diff --git a/ieee1364-notes.txt b/ieee1364-notes.txt
index 3b5427ed9..9253cb0e6 100644
--- a/ieee1364-notes.txt
+++ b/ieee1364-notes.txt
@@ -318,8 +318,32 @@ the right length, and will therefore flag instances u5 and u6 as
 errors. The IEEE1364 standard should be more specific one way or the
 other.
 
-$Id: ieee1364-notes.txt,v 1.8 2001/08/01 05:17:31 steve Exp $
+* UNKNOWN VALUES IN L-VALUE BIT SELECTS
+
+Consider this example:
+
+	reg [7:0] vec;
+	wire [4:0] idx = <expr>;
+	[...]
+	vec[idx] = 1;
+
+So long as the value of idx is a valid bit select address, the
+behavior of this assignment is obvious. However, there is no explicit
+word in the standard as to what happens if the value is out of
+range. The standard clearly states the value of an expression when the
+bit-select or part select is out of range (the value is x) but does
+not address the behavior when the expression is an l-value.
+
+Icarus Verilog will take the position that bit select expressions in
+the l-value will select oblivion if it is out of range. That is, if
+idx has a value that is not a valid bit select of vec, then the
+assignment will have no effect.
+
+$Id: ieee1364-notes.txt,v 1.9 2002/01/26 02:08:07 steve Exp $
 $Log: ieee1364-notes.txt,v $
+Revision 1.9  2002/01/26 02:08:07  steve
+ Handle x in l-value of set/x
+
 Revision 1.8  2001/08/01 05:17:31  steve
  Accept empty port lists to module instantiation.
 
diff --git a/vvp/opcodes.txt b/vvp/opcodes.txt
index 0b6b53481..2cff04868 100644
--- a/vvp/opcodes.txt
+++ b/vvp/opcodes.txt
@@ -1,7 +1,7 @@
 /*
  * Copyright (c) 2001 Stephen Williams (steve@icarus.com)
  *
- *  $Id: opcodes.txt,v 1.31 2001/11/01 03:00:19 steve Exp $
+ *  $Id: opcodes.txt,v 1.32 2002/01/26 02:08:07 steve Exp $
  */
 
 
@@ -379,6 +379,10 @@ This sets the bit of a variable functor, the address calculated by
 using the index register <idx> (0, 1, 2 or 3) to index the functor
 address of <var-label>.
 
+If the index value in index register <idx> is <0 (for example if
+%ix/get converted an unknown value into the register) then the set is
+not performed.
+
 
 * %shiftl/i0 <bit>, <wid>
 
diff --git a/vvp/vthread.cc b/vvp/vthread.cc
index 618a456d9..51429d27f 100644
--- a/vvp/vthread.cc
+++ b/vvp/vthread.cc
@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT)
-#ident "$Id: vthread.cc,v 1.65 2001/12/31 00:01:16 steve Exp $"
+#ident "$Id: vthread.cc,v 1.66 2002/01/26 02:08:07 steve Exp $"
 #endif
 
 # include  "vthread.h"
@@ -275,6 +275,7 @@ void vthread_run(vthread_t thr)
 	    vvp_code_t cp = codespace_index(thr->pc);
 	    thr->pc += 1;
 
+	    assert(cp);
 	    assert(cp->opcode);
 
 	      /* Run the opcode implementation. If the execution of
@@ -1490,10 +1491,31 @@ bool of_SET_MEM(vthread_t thr, vvp_code_t cp)
       return true;
 }
 
+/*
+ * Implement the %set/x instruction:
+ *
+ *      %set/x <functor>, <bit>, <idx>
+ *
+ * The single bit goes into the indexed functor. Abort the instruction
+ * if the index is <0.
+ */
 bool of_SET_X(vthread_t thr, vvp_code_t cp)
 {
       unsigned char bit_val = thr_get_bit(thr, cp->bit_idx[0]);
-      vvp_ipoint_t itmp = ipoint_index(cp->iptr, thr->index[cp->bit_idx[1]&3]);
+      long idx = thr->index[cp->bit_idx[1]&3];
+
+	/* If idx < 0, then the index value is probably generated from
+	   an undefined value. At any rate, this is defined to have no
+	   effect so quit now. */
+      if (idx < 0) {
+	    return true;
+      }
+
+	/* Form the functor pointer from the base pointer and the
+	   index from the index register. */
+      vvp_ipoint_t itmp = ipoint_index(cp->iptr, idx);
+
+	/* Set the value. */
       functor_set(itmp, bit_val, strong_values[bit_val], true);
 
       return true;
@@ -1696,6 +1718,9 @@ bool of_ZOMBIE(vthread_t thr, vvp_code_t)
 
 /*
  * $Log: vthread.cc,v $
+ * Revision 1.66  2002/01/26 02:08:07  steve
+ *  Handle x in l-value of set/x
+ *
  * Revision 1.65  2001/12/31 00:01:16  steve
  *  Account for negatives in cmp/s
  *