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Bus Operations
9.5.10
Storage Reservation
Reservation occurs when a master loads data from memory. The memory location must not
be overwritten until the master finishes processing the data and writing the results back to
the reserved location. The MPC533 storage reservation protocol supports a multi-level bus
structure. For each local bus, storage reservation is handled by the local reservation logic.
The protocol tries to optimize reservation cancellation such that an MPC500 processor is
notified of storage reservation loss on a remote bus only when it has issued a conditional
storeword (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 MPC500 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 existing
reservation clears the reservation flag
• In case the storage reservation is lost, it is guaranteed that stwcx will not modify the
storage
The reservation protocol for a single-level (local) bus is illustrated in Figure 9-30. The
protocol assumes that an external logic on the bus carries out the following functions:
• Snoops accesses to all local bus slaves
• Holds one reservation for each local master capable of storage reservations
• Sets the reservation when that master issues a load and reserve request
• Clears the reservation when some other master issues a store to the reservation
address
9-44
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
MPC533 Reference Manual
MOTOROLA
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