MPC555
/
MPC556
L-BUS TO U-BUS INTERFACE (L2U)
MOTOROLA
USER’S MANUAL
Rev. 15 October 2000
11-7
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 ma-
chine check exceptions.
11.6 Reservation Support
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 a PowerPC 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 indica-
tion comes as part of the
stwcx
cycle.
11.6.1 The 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 same processor clears the reservation flag
• A store instruction by 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.
The reservation logic in the L2U performs the following functions:
• Snoops accesses to all L-bus and U-bus slaves
• Holds one reservation (address) for the core
• Sets the reservation flag when the CPU issues a load-with-reservation request
The unit for reservation is one word. A byte or half-word store request by another mas-
ter will clear the reservation flag.
A load-with-reservation request by the CPU updates the reservation address related
to a previous load-with-reservation request and sets the reservation flag for the new
location. A store-with-reservation request by the CPU clears the reservation flag. A
store request by the CPU does not clear the flag. A store request by some other master
to the reservation address clears the reservation flag.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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