Added command to select RL11 drive

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' %}