IBM PowerPRS Q-64G
Preliminary
Packet Routing Switch
The extended output queue grant flywheel and the output queue grant priority flywheel carry related informa-
tion as do the subport grant type/priority flywheel and the subport grant port flywheel.
3.3.6.1 Extended Output Queue Grant Flywheel
The extended output queue grant flywheel determines the set of ports for the output queue grants carried by
the current egress packet cycle. For the 256-Gbps configuration, this flywheel cycles as follows:
0
1
Ports 0 to 7
Ports 8 to 15
For the 512-Gbps configuration, it cycles as follows:
0
1
Ports 0 to 15
Ports 16 to 31
There is one output queue grant per port and per priority. The particular set of output queue grants carried by
the current packet cycle is determined by this flywheel in combination with the output queue grant priority
flywheel. This flywheel is incremented every packet cycle. One packet cycle carries the grants for half the
ports and one priority. When all four priorities are enabled, eight packet cycles are required to update the
complete set of output queue grants.
To synchronize the PowerPRS Q-64G flywheels with those in the attached devices, the extended output
queue grant flywheel status and output queue grant priority flywheel status are inserted into the packet quali-
fier byte (H0) of egress idle packets carried on the low channel. The output queue grants are inserted into
header bytes H1 and, for the 512-Gbps configuration, H2 of egress data, control, service, and idle packets
carried on the low channel. See the following packet format descriptions for more information.
3.3.6.2 Output Queue Grant Priority Flywheel
The output queue grant priority flywheel determines the priority of the output queue grants carried by the
current egress packet cycle. This flywheel cycles as follows:
00
01
10
11
Priority 0
Priority 1
Priority 2
Priority 3
This flywheel is incremented each time the extended output queue grant flywheel returns to ‘0’. This flywheel
returns to ‘00’ after reaching the number of priorities defined in the number of priorities field in the
Configuration 1 Register (page 112).
3.3.6.3 Subport Grant Type/Priority Flywheel
The subport grant type/priority flywheel determines the type of subport grants carried by the current egress
packet cycle. This flywheel cycles as follows:
000
001
010
011
100
Subport output queue grants, priority 0
Subport output queue grants, priority 1
Subport output queue grants, priority 2
Subport output queue grants, priority 3
Subport multicast grants
prsq-64g.01.fm
December 20, 2001
Functional Description
Page 37 of 199
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