Clean up the CPU/ChipSet interface, and get the PC8080 Video component ready to actually do something

modules/pc8080/lib/cpusim.js

@@ -450,6 +450,18 @@ CPUSim.prototype.getPC = function()
     return this.regPC;
 };
 
+/**
+ * offPC()
+ *
+ * @this {CPUSim}
+ * @param {number} off
+ * @return {number}
+ */
+CPUSim.prototype.offPC = function(off)
+{
+    return (this.regPC + off) & 0xffff;
+};
+
 /**
  * setPC(off)
  *