diff --git a/sim/mips/ChangeLog b/sim/mips/ChangeLog
index be4ce2ac24..a91b13725d 100644
--- a/sim/mips/ChangeLog
+++ b/sim/mips/ChangeLog
@@ -1,3 +1,8 @@
+2004-04-09  Chris Demetriou  <cgd@broadcom.com>
+
+	* sb1.igen (check_sbx): New function.
+	(PABSDIFF.fmt, PABSDIFC.fmt, PAVG.fmt): Use check_sbx.
+
 2004-03-29  Chris Demetriou  <cgd@broadcom.com>
 	    Richard Sandiford  <rsandifo@redhat.com>
 
diff --git a/sim/mips/sb1.igen b/sim/mips/sb1.igen
index 33c5141e77..7a718c736b 100644
--- a/sim/mips/sb1.igen
+++ b/sim/mips/sb1.igen
@@ -22,6 +22,20 @@
 // 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 
 
+// Helper:
+//
+// Check that the SB-1 extension instruction can currently be used, and
+// signal a ReservedInstruction exception if not.
+//
+
+:function:::void:check_sbx:instruction_word insn
+*sb1:
+{
+  if ((SR & status_SBX) == 0)
+    SignalException(ReservedInstruction, insn);
+}
+
+
 //  MDMX ASE Instructions
 //  ---------------------
 //
@@ -152,13 +166,9 @@
 *sb1:
 {
   check_mdmx (SD_, instruction_0);
-  if (SR & status_SBX)
-    {
-      check_mdmx_fmtsel (SD_, instruction_0, FMTSEL);
-      StoreFPR(VD,fmt_mdmx,MX_AbsDiff(ValueFPR(VS,fmt_mdmx),VT,FMTSEL));
-    }
-  else
-    SignalException(ReservedInstruction, instruction_0);
+  check_sbx (SD_, instruction_0);
+  check_mdmx_fmtsel (SD_, instruction_0, FMTSEL);
+  StoreFPR(VD,fmt_mdmx,MX_AbsDiff(ValueFPR(VS,fmt_mdmx),VT,FMTSEL));
 }
 
 
@@ -167,13 +177,9 @@
 *sb1:
 {
   check_mdmx (SD_, instruction_0);
-  if (SR & status_SBX)
-    {
-      check_mdmx_fmtsel (SD_, instruction_0, FMTSEL);
-      MX_AbsDiffC(ValueFPR(VS,fmt_mdmx),VT,FMTSEL);
-    }
-  else
-    SignalException(ReservedInstruction, instruction_0);
+  check_sbx (SD_, instruction_0);
+  check_mdmx_fmtsel (SD_, instruction_0, FMTSEL);
+  MX_AbsDiffC(ValueFPR(VS,fmt_mdmx),VT,FMTSEL);
 }
 
 
@@ -182,11 +188,7 @@
 *sb1:
 {
   check_mdmx (SD_, instruction_0);
-  if (SR & status_SBX)
-    {
-      check_mdmx_fmtsel (SD_, instruction_0, FMTSEL);
-      StoreFPR(VD,fmt_mdmx,MX_Avg(ValueFPR(VS,fmt_mdmx),VT,FMTSEL));
-    }
-  else
-    SignalException(ReservedInstruction, instruction_0);
+  check_sbx (SD_, instruction_0);
+  check_mdmx_fmtsel (SD_, instruction_0, FMTSEL);
+  StoreFPR(VD,fmt_mdmx,MX_Avg(ValueFPR(VS,fmt_mdmx),VT,FMTSEL));
 }