diff --git a/vvm/vpi_bit.c b/vvm/vpi_bit.c
index 722158870..a7732542e 100644
--- a/vvm/vpi_bit.c
+++ b/vvm/vpi_bit.c
@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT) && !defined(macintosh)
-#ident "$Id: vpi_bit.c,v 1.4 2000/05/09 21:16:35 steve Exp $"
+#ident "$Id: vpi_bit.c,v 1.5 2000/05/11 01:37:33 steve Exp $"
 #endif
 
 # include  "vpi_priv.h"
@@ -28,7 +28,7 @@
  * bits are identical. This means that the VSSSvsss bits of the 8bit
  * value have V==v and SSS==sss.
  */
-#define UNAMBIG(v) (((v)&0x0f) == (((v)>>4)&0x0f))
+#define UNAMBIG(v) (! B_ISAMBIG(v))
 
 
 # define STREN1(v) ( ((v)&0x80)? ((v)&0xf0) : (0x70 - ((v)&0xf0)) )
@@ -134,6 +134,9 @@ vpip_bit_t vpip_bits_resolve(const vpip_bit_t*bits, unsigned nbits)
 
 /*
  * $Log: vpi_bit.c,v $
+ * Revision 1.5  2000/05/11 01:37:33  steve
+ *  Calculate the X output value from drive0 and drive1
+ *
  * Revision 1.4  2000/05/09 21:16:35  steve
  *  Give strengths to logic and bufz devices.
  *
diff --git a/vvm/vpi_priv.h b/vvm/vpi_priv.h
index d9fbdeff2..ae73600be 100644
--- a/vvm/vpi_priv.h
+++ b/vvm/vpi_priv.h
@@ -19,7 +19,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT) && !defined(macintosh)
-#ident "$Id: vpi_priv.h,v 1.19 2000/05/09 21:16:35 steve Exp $"
+#ident "$Id: vpi_priv.h,v 1.20 2000/05/11 01:37:33 steve Exp $"
 #endif
 
 /*
@@ -87,15 +87,24 @@ typedef unsigned char vpip_bit_t;
 # define StX 0xe6
 # define HiZ 0x08
 
-      /* Compare the logic values of two vpip_bit_t variables, or test
-	 the logical value of the bit. */
+	/* Compare the logic values of two vpip_bit_t variables. This
+	   is like the === operator of Verilog, it ignored strengths. */
 # define B_EQ(l,r) (((l)&0x88) == ((r)&0x88))
+
+	/* Test and return true if the value has ambiguous
+	   strength. The logic value may yet be knowable. */
+# define B_ISAMBIG(v) (((v)&0x0f) != (((v)>>4)&0x0f))
+
+	/* Test whether the value is of the specified logic value. It
+	   is possible for even ambiguous signals to have a known
+	   logic value. */
 # define B_IS0(v)  (((v)&0x88) == 0x00)
 # define B_IS1(v)  (((v)&0x88) == 0x88)
 # define B_ISX(v)  (((v)&0x88) == 0x80)
 # define B_ISZ(v)  ((v) == HiZ)
 # define B_ISXZ(v) (1 & (((v)>>7) ^ ((v)>>3)))
 
+
       /* Take as input an array of bits, and return the resolved
 	 value. The result accounts for the strengths involved. */
 extern vpip_bit_t vpip_pair_resolve(vpip_bit_t a, vpip_bit_t b);
@@ -368,6 +377,9 @@ extern int vpip_finished();
 
 /*
  * $Log: vpi_priv.h,v $
+ * Revision 1.20  2000/05/11 01:37:33  steve
+ *  Calculate the X output value from drive0 and drive1
+ *
  * Revision 1.19  2000/05/09 21:16:35  steve
  *  Give strengths to logic and bufz devices.
  *
diff --git a/vvm/vvm_gates.cc b/vvm/vvm_gates.cc
index 4e1527885..cc353cec0 100644
--- a/vvm/vvm_gates.cc
+++ b/vvm/vvm_gates.cc
@@ -17,7 +17,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT) && !defined(macintosh)
-#ident "$Id: vvm_gates.cc,v 1.15 2000/05/09 21:16:35 steve Exp $"
+#ident "$Id: vvm_gates.cc,v 1.16 2000/05/11 01:37:33 steve Exp $"
 #endif
 
 # include  "vvm_gates.h"
@@ -52,16 +52,13 @@ vvm_1bit_out::~vvm_1bit_out()
 void vvm_1bit_out::drive0(vpip_bit_t v)
 {
       drive0_ = v;
+      driveX_ = (drive1_&0xf0) | (drive0_&0x0f);
 }
 
 void vvm_1bit_out::drive1(vpip_bit_t v)
 {
       drive1_ = v;
-}
-
-void vvm_1bit_out::driveX(vpip_bit_t v)
-{
-      driveX_ = v;
+      driveX_ = (drive1_&0xf0) | (drive0_&0x0f);
 }
 
 void vvm_1bit_out::driveZ(vpip_bit_t v)
@@ -353,6 +350,9 @@ void vvm_not::take_value(unsigned, vpip_bit_t val)
 
 /*
  * $Log: vvm_gates.cc,v $
+ * Revision 1.16  2000/05/11 01:37:33  steve
+ *  Calculate the X output value from drive0 and drive1
+ *
  * Revision 1.15  2000/05/09 21:16:35  steve
  *  Give strengths to logic and bufz devices.
  *
diff --git a/vvm/vvm_gates.h b/vvm/vvm_gates.h
index 832a93a56..c0571107b 100644
--- a/vvm/vvm_gates.h
+++ b/vvm/vvm_gates.h
@@ -19,7 +19,7 @@
  *    Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  */
 #if !defined(WINNT) && !defined(macintosh)
-#ident "$Id: vvm_gates.h,v 1.62 2000/05/09 21:16:35 steve Exp $"
+#ident "$Id: vvm_gates.h,v 1.63 2000/05/11 01:37:33 steve Exp $"
 #endif
 
 # include  "vvm.h"
@@ -71,7 +71,6 @@ class vvm_1bit_out  : public vvm_nexus::drive_t  {
 
       void drive0(vpip_bit_t);
       void drive1(vpip_bit_t);
-      void driveX(vpip_bit_t);
       void driveZ(vpip_bit_t);
 
     private:
@@ -940,6 +939,9 @@ class vvm_posedge  : public vvm_nexus::recvr_t {
 
 /*
  * $Log: vvm_gates.h,v $
+ * Revision 1.63  2000/05/11 01:37:33  steve
+ *  Calculate the X output value from drive0 and drive1
+ *
  * Revision 1.62  2000/05/09 21:16:35  steve
  *  Give strengths to logic and bufz devices.
  *