be as integrated with the operating systems, therefore not requiring a full reboot.  The same principles apply when uninstalling a program, which is why the program may not be fully erased from the machine until it is rebooted.

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