diff --git a/devices/pdp11/machine/1170/panel/debugger/xxdp/README.md b/devices/pdp11/machine/1170/panel/debugger/xxdp/README.md
index 513b240f9..fa9d1a838 100644
--- a/devices/pdp11/machine/1170/panel/debugger/xxdp/README.md
+++ b/devices/pdp11/machine/1170/panel/debugger/xxdp/README.md
@@ -11,6 +11,7 @@ machines:
     autoMount:
       RL0:
         path: http://archive.pcjs.org/disks/dec/rl02k/RL02K-XXDP.json
+    sticky: top
 ---
 
 This machine is ready to boot [XXDP+ Diagnostics](/disks/dec/rl02k/xxdp/) ("BOOT RL0") and run
@@ -20,10 +21,14 @@ diagnostics (e.g., "R EKBEE1"):
 - [EKBBF0: 11/70 CPU DIAGNOSTIC (PART 2)](/disks/dec/rl02k/xxdp/ekbbf0/)
 - [EKBEE1: 11/70 MEMORY MANAGEMENT DIAGNOSTIC](/disks/dec/rl02k/xxdp/ekbee1/)
 
-For more information about booting and running these diagnostics, see [XXDP+ Diagnostics](/disks/dec/rl02k/xxdp/).
+Step-by-step instructions are included below.  Other interesting things to know about this machine:
+
+* It includes an [M9312 ROM](/devices/pdp11/rom/M9312/) at address 165000.  The exact ROM is [23-616F1](/devices/pdp11/rom/M9312/23-616F1.txt).
 
 {% include machine.html id="test1170" %}
 
-Other interesting things to know about this machine:
+Step 1: We need to select a drive to load the XXDP+ Diagnostics disk, and since it is an RL02K disk,
+we need to use an RL02 drive.  A typical PDP-11 machine with a single RL11 disk controller could contain
+up to four such drives, which we refer to as RL0 through RL3.
 
-* It includes an [M9312 ROM](/devices/pdp11/rom/M9312/) at address 165000.  The exact ROM is [23-616F1](/devices/pdp11/rom/M9312/23-616F1.txt).
+To select drive RL0, press {% include machine-command.html type='button' label='Select RL0' machine='test1170' component='RL11' command='selectDrive' value='RL0' %}
diff --git a/modules/pdp11/lib/rl11.js b/modules/pdp11/lib/rl11.js
index 1e940bd0c..fc0053932 100644
--- a/modules/pdp11/lib/rl11.js
+++ b/modules/pdp11/lib/rl11.js
@@ -83,6 +83,10 @@ class RL11 extends Component {
          * Define all the properties to be initialized by initController()
          */
         this.regRLCS = this.regRLBA = this.regRLDA = this.tmpRLDA = this.regRLMP = this.regRLBE = 0;
+
+        this['exports'] = {
+            'selectDrive': this.selectDrive
+        };
     }
 
     /**
@@ -756,15 +760,20 @@ class RL11 extends Component {
     /**
      * displayDisk(iDrive, fUpdateDrive)
      *
+     * This ensures that the selected disk matches the drive's sDiskPath property, and if fUpdateDrive is set,
+     * it also ensures that the selected drive matches the specified drive number.
+     *
      * @this {RL11}
-     * @param {number} iDrive (unvalidated)
+     * @param {number} iDrive
      * @param {boolean} [fUpdateDrive] is true to update the drive list to match the specified drive (eg, the auto-mount case)
+     * @return {boolean} true if successful, false if not
      */
     displayDisk(iDrive, fUpdateDrive)
     {
         /*
          * First things first: validate iDrive.
          */
+        var fSuccess = false;
         if (iDrive >= 0 && iDrive < this.aDrives.length) {
             var drive = this.aDrives[iDrive];
             var controlDisks = this.bindings["listDisks"];
@@ -774,9 +783,24 @@ class RL11 extends Component {
              */
             if (controlDisks && controlDrives && controlDisks.options && controlDrives.options) {
                 /*
-                 * Next, make sure the drive whose disk we're updating is the currently selected drive.
+                 * Next, update the drive if the caller has requested it.
                  */
                 var i;
+                if (fUpdateDrive) {
+                    this.assert(iDrive == drive.iDrive);
+                    for (i = 0; i < controlDrives.options.length; i++) {
+                        if (Str.parseInt(controlDrives.options[i].value, 10) == drive.iDrive) {
+                            if (controlDrives.selectedIndex != i) {
+                                controlDrives.selectedIndex = i;
+                            }
+                            fSuccess = true;
+                            break;
+                        }
+                    }
+                }
+                /*
+                 * Next, make sure the drive whose disk we're updating is the currently selected drive.
+                 */
                 var iDriveSelected = Str.parseInt(controlDrives.value, 10);
                 var sTargetPath = (drive.fLocal? RL11.SOURCE.LOCAL : drive.sDiskPath);
                 if (!isNaN(iDriveSelected) && iDriveSelected == iDrive) {
@@ -785,23 +809,39 @@ class RL11 extends Component {
                             if (controlDisks.selectedIndex != i) {
                                 controlDisks.selectedIndex = i;
                             }
+                            fSuccess = true;
                             break;
                         }
                     }
                     if (i == controlDisks.options.length) controlDisks.selectedIndex = 0;
                 }
-                if (fUpdateDrive) {
-                    for (i = 0; i < controlDrives.options.length; i++) {
-                        if (Str.parseInt(controlDrives.options[i].value, 10) == drive.iDrive) {
-                            if (controlDrives.selectedIndex != i) {
-                                controlDrives.selectedIndex = i;
-                            }
-                            break;
-                        }
-                    }
+            }
+        }
+        return fSuccess;
+    }
+
+    /**
+     * selectDrive(sDrive)
+     *
+     * Used to programmatically select a drive by name.
+     *
+     * @this {RL11}
+     * @param {number} sDrive
+     * @return {boolean} true if successful, false if not
+     */
+    selectDrive(sDrive)
+    {
+        var controlDrives = this.bindings["listDrives"];
+        if (controlDrives && controlDrives.options) {
+            var nDrives = controlDrives.options.length;
+            for (var i = 0; i < nDrives; i++) {
+                if (controlDrives.options[i].innerHTML == sDrive) {
+                    var iDrive = Str.parseInt(controlDrives.options[i].value, 10);
+                    return this.displayDisk(iDrive, true);
                 }
             }
         }
+        return false;
     }
 
     /**
diff --git a/modules/shared/lib/sticky.js b/modules/shared/lib/sticky.js
index 38b2ad9b5..d20ff0c86 100644
--- a/modules/shared/lib/sticky.js
+++ b/modules/shared/lib/sticky.js
@@ -117,7 +117,9 @@ function commandMachine(idMachine, sType, sCommand, sValue)
                 var fnCommand = exports[sCommand];
                 if (fnCommand) {
                     //noinspection JSUnresolvedFunction
-                    fnCommand.call(component, sValue);
+                    if (!fnCommand.call(component, sValue)) {
+                        window.alert("Error calling " + idMachine + "." + sType + "." + sCommand + "(" + sValue + ")");
+                    }
                     return;
                 }
             }