diff --git a/boot.py b/boot.py
index 67b33db..0455949 100644
--- a/boot.py
+++ b/boot.py
@@ -292,10 +292,19 @@ if __name__ == "__main__":
 
     pdpoptions = {'drivenames': args.drives}
     runoptions = {}
+    bkpts = []
     if args.debug:
         pdpoptions['loglevel'] = 'DEBUG'
+        bkpts.append(breakpoints.MemChecker(1000000))
     if args.instlog:
-        runoptions['breakpoint'] = breakpoints.Logger()
+        bkpts.append(breakpoints.Logger())
+
+    if bkpts:
+        # if there is just one, use it directly, else combine 'em
+        if len(bkpts) == 1:
+            runoptions['breakpoint'] = bkpts[0]
+        else:
+            runoptions['breakpoint'] = breakpoints.MultiBreakpoint(*bkpts)
 
     p = make_unix_machine(**pdpoptions)
 
diff --git a/breakpoints.py b/breakpoints.py
index beb847f..eb1a8ae 100644
--- a/breakpoints.py
+++ b/breakpoints.py
@@ -154,3 +154,33 @@ class Logger(Breakpoint):
         inst_s = "None" if inst is None else oct(inst)
         self.__logger.debug(f"{pc_s}/{m_s} :: {inst_s}")
         return self.__bp(pdp, xinfo)
+
+# Useful for ensuring the emulation implementation isn't broken in
+# some way - making sure all values in physical memory are 16 bits (only).
+# This will run an expensive check every nth instructions.
+# Obviously, if bogus values are found it will be nearly impossible to
+# figure out HOW they got in there; put some checks back into readRW/etc
+# functions as necessary to debug that (they are removed for performance).
+
+
+class MemChecker(Breakpoint):
+    def __init__(self, nth):
+        self.check_every = nth
+        self.togo = nth
+
+    def __call__(self, pdp, xinfo):
+        self.togo -= 1
+        if self.togo == 0:
+            self.check16(pdp)
+            self.togo = self.check_every
+        return False
+
+    def check16(self, pdp):
+        """Verifies no illegal values ended up in physical memory or regs."""
+        pdp.logger.debug("Check16 checking physical memory")
+        for a, w in enumerate(pdp.physmem):
+            if w < 0 or w > 65535:
+                raise ValueError(f"Illegal physmem value {w} @ {oct(a<<2)}")
+        for r, w in enumerate(pdp.r):
+            if w < 0 or w > 65535:
+                raise ValueError(f"Illegal reg value {w} @ {r}")