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ED Block
The ED Block is responsible for processing Endpoint Descriptors. This includes reading EDs from
memory, servicing an active TD if necessary, and writing back information if necessary.
1) List Service Flow
This section describes the actions required of the Host Controller during list processing. These
actions are taken after the Host Controller has determined which particular list is to be serviced.
The general list service flow is depicted in Figure 18-4.
List Enabled Check
The first action the Host Controller takes when processing a list is to check that the list is
enabled. Periodically, lists are disabled by the Host Controller Driver for the purpose of
altering an Endpoint Descriptor (or other reasons). If the list is enabled, the Host Controller
may service the list. If the list is disabled, the Host Controller skips that list and moves on
to the next list. Lists are enabled/disabled with the list enable bits of the HcControl register.
When a list is disabled during a frame, the Host Controller must not process the list beyond
the next frame boundary. When a list is enabled, it is not available for processing until the
next frame. In addition, when a list is enabled after being previously disabled, the only
piece of information the Host Controller may assume is valid is the list HeadED pointer
and, if a non-periodic list, the list CurrentED pointer.
The IsochronousListEnable bit is used to disable processing of the Isochronous list which
is always at the tail of the periodic list. If the Host Controller finds an Isochronous
Endpoint Descriptor while servicing the Periodic list and the IsochronousListEnable bit is
‘0’, the Host Controller stops processing the list.
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