diff --git a/include/opcode/ChangeLog b/include/opcode/ChangeLog
index e1b96f149..82bdc8f45 100644
--- a/include/opcode/ChangeLog
+++ b/include/opcode/ChangeLog
@@ -1,3 +1,7 @@
+2010-07-03  Alan Modra  <amodra@gmail.com>
+
+	* ppc.h (PPC_OPCODE_COMMON): Expand comment.
+
 2010-06-29  Alan Modra  <amodra@gmail.com>
 
 	* maxq.h: Delete file.
diff --git a/include/opcode/ppc.h b/include/opcode/ppc.h
index 2d99cd493..6e3a84b0c 100644
--- a/include/opcode/ppc.h
+++ b/include/opcode/ppc.h
@@ -89,7 +89,10 @@ extern const int powerpc_num_opcodes;
 #define PPC_OPCODE_601		      0x20
 
 /* Opcode is supported in both the Power and PowerPC architectures
-   (ie, compiler's -mcpu=common or assembler's -mcom).  */
+   (ie, compiler's -mcpu=common or assembler's -mcom).  More than just
+   the intersection of PPC_OPCODE_PPC with the union of PPC_OPCODE_POWER
+   and PPC_OPCODE_POWER2 because many instructions changed mnemonics
+   between POWER and POWERPC.  */
 #define PPC_OPCODE_COMMON	      0x40
 
 /* Opcode is supported for any Power or PowerPC platform (this is