Assorted improvements to new opcode decoders

modules/pdp11/lib/defines.js

@@ -225,6 +225,14 @@ var PDP11 = {
         READ:       0x2,
         WRITE:      0x4,
         UPDATE:     0x6
+    },
+    /*
+     * Internal flags passed to writeByteByMode(), etc.
+     */
+    WRITE: {
+        NORMAL:     0x0,        // write byte or word normally
+        ZERO:       0x1,        // zero byte or word
+        SIGNEXT:    0x2         // sign-extend a byte to a word
     }
 };