Sun Microsystems UltraSPARC-I User Manual page 102

7. The C
the cycle in which it deasserts its request.
8. If the C
one or more other requests were asserted, arbitration occurs during this
cycle to decide who can drive during the next cycle.
9. During an arbitration cycle, the highest priority request from the last cycle
is determined, as shown in Table 7-6. During the next cycle, the value of
C
URRENT
During the next cycle, the value of L
value of C
this case, L
Note that the round robin protocol is unfair by design, favoring the L
P
ORT
for the L
being dependent on possible simultaneously asserted requests. Fairness is
provided by the release request in presence of another request rule; for
example, a request from another port.
10. If during an arbitration cycle, an SC request was asserted last cycle, it has
the highest priority and SC becomes the C
SC request does not modify the L
affect the round-robin turn for other interconnect ports, as shown in
Table 7-6.
Table 7-6
L P D
AST ORT RIVER
port_ID=0
port_ID=1
port_ID=2
port_ID=3
7.4.3.2 Latency Optimization in Uniprocessor Systems
Normally the C
ing requests. This rule minimizes the arbitration latency for other bus masters.
In uniprocessor systems, where SYSADDR is shared only by one processor, the
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
SC, and at most one I/O device, it is advantageous to minimize the latency for
D may drive SYSADDR at any time up to and including
URRENT RIVER
D 's request was deasserted during the last cycle and
URRENT RIVER
D is changed to match the highest priority request.
RIVER
D , unless the SC is the new C
URRENT RIVER
P D
AST ORT RIVER
D . This feature is required; it enables the request-then-drive rule
RIVER
P D , since the L
AST ORT RIVER
Round Robin Arbitration Priority, without SC Request
Arbitration Priority
Highest-to-Lowest
0 1 2 3
1 2 3 0
2 3 0 1
3 0 1 2
D must drop its request when it has no more pend-
URRENT RIVER
7. UltraSPARC External Interfaces
P D
AST ORT RIVER
retains its current state.
P D
AST ORT RIVER
URRENT
P D
AST ORT RIVER
will change to the
D . In
URRENT RIVER
AST
can drive without
D next cycle. The
RIVER
variable and does not