QNX interprocess communication consists of sending a message from one process to another and waiting for a reply. This is a single operation, called codice_3. The message is copied, by the kernel, from the address space of the sending process to that of the receiving process. If the receiving process is waiting for the message, control of the CPU is transferred at the same time, without a pass through the CPU scheduler. Thus, sending a message to another process and waiting for a reply does not result in "losing one's turn" for the CPU. This tight integration between message passing and CPU scheduling is one of the key mechanisms that makes QNX message passing broadly usable. Most Unix and Linux interprocess communication mechanisms lack this tight integration, although a user space implementation of QNX-type messaging for Linux does exist. Mishandling of this subtle issue is a primary reason for the disappointing performance of some other microkernel systems such as early versions of Mach. The recipient process need not be on the same physical machine.