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2) Endpoint Description Processing
During the processing of a list, the Host Controller is required to interpret and service the
Endpoint Descriptors present on that list. The flow for service of an Endpoint Descriptor is
shown in Figure 18-5.
SERVICE
ENDPOINT
DESCRIPTOR
Service
Transfer Descriptor
HALT = 1
or
SKIP = 1?
NO
NO
FINISHED
YES
YES
Set
_ _ _ _Filled = 1
(Bulk or Control)
Periodic List?
YES
NO
NextTD =
Tail Pointer ?
Figure 18.5 Endpoint Descriptor Service Flow
When the Host Controller reads an Endpoint Descriptor, it first determines if the descriptor
should be skipped. If either the sKip bit or the Halt bit in the Endpoint Descriptor is a ‘1,’ the
Endpoint Descriptor is skipped and the Host Controller proceeds normally with the next
Endpoint Descriptor or the next list. If the Endpoint Descriptor is not skipped, the Host
Controller performs a check to determine if there is a Transfer Descriptor on the queue. If not,
the Host Controller proceeds to the next Endpoint Descriptor or the next list.
To determine if there is a Transfer Descriptor on the queue that can be processed, the Host
Controller compares the Endpoint Descriptor? TailPointer and NextTransferDescriptor
fields. If the fields are different, there is a Transfer Descriptor available for processing. If they
are equal, there is not a valid Transfer Descriptor on the list. If a valid Transfer Descriptor is
present on the queue, the Host Controller attempts to service that Transfer Descriptor. Service
of the Transfer Descriptor involves making only a single transaction attempt.
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