includes an external handshake that will be exercised upon execution of the fence.i instruction. The handshake is composed of the signals fencei_flush_req_o and fencei_flush_ack_i and can for example be used to flush an externally connected cache.
The fencei_flush_req_o signal will go high upon executing a fence.i instruction once possible earlier store instructions have fully completed (including emptying of the the write buffer). The fencei_flush_req_o signal will go low again the cycle after sampling both fencei_flush_req_o and fencei_flush_ack_i high. Once fencei_flush_req_o has gone low again a branch will be taken to the instruction after the fence.i thereby flushing possibly prefetched instructions.
Fence instructions are not impacted by the distinction between main and I/O regions (defined in pma) and execute as a conservative fence on all operations, ignoring the predecessor and successor fields.
Note
If the fence.i external handshake is not used by the environment of , then it is recommended to tie the fencei_flush_ack_i to 1 in order to avoid stalling fence.i instructions indefinitely.