--- pdb.py	2010-06-24 07:29:45.912957098 -0700
+++ pdb_new.py	2010-06-24 08:30:25.348547160 -0700
@@ -604,45 +604,49 @@
     do_w = do_where
     do_bt = do_where
 
+    def steps_arg(self, cmd, arg):
+        """Parse up/down #steps numeric argument.
+        Return None if bad numeric argument, 1 if no argument,
+        otherwise the value of the argument.  If the user specifies a
+        negative number, return a large integer, which is then
+        capped by our caller at the actual number of steps
+        possible."""
+        val = 1
+        if arg:
+            try:
+                val = int(arg)
+            except:
+                print >>self.stdout, "Usage : %s [#steps]" % cmd
+                return None
+            if val < 0:
+                val = 999999999
+        return val
+
     def do_up(self, arg):
-	nup = 1
-	if arg:
-	    try:
-		nup = int(arg)
-		if nup < 0:
-		    nup = self.curindex
-	    except:
-		pass
-
-	for _ignore in xrange(nup):
-	    if self.curindex is 0:
-		print >>self.stdout, '*** Oldest frame'
-		break
-            self.curindex -= 1
-            self.curframe = self.stack[self.curindex][0]
-            self.lineno = None
-	self.print_stack_entry(self.stack[self.curindex])
+        nup = self.steps_arg("up", arg)
+        if nup is None:
+            return
+        nup = min(nup, self.curindex)
+        self.curindex -= nup
+        self.curframe = self.stack[self.curindex][0]
+        self.lineno = None
+        if self.curindex is 0:
+            print >>self.stdout, '*** Oldest frame'
+        self.print_stack_entry(self.stack[self.curindex])
 
     do_u = do_up
 
     def do_down(self, arg):
-	ndown = 1
-	if arg:
-	    try:
-		ndown = int(arg)
-		if ndown < 0:
-		    ndown = len(self.stack) - self.curindex
-	    except:
-		pass
-
-	for _ignore in xrange(ndown):
-	    if self.curindex + 1 is len(self.stack):
-		print >>self.stdout, '*** Newest frame'
-		break
-            self.curindex += 1
-            self.curframe = self.stack[self.curindex][0]
-            self.lineno = None
-	self.print_stack_entry(self.stack[self.curindex])
+        ndown = self.steps_arg("down", arg)
+        if ndown is None:
+            return
+        ndown = min(ndown, len(self.stack) - self.curindex - 1)
+        self.curindex += ndown
+        self.curframe = self.stack[self.curindex][0]
+        self.lineno = None
+        if (self.curindex + 1) is len(self.stack):
+            print >>self.stdout, '*** Newest frame'
+        self.print_stack_entry(self.stack[self.curindex])
 
     do_d = do_down