8.6. Locked access

The AXI protocol specifies that, when a locked transaction occurs, the master must follow the locked transaction with an unlocked transaction to remove the lock of the interconnect. The locked sequence is not complete until the end of the locking transaction. The SWP{B,H} instructions include separate read and write transactions on the AXI. The read transaction is marked as a locked transaction while the write transaction is not marked as a locked transaction. Therefore, the write transaction serves as the unlocking transaction and the AXI interconnect is unlocked when the write response is generated.

The SWP{B,H} instructions can access cacheable or noncacheable memory. If it is to cacheable memory, the bus transaction is not marked as a locked transaction. If it is to noncacheable memory, both the read and write transactions are treated as strongly ordered memory type, and the bus transaction is marked as a locked transaction.

If an abort occurs, the swapping of data between the register and memory is unsuccessful. To clear the lock, the processor issues a write transaction on the AXI interface without any byte strobes active.

Note

All transactions related to the swap instructions are issued with the lock indicator on its respective port, ARLOCK or AWLOCK.

Copyright © 2006-2009 ARM Limited. All rights reserved.ARM DDI 0344I
Non-Confidential