fix most interpreter tests

lib/risc/interpreter.rb

@@ -267,7 +267,7 @@ module Risc
     end
 
     def handle_putstring
-      str = get_register( :"syscall_1" ) # should test length, ie r2
+      str = get_register( :r0 ) # should test length, ie r2
       case str
       when Symbol
         @stdout << str.to_s

lib/risc/platform.rb

@@ -40,6 +40,8 @@ module Risc
         :r0
       when :syscall_2
         :r1
+      when :saved_message
+        :r15
       else
         nil
       end