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Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
August 2006
Developer’s Manual
Order Number: 306262--, Revision: 004US
403
USB 2.0 Host Controller—Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
9.13.5.4
qTD Buffer Page Pointer List
The last five DWords of a queue element transfer descriptor is an array of physical
memory address pointers. These pointers reference the individual pages of a data
buffer.
System software initializes Current Offset field to the starting offset into the current
page, where current page is selected via the value in the C_Page field.
Table 159.
qTD Buffer Pointer(s) (DWords 3-7)
Bit
Description
31:12
Buffer Pointer List. Each element in the list is a 4K page aligned physical memory address. The
lower 12 bits in each pointer are reserved (except for the first one), as each memory pointer
must reference the start of a 4K page. The field C_Page specifies the current active pointer.
When the transfer element descriptor is fetched, the starting buffer address is selected using
C_Page (similar to an array index to select an array element). If a transaction spans a 4K buffer
boundary, the host controller must detect the page-span boundary in the data stream, increment
C_Page and advance to the next buffer pointer in the list, and conclude the transaction via the
new buffer pointer.
11:0
Current Offset (Reserved). This field is reserved in all pointers except the first one (e.g. Page
0). The host controller should ignore all reserved bits. For the page 0 current offset
interpretation, this field is the byte offset into the current page (as selected by C_Page). The host
controller is not required to write this field back when the qTD is retired. Software should ensure
the Reserved fields are initialized to zeros.