
MOTOROLA
Chapter 8. PCI Bus Interface
8-27
Exclusive Access
initiator must hold LOCK negated during the address phase of a read command and assert
LOCK in the clock cycle following the address phase. Note that the Trst transaction of a
locked operation must be a read transaction.
The locked operation is not established on the PCI bus until the Trst data transfer (IRDY
and TRDY asserted) completes. Once the lock is established, the initiator may retain
ownership of the LOCK signal and the target may remain locked beyond the end of the
current transaction. the initiator holds LOCK asserted until either the locked operation
completes or until an error (master-abort or target-abort) causes an early termination. A
target remains in the locked state until both FRAME and LOCK are negated. If the target
retries the Trst transaction without a data phase completing, the initiator should not only
terminate the transaction but should also release LOCK.
8.5.2 Continuing an Exclusive Access
When the LOCK owner is granted access to the bus for another exclusive access to the
previously-locked target, it negates the LOCK signal during the address phase to reestablish
the lock. The locked target accepts the transaction and claims the transaction. the initiator
asserts LOCK in the clock cycle following the address phase. If the initiator plans to
continue the locked operation, it continues to assert LOCK.
8.5.3 Completing an Exclusive Access
When an initiator is ready to complete an exclusive access, it should negate LOCK when
IRDY is negated following the completion of the last data phase of the locked operation.
This is to insure that the target is released prior to any other operation, and to insure that the
resource is no longer blocked.
8.5.4 Attempting to Access a Locked Target
If LOCK is asserted during the address phase to a locked target, the locked target signals a
retry, terminating the transaction without transferring any data. (The lock master always
negates LOCK during the address phase of a transaction to a locked target.) Nonlocked
targets ignore the LOCK signal when decoding the address. This allows other PCI agents
to initiate and respond to transactions while maintaining exclusive access to the locked
target.
8.5.5 Exclusive Access and the MPC8240
As an initiator, the MPC8240 does not generate locked operations. As a target, the
MPC8240 responds to locked operations by guaranteeing complete access exclusion to
local memory from the point-of-view of the PCI bus. From the point of view of the
processor core, only the cache line (32 bytes) of the transaction is locked.
If an initiator on the PCI bus asserts LOCK for a read transaction to local memory, the
MPC8240 completes the snoop transactions for any previous PCI-to-local-memory write
operations and performs a snoop transaction for the locked read operation on the internal