MPC555
/
MPC556
EXTERNAL BUS INTERFACE
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
9-40
Figure 9-28 Termination Signals Protocol Timing Diagram
9.5.9 Storage Reservation
The MPC555
/ MPC556 storage reservation protocol supports a multi-level bus struc-
ture. For each local bus, storage reservation is handled by the local reservation logic.
The protocol tries to optimize reservation cancellation such that a PowerPC processor
is notified of storage reservation loss on a remote bus only when it has issued a
stwcx
cycle to that address. That is, the reservation loss indication comes as part of the
stwcx
cycle. This method avoids the need to have very fast storage reservation loss
indication signals routed from every remote bus to every PowerPC master.
The storage reservation protocol makes the following assumptions:
• Each processor has, at most, one reservation flag
•
lwarx
sets the reservation flag
•
lwarx
by the same processor clears the reservation flag related to a previous
lwarx
instruction and again sets the reservation flag
•
stwcx
by the same processor clears the reservation flag
• Store by the same processor does
not
clear the reservation flag
• Some other processor (or other mechanism) store to the same address as an ex-
isting reservation clears the reservation flag
• In case the storage reservation is lost, it is guaranteed that
stwcx
will not modify
the storage
CLKOUT
ADDR[0:31]
TS
TA, BI, TEA
RD/WR
TSIZ[0:1]
Slave 1
Slave 2
Slave 1
allowed to drive
acknowledge signals
Slave 1
negates acknowledge
signals and turns off
Slave 2
allowed to drive
acknowledge signals
Slave 2
negates acknowledge
signals and turns off
Data
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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