Which Dec Emulation is the MOST useful and Versatile?

You and me both! I think part of the problem is that there is no generally-agreed-upon definition of the two terms. I like this one a lot, though: https://stackoverflow.com/questions/1584617/simulator-or-emulator-what-is-t… Emulation is the process of mimicking the outwardly observable behavior to match an existing target. The internal state of the emulation mechanism does not have to accurately reflect the internal state of the target which it is emulating. Simulation, on the other hand, involves modeling the underlying state of the target. The end result of a good simulation is that the simulation model will emulate the target which it is simulating. Ideally, you should be able to look into the simulation and observe properties that you would also see if you looked into the original target. In practice, there may some shortcuts to the simulation for performance reasons -- that is, some internal aspects of the simulation may actually be an emulation. ... EDIT: Other responses have pointed out that the goal of an emulation is to able to substitute for the object it is emulating. That's an important point. A simulation's focus is more on the modelling of the internal state of the target - and the simulation does not necessarily lead to emulation. ... SPICE, for example, cannot substitue for an actual electronics circuit There's also the question of what's being emulated. Ersatz-11, for example, does a good job of looking like a PDP-11 - for the software. However, it does not like a PDP-11 for the hardware (although John used to sell boards you could plug into a PC, which provided a QBUS, IIRC). So is it a simulator or an emulator? Good question. About the only _generally-agreed_ example of the terminology I can think of are 'in-circuit emulators', which _exactly_ match the behaviour of a given chip. Noel