guard against SET PC, [SP++], and SET PC, [--SP]

dcpu_16.py

@@ -334,6 +334,7 @@ class Memory(object):
 class CPU(object):
     """DCPU-16"""
     def __init__(c, memory=Memory(), debug=False):
+        """If you don't specify memory, CPUs will share the same, default memory object"""
         c.m = memory
         c.clear()
         c.reset()
@@ -414,8 +415,10 @@ class CPU(object):
     def run(c):
         """step until PC is constant"""
         last_pc = 0xFFFF
-        while c.pc != last_pc:
+        last_sp = 0xFFFF
+        while c.pc != last_pc or c.sp != last_sp:
             last_pc = c.pc
+            last_sp = c.sp
             c.step()
         log("Infinite loop")