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3. If the current list is the Control List, the Host Controller next action is dependent on
whether or not the number of Control Endpoint Descriptors dictated by the Control/Bulk
Service Ratio have been serviced. If the Control/Bulk Service Ratio has been satisfied,
the Host Controller moves on to the Bulk list; otherwise, service of another Control
Endpoint Descriptor is attempted.
Endpoint Description Priority
Within a list, Endpoint Descriptors are serviced with a round robin priority scheme. The
Host Controller must initially begin service at the head of the list and service each Endpoint
Descriptor on the list sequentially. For non-periodic lists, when the Host Controller reaches
the end of the list, it reads the list? Head Pointer and starts again with the first Endpoint
Descriptor on the list. Servicing an Endpoint Descriptor is defined as making a single
transaction attempt from the first Transfer Descriptor in the queue. Once a transaction
attempt is made, whether successful or not, and the appropriate actions are taken to
complete that transaction, the Host Controller will service the next Endpoint Descriptor
rather than make a second transaction attempt on the current Endpoint Descriptor. The only
exception to this is for an Isochronous TD that is late. The late Isochronous TD will be
retired and the next Isochronous TD connected to the Endpoint Descriptor will be processed
immediately.
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