The Main VDD provides approximately fifty functions that a mini-VDD can hook out to provide functionality other than what is supplied by the Main VDD. When a function is hooked out, the Main VDD does not call its default function, and instead calls the mini-VDD function. Most of the mini-VDD functions are easy to implement (such as changing banks on a video card). The big challenge in writing a mini-VDD is to decide what functions your hardware needs in order to get good full-screen and windowed MS-DOS support.