Fix for #14 https://github.com/outofmbufs/python-pdp11-emulator/issues/14

kl11.py

@@ -123,7 +123,16 @@ class KL11:
                         self.ub.intmgr.simple_irq(pri=4, vector=0o64)
 
             # transmit buffer
-            case 6 if value is not None:              # tbuf
+            case 6:
+                if value is None:
+                    # *** READING ***
+                    # manual says this is load-only; however automatic
+                    # byte write support (byteme/mmio) requires this
+                    # be readable. Probably byteme should be fixed instead
+                    # to catch traps from unreadables and synthesize
+                    # a zero there (?)
+                    value = 0
+                else:
                     # *** WRITING ***
                     value &= 0o177
                     if (value != 0o177):