Sun Microelectronics
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7. UltraSPARC External Interfaces
7. The CURRENT DRIVER may drive SYSADDR at any time up to and including
the cycle in which it deasserts its request.
8. If the CURRENT DRIVER’s request was deasserted during the last cycle and
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
CURRENT DRIVER is changed to match the highest priority request.
During the next cycle, the value of LAST PORT DRIVER will change to the
value of CURRENT DRIVER, unless the SC is the new CURRENT DRIVER. In
this case, LAST PORT DRIVER retains its current state.
Note that the round robin protocol is unfair by design, favoring the LAST
PORT DRIVER. This feature is required; it enables the request-then-drive rule
for the LAST PORT DRIVER, since the LAST PORT DRIVER can drive without
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 CURRENT DRIVER next cycle. The
SC request does not modify the LAST PORT DRIVER variable and does not
affect the round-robin turn for other interconnect ports, as shown in
Table 7-6.
7.4.3.2 Latency Optimization in Uniprocessor Systems
Normally the CURRENT DRIVER must drop its request when it has no more pend-
ing requests. This rule minimizes the arbitration latency for other bus masters.
In uniprocessor systems, where SYSADDR is shared only by one processor, the
SC, and at most one I/O device, it is advantageous to minimize the latency for
the processor at the expense of latency for SC or the I/O device. To support this,
Table 7-6 Round Robin Arbitration Priority, without SC Request
LAST PORT DRIVER
Arbitration Priority
Highest-to-Lowest
port_ID=0 0 1 2 3
port_ID=1 1 2 3 0
port_ID=2 2 3 0 1
port_ID=3 3 0 1 2
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