An option to let write-to-zero pass (for the @glk opcode).

This is not turned on by default.

exec.c

@@ -622,6 +622,10 @@ void execute_loop()
         value = inst[1].value;
         arglist = pop_arguments(value, 0);
         val0 = perform_glk(inst[0].value, value, arglist);
+#ifdef TOLERATE_SUPERGLUS_BUG
+        if (inst[2].desttype == 1 && inst[2].value == 0)
+            inst[2].desttype = 0;
+#endif /* TOLERATE_SUPERGLUS_BUG */
         store_operand(inst[2].desttype, inst[2].value, val0);
         profile_out(stackptr);
         break;

glulxe.h

@@ -44,6 +44,11 @@ typedef int16_t glsi16;
    game files from crashing the interpreter. */
 #define VERIFY_MEMORY_ACCESS (1)
 
+/* Uncomment this definition to permit an exception for memory-address
+   checking for @glk opcodes that try to write to memory address 0.
+   This was a bug in old Superglus-built game files. */
+/* #define TOLERATE_SUPERGLUS_BUG (1) */
+
 /* Uncomment this definition to turn on Glulx VM profiling. In this
    mode, all function calls are timed, and the timing information is
    written to a data file called "profile-raw".