The internal clock of a PC generates a interrupt every 1ms. This interrupt may be acted upon or ignored depending on the current state of the PC. It could be that the PC needs to refresh the display or send more sound to the sound card. When the hardware interrupt is received it will inform the OS which will then act upon that interrupt. In windows there is a timer event which is generated based upon this interrupt. Applications have the option of receiving this interrupt or to ignore it. Applications which have animation will rely on this interrupt to synchronize the animation.