Add XCHG to the list of instructions extended to support cap operands.

Explicitly note that NOP (0x90) needs to leave CAX unchanged.

Author: bsdjhb

chap-cheri-x86-64.tex

@@ -622,6 +622,17 @@ \subsubsection{Extending Existing Instructions to Support Capability Operands}
     \insnnoref{CMPXCHG} should default to integer operands in
     capability mode.
 
+  \item \insnnoref{XCHGC} will also be required to support atomic
+    operations on capabilities.
+
+    The opcodes \texttt{87} and \texttt{90 + rc} would be extended to
+    support capability operands via the capability operand prefix.
+    The specific opcode \texttt{90} would remain a single byte
+    \insnnoref{NOP} which would not alter the value of \CAX{}.
+
+    \insnnoref{XCHG} should default to integer operands in capability
+    mode.
+
   \item It may also be desirable to support \insnnoref{XADDC}.  For
     this instruction, only the integer portion of the second (source)
     operand would be added to the integer value of the first