Improve description of FENCE.TSO

The description of FENCE.TSO as "optional" was incorrect.  It has always
been a mandatory instruction (because of the requirement that unused
patterns in the fm field be treated as though fm=0).

1 files changed, 4 insertions, 5 deletions

diff --git a/src/rv32.tex b/src/rv32.tex
index 99d58f7..70e27d6 100644
--- a/src/rv32.tex
+++ b/src/rv32.tex
@@ -1266,7 +1266,7 @@ The fence mode field {\em fm} defines the semantics of the FENCE.  A
 FENCE with {\em fm}=0000 orders all memory operations in its
 predecessor set before all memory operations in its successor set. 
 
-The optional FENCE.TSO instruction is encoded as a FENCE instruction
+The FENCE.TSO instruction is encoded as a FENCE instruction
 with {\em fm}=1000, {\em predecessor}=RW, and {\em successor}=RW.
 FENCE.TSO orders all load
 operations in its predecessor set before all memory operations in its
@@ -1276,10 +1276,9 @@ operations in the FENCE.TSO's predecessor set unordered with non-AMO
 loads in its successor set.
 
 \begin{commentary}
-  The FENCE.TSO encoding was added as an optional extension to the
-  original base FENCE instruction encoding.  The base definition
-  requires that implementations ignore any set bits and treat the
-  FENCE as global, and so this is a backwards-compatible extension.
+  Because \mbox{FENCE RW,RW} imposes a superset of the orderings that
+  FENCE.TSO imposes, it is correct to ignore the {\em fm} field and
+  implement FENCE.TSO as \mbox{FENCE RW,RW}.
 \end{commentary}
 
 The unused fields in the FENCE instructions---{\em rs1} and {\em rd}---are