IBM PowerPC 604 User Manual page 321

latency for a transaction. The 604 always negates ABB for at least one bus clock cycle after
AACK is asserted, even if it is parked and has another transaction pending.
Typically, bus parking is provided
to
the device that was the most recent bus master;
however, system designers may choose other schemes such as providing unrequested bus
grants in situations where it is easy to correctly predict the next device requesting bus
mastership.
neid_bus 1
qua/BG
I
-1 0
XO .... 1 _ _ _ __..., _ _ _ _,,,,,."""'
I
........i----~
Figure 8-5. Address Bus Arbitration Showing Bus Parking
When the 604 receives
a~ified
bus grant, it assumes address bus mastership by asserting
ABB and negating the BR output signal. Meanwhile, the 604 drives the address for the
requested access onto the address bus
and
asserts TS to indicate the start of a new
transaction.
When designing external bus arbitration logic, note that the 604 may assert BR without
using the bus after it receives the qualified bus grant. For example, in a system using bus
snooping, if the 604 asserts BR to perform a queued read-with-intent-to-modify-atomic
(RWITMA), and the 604 snoops an access which cancels the reservation associated with
the RWITMA. Once the 604 is granted the bus, it no longer needs to perform the RWITMA;
therefore, the 604 does not assert ABB and does not use the bus for the read operation. Note
that the 604 asserts BR for at least one clock cycle in these instances.
Chapter 8. System Interface Operation 8-11