A method of controlling transmission of data from a computer to a video client via an interface device, comprising: reading a register on the interface device to obtain a value indicating temporal proximity to an occurrence of a vertical blanking interval occurs, the value increased incrementally until a vertical blanking interval occurs, and then being reset; deriving a time value, the time value indicating the occurrence of a vertical blanking interval; sending an interrupt to a processor on the computer at the occurrence of the vertical blanking interval; and invoking code in response to an occurrence of the video blanking interval.