82801DB Datasheet, PDF (173/587 Pages) Intel Corporation – Intel 82801DB I/O Controller Hub 4 (ICH4)

Functional Description
5.16.1.2 Transfer Descriptor (TD)
Transfer Descriptors (TDs) express the characteristics of the transaction requested on USB by a
client. TDs are always aligned on 16-byte boundaries, and the elements of the TD are shown in
Figure 5-15. The 4 different USB transfer types are supported by a small number of control bits in
the descriptor that the ICH4 interprets during operation. All Transfer Descriptors have the same
basic, 32-byte structure. During operation, the ICH4 hardware performs consistency checks on
some fields of the TD. If a consistency check fails, the ICH4 halts immediately and issues an
interrupt to the system. This interrupt cannot be masked within the ICH4.
Figure 5-15. Transfer Descriptor
31 30 29 28
27 26 25 24 23
21 20 19 18
Link Pointer
16 15 14
11 10 8 7
43 2 1 0
0 Vf Q T
R SPD C_ERR LS ISO ISC
MaxLen
Status
RD
EndPt
R
Device Address
ActLen
PID
Buffer Pointer
R = Reserved
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Table 5-57. TD Link Pointer
Bit
Description
31:4
Link Pointer (LP). Bits [31:4] Correspond to memory address signals [31:4], respectively. This field
points to another TD or QH.
3 Reserved. Must be 0 when writing this field.
Depth/Breadth Select (VF). This bit is only valid for queued TDs and indicates to the hardware
whether it should process in a depth first or breadth first fashion. When set to depth first, it informs
2 ^{the Intel® ICH4 to process the next transaction in the queue rather than starting a new queue.}
0 = Breadth first
1 = Depth first
QH/TD Select (Q). This bit informs the ICH4 whether the item referenced by the link pointer is
another TD or a QH. This allows the ICH4 to perform the proper type of processing on the item after
1 ^{it is fetched.}
0 = TD
1 = QH
Terminate (T). This bit informs the ICH4 that the link pointer in this TD does not point to another
valid entry. When encountered in a queue context, this bit indicates to the ICH4 that there are no
more valid entries in the queue. A TD encountered outside of a queue context with the T bit set
0 _{informs the ICH4 that this is the last TD in the frame.}
0 = Link Pointer field is valid.
1 = Link Pointer field not valid.
Intel^® 82801DB ICH4 Datasheet
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