MOTOROLA
Chapter 14. MPC860 External Bus Interface
14-35
Part IV. Hardware Interface
That is, the reservation loss indication comes as part of the STWCX cycle, which avoids
the need for fast memory reservation loss indication signals between each remote bus and
each PowerPC master. The memory reservation protocol assumes the following:
¥
Each processor has no more than one reservation ßag.
¥
lwarx sets the reservation ßag.
¥
lwarx by the same processor clears the reservation ßag related to a previous lwarx
instruction and again sets the reservation ßag.
¥
stwcx. by the same processor clears the reservation ßag.
¥
Store by the same processor does not clear the reservation ßag.
¥
Some other processor (or other mechanism) store to the same address as an existing
reservation clears the reservation ßag.
¥
If memory reservation is lost, it is guaranteed that stwcx. will not modify the
memory.
14.4.9.1 Cancel Reservation (CR)
CR is a point-to-point signal. To use it, reservation logic must remember speciÞcally which
bus master requested reservation for which address. If another master writes to the reserved
address, the reservation logic asserts CR only to the master that holds the associated
reservation, thus clearing its ßag.
The advantage of CR is that it preempts the stwcx. instruction if reservation is lost, thus
eliminating unnecessary trafÞc on the external bus.
Figure 14-27 shows the reservation protocol for a single-level (local) bus. It assumes that
an external logic on the bus handles the following:
¥
Snoops accesses to all local bus slaves.
¥
Holds one reservation for each local master capable of memory reservations.
¥
Sets the reservation when that master issues a load and reserve request.
¥
Clears the reservation when another master issues a store to the reservation address.
Содержание MPC860 PowerQUICC
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