do release only if cas succeeds

zacas.adoc

@@ -152,9 +152,18 @@ aligned for _quadwords_, eight-byte aligned for _doublewords_, and four-byte
 aligned for _words_). And the same exception options apply if the address
 is not naturally aligned.
 
+An `AMOCAS.W/D/Q` generates a memory read access and, if successful, generates
+a memory write access to store the _swap_ value. An unsuccessful `AMOCAS.W/D/Q`
+may either not perform a memory write or may write back the old value loaded
+from memory. An `AMOCAS.W/D/Q` instruction always requires write permissions.
+
 Just as for AMOs in the A extension, the `AMOCAS.W/D/Q` optionally provide
 release consistency semantics, using the `aq` and `rl` bits, to help implement
-multiprocessor synchronization.
+multiprocessor synchronization. The memory operation performed by an
+`AMOCAS.W/D/Q`, when successful, has acquire semantics if `aq` bit is 1 and has
+release semantics if `rl` bit is 1. The memory operation performed by an
+`AMOCAS.W/D/Q`, when not successful, has acquire semantics if `aq` bit is 1 but
+does not have release semantics, regardless of `rl`.
 
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