may not run concurrently, and are generally handled by an intermediary level of software (often called middleware); a common type being Message-oriented middleware (MOM). The buffer required in asynchronous communication can cause problems when it is full. A decision has to be made whether to block the sender or whether to discard future messages. A blocked sender may lead to deadlock. If messages are dropped, communication is no longer reliable. Hybrids Synchronous communication can be built on top of asynchronous communication by using a Synchronizer. For example, the α-Synchronizer works by ensuring that the sender always waits for an acknowledgement message from the receiver. The sender only sends the next message after the acknowledgement has been received. On the other hand, asynchronous communication can also be built on top of synchronous communication. For example, modern microkernels generally only provide a synchronous messaging primitive and asynchronous messaging can be implemented on top by using helper threads. Distributed objects Message-passing systems use either distributed or local objects. With distributed objects the sender and receiver may be on different computers, running different operating systems, using different programming languages, etc. In this case the bus layer takes care of details about converting data from one system to another, sending and receiving data across the network, etc. The Remote Procedure Call (RPC) protocol in Unix was an early example of this. Note that with this type of message passing it is not a requirement that sender nor receiver use object-oriented programming. Procedural language systems can be wrapped and treated as large grained objects capable of sending and receiving messages. Examples of systems that support distributed objects are: Emerald, ONC RPC, CORBA, Java RMI, DCOM, SOAP, .NET Remoting, CTOS, QNX Neutrino RTOS, OpenBinder and D-Bus. Distributed object systems have been called "shared nothing" systems because the message passing abstraction hides underlying state changes that may be used in the implementation of sending messages. Distributed, or asynchronous, message-passing has additional overhead compared to calling a procedure. In message-passing, arguments must be copied to the new message. Some arguments can contain megabytes of data, all of which must be copied and transmitted to the receiving object. Traditional procedure calls differ from message-passing in terms of memory usage, transfer time and locality. Arguments are passed to the receiver typically by general purpose registers requiring no additional storage nor transfer time, or in a parameter list containing the arguments' addresses