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CHAPTER 5 SAR FUNCTION
129
(1) Transmit queue
The host creates and manages a transmit queue for each channel (transmit VC). The transmit queue
consists of the packet descriptors for the corresponding channel. Each packet has one or more data
buffers. The read pointer of the transmit queue, in the corresponding transmit VC table of control memory,
is initialized by the host. Subsequently, the read pointer is updated and managed by the
μ
PD98405 each
time the transmit packet descriptor is fetched.
Do not release the transmit queue for a VC while that VC exists. A transmit queue is set for each VC. The
host sets the initial value of the read pointer for that queue (Tx Queue Read Pointer) in the transmit VC
table. The read pointer value is updated each time the
μ
PD98405 transmits a packet. During
transmission, the
μ
PD98405 reads the packet descriptor in system memory pointed to by the read pointer.
Packet descriptor control by the host and read access to a packet descriptor by the
μ
PD98405 are not
synchronized. If the host releases a transmit queue area in system memory, therefore, the
μ
PD98405
cannot obtain correct information from the packet descriptor, such that a malfunction occurs. Before
releasing a transmit queue area, therefore, first close the VC for that queue by executing the
Close_Channel command.
Figure 5-8 shows the format of the packet descriptor.
Figure 5-8. Format of Packet Descriptor
IM
AAL
CLPM
PTI
GFC
S/M
D/P
31
29
28
27
26
24
23
20
19
18
17
16
15
0
31
0
CPCS-UU
0
31
16
15
0
31
0
SIZE
MB
CPI
7
8
V
0
ADDRESS
C10
Word 0
Word 1
Word 2
Word 3
30
A "packet descriptor" consists of four words, and is located at a word boundary in system memory. One
packet descriptor is set for each transmit packet. The packet descriptor can store the pointer to a data
buffer or that to a packet directory. Each field of the packet descriptor is described below.