Transfer Descriptor Queue Processing Sample Clauses

Transfer Descriptor Queue Processing. For a transfer to or from an endpoint to occur, a TD must be linked to the queue associated with the ED. HeadP and TailP in an ED define the TD queue. If HeadP and TailP are not the same, then HeadP is a pointer to the TD that will be processed when the Host Controller reaches the ED. Software queues to the list by using the value of TailP to obtain the physical address of the last TD queued to the ED. Since the TD pointed to by TailP is not accessed by the HC, the Host Controller Driver can initialize that TD and link at least one other to it without creating a coherency or synchronization problem. After the new TDs are linked, TailP is updated, extending the list of TDs that can be accessed and processed by the HC, with TailP again pointing to a TD that can be initialized by software. Software may not alter in any way any of the TDs it has queued prior to the one pointed to by TailP until the Host Controller completes processing of the TD or the Host Controller Driver ensures that queue processing for the ED has been halted. When the Host Controller finishes processing a TD, it copies the NextTD value from the just completed TD into HeadP of the ED. For a General TD, the Host Controller also sets the toggle Carry bit to the value of the last used data toggle for the endpoint and sets the Halted bit to 0 if the TD completed without error or to 1 if an error occurred.