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Priority Mode Operation
7-5
Figure 7.4
Priority Mode - Calendar Table
7.2.2 Service Command
In Priority mode, the Scheduler maintains the head and tail of the
Connection List for each class at the current cell slot in internal registers
for each calendar. This reduces the number of memory accesses. When
the APU reads the SCD_Serv register, the Scheduler returns the head
of the highest priority nonempty list and advances the head pointer to the
next connection. If all lists are empty, the Scheduler returns Connection
Number 0.
In addition, most applications need to know the connection class and
whether the connection has any data to send. To support these
requirements, the Scheduler returns the
SCD_BuffPres
and
SCD_Class
fields along with the
Connection Number
field as shown in
Figure 7.5
. It
also reads the next VCD pointed to by the new head pointer to have the
VCD_BuffPres
and
VCD_Class
information ready for the next time the
service
command is issued.
A
1
C
0
2
3
4
5
6
7
8
9
1
E
2
B
1
D
3
NOW
F
1
G
3
Calendar Table - After Service, Schedule, and Tic Commands