Intel
82801BA ICH2 Datasheet
5-99
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 ICH2 interprets during operation. All Transfer Descriptors have the same
basic, 32-byte structure. During operation, the ICH2 hardware performs consistency checks on
some fields of the TD. If a consistency check fails, the ICH2 halts immediately and issues an
interrupt to the system. This interrupt cannot be masked within the ICH2.
Figure 5-15. Transfer Descriptor
Table 5-58. 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.
2
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
the ICH2 to process the next transaction in the queue rather than starting a new queue.
1 = Depth first.
0 = Breadth first.
1
QH/TD Select (Q).
This bit informs the ICH2 whether the item referenced by the link pointer is
another TD or a QH. This allows the ICH2 to perform the proper type of processing on the item after
it is fetched
1 = QH.
0 = TD.
0
Terminate (T).
This bit informs the ICH2 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 ICH2 that there are no
more valid entries in the queue. A TD encountered outside of a queue context with the T bit set
informs the ICH2 that this is the last TD in the frame.
1 = Link Pointer field not valid.
0 = Link Pointer field is valid.
T
f
d
Q
0
0
1
2
3
4
31
MaxLen
T
7
8
15
16
23
24
10
11
14
18
19
20
R
D
Status
R
25
26
27
28
29
30
21
ICH2 Read Only
Link Pointer
EndPt
Device Address
PID
ActLen
Vf
C_ERR LS
SPD
ISOISC
R
Buffer Pointer
R = Reserved
ICH2 Read/Write
ICminer.com Electronic-Library Service CopyRight 2003