move message to stack_pointer

changing a bunch of register names
some of which get names now

test/support/risc_interpreter.rb

@@ -47,9 +47,11 @@ module Risc
       Risc.init_length
     end
 
+    # the return of syscall should be an integer
     def get_return
-      assert_equal Parfait::Message , @interpreter.get_register(:r15).class
-      @interpreter.get_register(:r0)
+      saved_in = @interpreter.std_reg(:saved_message)
+      assert_equal Parfait::Message , @interpreter.get_register(saved_in).class
+      @interpreter.get_register(@interpreter.std_reg(:syscall_1))
     end
 
     # do as many as given ticks in the main, ie main_at more
@@ -98,7 +100,7 @@ module Risc
     # get the return from the message (not exit code)
     # exit code must be int
     def get_message_return
-      @interpreter.get_register(:r15).return_value
+      @interpreter.get_register(@interpreter.std_reg(:saved_message)).return_value
     end
 
     # wrap the input so it is a main, compile and run it