Fixed XXDP disk boot (haven't tried any tests yet, but it's still good progress)

modules/pdp11/lib/device.js

@@ -872,6 +872,11 @@ DevicePDP11.prototype.writeCTRL = function(data, addr)
  */
 DevicePDP11.prototype.readSIZE = function(addr)
 {
+    /*
+     * TODO: getMemorySize() returns an aggregate total, so if there are multiple discontiguous
+     * chunks of RAM, this could return the wrong result; another interface, getHighestAddress(),
+     * might be required.
+     */
     return addr == PDP11.UNIBUS.LSIZE? ((this.bus.getMemorySize(MemoryPDP11.TYPE.RAM) >> 6) - 1) : 0;
 };