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CHAPTER 5 SAR FUNCTION
142
BACKWARD
POINTER
Pointer to the preceding VC in the shaper link list.
LST
Last VC.
"1" indicates that this VC is the last VC in the shaper link list.
FORWARD
POINTER
Pointer to the next VC in the shaper link list.
Remarks 1.
Those fields indicated as "Reserved" are reserved for the
μ
PD98405. Do not modify these
fields except at initialization. Reset all of them to 0 at initialization.
2.
Do not modify Word 15 even at initialization for open VC tables. Word 15 of the transmit VC
table holds the pointers and flags required for the
μ
PD98405 to manage the VC table. If
Word 15 is modified by an external host, it becomes impossible for the
μ
PD98405 to
manage correctly.
3.
The rate fields (MCR, PCR, ICR, and ACR) are represented as follows:
Rate = 2
e
*(1 + m/512)*nz cells/s
14
0
m
15
Re-
9
8
nz
e
13
4.
See also
Section 5.8.4
for details of the ABR parameter.
(3) Status transition of transmit channel
The transmit channel can assume one of three statuses: non-existent, idle, and active. The host opens a
channel by issuing an Open_Channel command. After the
μ
PD98405 receives this command, it reports
the address of the transmit VC table to the host, as a command indication from the free block pool of
control memory. Next, the host writes appropriate parameters (such as VPI/VCI, the shaper to be used,
and the transmit queue read pointer) to the allocated block by using the Indirect_Access command. Thus,
this block is set as a transmit VC table, and the channel enters the idle status.
While the channel is in the idle status (once all parameters have been set), the VC is ready to perform
transmission. In this status, once the data to be transmitted has been generated and the host has
prepared a valid packet descriptor, the host issues a Tx_Ready command to the
μ
PD98405. Upon
receiving this command, the
μ
PD98405 sets bit "A" in Word 15 of the VC table to "1," then adds the VC
table to the link list of the shaper. For the VC that uses the ABR service, that VC is linked to the ABR
scheduler. As a result, the channel enters the active status.