adding register_names to instruction protocol

lib/risc/instructions/function_call.rb

@@ -10,6 +10,11 @@ module Risc
     end
     attr_reader :method
 
+    # return an array of names of registers that is used by the instruction
+    def register_names
+      []
+    end
+
     def to_s
       class_source method.name
     end