Transfer Schedule

Perhaps the most important aspect of transferring data on the USB is generation of a suitable schedule. A schedule is a sequential representation of the transfers that need to take place on the bus. The idea of a transfer is encapsulated by what is called a frame. The host controller executes each of these frames for a duration of 1 millisecond. At the completion of the 1 ms interval, it moves onto the next frame irrespective of the present frame being fully executed. These frames are stored in a contiguous section in memory, known as the Frame List. Each frame is of size 4 bytes, and the frame list has a total of 1024 frames, giving the list a size of 4K (4096 bytes). The host controller treats the frame list as a cyclical list, wrapping around on reaching the end.

 

Figure 3: Frame List