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CHAPTER 5 SAR FUNCTION
155
(b) Unlinking
A VC is unlinked if the status of that VC changes from active to idle. If the packet descriptor of the
fetched packet is vacant, and all the data in the packet to be transmitted has been read, rewriting of
the "Forward Pointers" and "Backward Pointers" of the VC, and the VCs before and after it, is started.
As a result, the VC is released from the link list. At this time, active bit A of the VC table is switched
from 1 to 0.
If the VC to be unlinked is the last VC remaining linked to the shaper, the "A bit" in the scheduler
register and the "a bit" of the shaper pointer entry are switched from 1 to 0 and the shaper itself is
made inactive as soon as it has been unlinked.
The
μ
PD98405 uses the "Forward Pointer" in the link list to control transmission processing for each
VC. The
μ
PD98405 stores the "VC NUMBER" in the "Forward Pointer" of the VC table, i.e., the "VC
NUMBER" of the VC to be processed next, into the shaper pointer of control memory when it
executes transmission processing for a certain VC. In this way, the
μ
PD98405 determines the VC
from which it should start processing after moving from one shaper to another.
(6) Shaper pointer entry
The shaper pointer entry is a table for indicating which VCs have been processed when the
μ
PD98405
moves from one shaper to another. It is located in the shaper pointer entry register (SPE0 to SPE15).
SPE0 is the shaper pointer entry for shaper 0, while SPE15 is the shaper pointer entry for shaper 15.
The shaper pointer entry is used and managed by the
μ
PD98405 as a table. The host can read/write this
register by using the Indirect_Access command. However, the host accesses this register only when it
uses the unassigned cell generator function. It does not need to access this area otherwise.
The format of the shaper pointer entry is as follows:
Figure 5-25. Shaper Pointer Entry Format
30
31
- 0 -
0
14
15
VC NUMBER
a
u
a
Active flag. This bit is used by the
μ
PD98405 as an internal flag.
1 - At least one active VC is linked to the shaper.
0 - No active VC is linked to the shaper.
When this shaper is used as an unassigned cell generator, this bit and the u bit are set
to 1 by the host.
u
Unassigned cell generator. Only when this shaper is used as an unassigned cell
generator, this bit is set to 1 by the host. (See
Section 5.4.4 (8)
for details.)
VC NUMBER
This field is used as a table to stack the "VC NUMBER" of the VC the
μ
PD98405 is to
transmit next. If no VC is linked to the shaper, this field consists entirely of zeros.