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MPC561/MPC563 Reference Manual
MOTOROLA
Reservation Support
11.5.3
L-Bus Memory Access Violations
All L-bus slaves have their own access protection logic. For consistency, all storage access
violations have the same termination result. Thus access violations for load/store accesses
started by the RCPU always have the same termination from all slaves: assertion of the data
storage exception. All other L-bus masters cause machine check exceptions.
11.6 Reservation Support
In general terms, a reservation activity is the process whereby a load and store instruction
pair is accompanied by a reservation of the data, the goal being to achieve an atomic
operation. If a bus master other than the one holding the reservation accesses the data (or
Protocol”) the reservation is lost and is indicated accordingly.
The RCPU 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 (RCPU) is notified of
storage reservation loss on a remote bus (U-bus, IMB or external 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.
11.6.1
Reservation Protocol
The reservation protocol operates under the following assumptions:
Each processor has at most 1 reservation flag
A lwarx instruction sets the reservation flag
Another lwarx instruction by same processor clears the reservation flag related to a
previous lwarx instruction and sets again the reservation flag
A stwcx instruction by the same processor clears the reservation flag
A store instruction by the same processor does not clear the reservation flag
Some other processor (or other mechanism) store to an address with an existing
reservation clears the reservation flag
In case the storage reservation is lost, it is guaranteed that stwcx will not modify the
storage
11.6.2
L2U Reservation Support
The L2U is responsible for handling the effects of reservations on the L-bus and the U-bus.
For the L-bus and the U-bus, the L2U detects reservation losses.