The multiplexer/ demultiplexer (MDM) emulator is the first virtual machine that can emulate an avionic computer. New flight software can be easily “dropped in,” increasing operational flexibility. The MDM makes it possible to perform integration more quickly, reducing the need for additional hardware. The MDM emulator will be used extensively in the Space Station Training Facility (SSTF), where teams of astronauts and ground controllers will be trained in operation and utilization of the station — the first use of virtual-machine techniques for training of this type. In addition to being a major advance in virtual machines, the MDM emulator is economical: Although the cost of its hardware is estimated at $4 M (an estimate, as of year 2001, that includes the cost of development and testing), it has been estimated that the MDM emulator will save $12 M in labor costs.

The MDM emulator includes a ‘486 portable-computer-compatible virtual memory emulator board as its processor. This board can transfer data at rate large enough and with a latency small enough for running a simulator in real time. The simulator, in turn, makes it possible to load MDMs, personal computer systems, and robotic workstations in orbit. The MDM simulator is very flexible in that computers of all other types can also be functionally simulated.

The MDM emulator software is equally flexible. It consists of a boot subsystem, a kernel subsystem, and a device- simulation subsystem. The boot subsystem includes a self-test component and has sufficient “intelligence” to begin communicating with a host computer so that the rest of the software can be loaded. The kernel contains an executive component, a message component, and virtual-machine-setup and protection trap-handling routines. Device- simulating capabilities include the capability to (1) model missing hardware, (2) handle interfaces to the host computer and to devices outside the ‘486 card, and (3) use the message component to communicate with host-computer models of firmware controllers, sensors, and actuators.