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Transfer Description Priority
The priority of Transfer Descriptors on a queue is first-come-first-serve. The Transfer
Descriptors the Host Controller services are always part of a queue attached to an Endpoint
Descriptor. The Host Controller services the first Transfer Descriptor on the queue which is
pointed to by the NextTransferDescriptor field of the Endpoint Descriptor. When that
Transfer Descriptor is retired, it is removed from the queue and the Transfer Descriptor
linked with the NextTransferDescriptor field of that Transfer Descriptor is moved to the
front of the queue.
3) Interface to the TD Block
The ED Block is responsible for providing the TD block with a "o" signal. When the ED Block
determines there is a Transfer Descriptor needing service it signals the TD Block to fetch that
TD and begin processing. The ED Block provides the address of the Transfer Descriptor and all
other required information from the Endpoint Descriptor to the TD Block along with the "o"
flag. When processing of the Transfer Descriptor is complete the TD Block responds with a
"ED complete" flag.
4) Operational Registers
HcHCCA
This register is a pointer to the base address of the OpenHCI defined HCCA and is defined
by software. When the Host Controller indexes into the Interrupt Table to fetch the
Periodic List head pointer, the index offset is added to this value to determine the physical
address of the list head pointer.
HcBulkHeadED
This register is a pointer to the head ED of the Bulk list and is maintained by software.
HcControlHeadED
This register is a pointer to the head ED of the Control list and is maintained by software.
HcPeriodCurrentED
When the Host Controller processes the Periodic List, the address of the ED being serviced
is stored here. The Host Controller maintains this register without interference from
software.
HcBulkCurrentED
When the Host Controller processes the Bulk List, the address of the ED being serviced is
stored here. When the Host Controller moves to the next ED, the NextED field of the
previous ED is copied into this register. This allows the Host Controller to keep its place
when processing of the Bulk List is interrupted.
HcControlCurrentED
When the Host Controller processes the Control List, the address of the ED being serviced
is stored here. When the Host Controller moves to the next ED, the NextED field of the
previous ED is copied into this register. This allows the Host Controller to keep its place
when processing of the Control List is interrupted.
Rev. 3.0, 03/01, page 272 of 390
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