将两个寄存器合二为一

Question

我试图在edx：eax中获取双字乘法的结果，并将它们组合成一个寄存器。我试过这个：

                 PATCH    /users(.:format)                            users/registrations#update
                 PUT      /users(.:format)                            users/registrations#update
                 DELETE   /users(.:format)                            users/registrations#destroy
          charge POST     /charge/:room_id(.:format)                  transactions#create
         surveys POST     /surveys(.:format)                          surveys#create
        archives POST     /archives(.:format)                         archives#create
  stop_recording POST     /stop_recording(.:format)                   archives#stop_recording
           rooms GET      /rooms(.:format)                            rooms#index
                 POST     /rooms(.:format)                            rooms#create
        new_room GET      /rooms/new(.:format)                        rooms#new
    room_receipt GET      /rooms/:guid/receipt(.:format)              rooms#receipt
            room GET      /rooms/:guid(.:format)                      rooms#show
     room_finish GET      /rooms/:guid/session_ended(.:format)    

    rooms#finish
  customer_heartbeat POST     /customers/:customer_id/heartbeat(.:format) customers#heartbeat
      customer_start POST     /customers/:customer_id/start(.:format)     customers#start
           customers POST     /customers(.:format)                        customers#create

最高有效位移位很好，但最后一行会覆盖整个寄存器。我认为mov r15d只能覆盖r15的下半部分。我做错了什么？