Simulator or Emulator? What is the difference?

Question

While I understand what simulation and emulation mean in general, I almost always get confused about them. Assume that I create a piece of software that mimics existing hardware/software, what should I call it? A simulator or an emulator?

Could anyone explain the difference in terms of programming?

Bonus: What is the difference in English between these two terms? (Sorry, I am not a native speaker :))

Answer 1

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.

MAME is an arcade game emulator; Hyperterm is a (not very good) terminal emulator. There's no need to model the arcade machine or a terminal in detail to get the desired emulated behavior.

Flight Simulator is a simulator; SPICE is an electronics simulator. They model as much as possible every detail of the target to represent what the target does in reality.

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. In particular, a simulation may run far slower than real time. SPICE, for example, cannot substitue for an actual electronics circuit (even if assuming there was some kind of magical device that perfectly interfaces electrical circuits to a SPICE simulation.) A simulation Simulation does not always lead to emulation --

Answer 2

Simulator: it is similar to interpreter. i.e. it actually executes the real code in line by line to mimic the behaviour

Emulator: it is similar executable. i.e. it takes compiled code and executes it.

Answer 3

A Virtual PC tries to emulate a Computer, from the point of view of a Programmer BUT, at the same time, it simulates a Computer from the point of view of a Electrical Engineer.

Answer 4

I don't think emulator and simulator can be compared. Both mimic something, but are not part of the same scope of reasonning, they are not used in the same context.

In short: an emulator is designed to copy some features of the orginial and can even replace it in the real environment. A simulator is not desgined to copy the features of the original, but only to appear similar to the original to human beings. Without the features of the orginal, the simulator cannot replace it in the real environment.

An emulator is a device that mimics something close enough so that it can be substituted to the real thing. E.g you want a circuit to work like a ROM (read only memory) circuit, but also wants to adjust the content until it is what you want. You'll use a ROM emulator, a black box (likely to be CPU-based) with a physical and electrical interfaces compatible with the ROM you want to emulate. The emulator will be plugged into the device in place of the real ROM. The motherboard will not see any difference when working, but you will be able to change the emulated-ROM content easily. Said otherwise the emulator will act exactly as the actual thing in its motherboard context (maybe a little bit slower due to actual internal model) but there will be additional functions (like re-writing) visible only to the designer, out of the motherboard context. So emulator definition would be: something that mimic the original, has all of its functional features, can actually replace it to some extend in the real world, and may have additional features not visible in the normal context.

A simulator is used in another thinking context, e.g a plane simulator, a car simulator, etc. The simulation will take care only of some aspect of the actual thing, usually those related to how a human being will perceive and control it. The simulator will not perform the functions of the real stuff, and cannot be sustituted to it. The plane simulator will not fly or carry someone, it's not its purpose at all. The simulator is not intended to work, but to appear to the pilot somehow like the actual thing for purposes other than its normal ones, e.g. to allow ground training (including in unusual situations like all-engine failure). So simulator definition would be: something that can appear to human, to some extend, like the original, but cannot replace it for actual use. In addition the pilot will know that the simulator is a simulator.

I don't think we'll see any ROM simulator, because ROM are not interacting with human beings, nor we'll see any plane emulator, because planes cannot have a replacement performing the same functions in the real world.

In my view the model inside an emulator or a simulator can be anything, and has not to be similar to the model of the original. A ROM emulator model will likely be software instead of hardware, MS Flight Simulator cannot be more software than it is.

This comparison of both terms will contradict the currently selected answer (from Toybuilder) which puts the difference on the internal model, while my suggestion is that the difference is whether the fake can or cannot be used to perform the actual function in the actual world (to some accepted extend, indeed).

Note that the plane simulator will have also to simulate the earth, the sun, the wind, etc, which are not part of the plane, so a plane simulator will have to mimic some aspects of the plane, as well as the environment of the plane because it is not used in this actual environment, but in a training room.

This is a big difference with the emulator which emulates only the orginal, and its purpose is to be used in the environment of the original with no need to emulate it. Back to the plane context... what could be a plane emulator? Maybe a train that will connect two airports -- actually two plane steps -- carrying passengers, with stewardesses onboard, with car interior looking like an actual plane cabin, and with captain saying "ladies and gentlemen our altitude is currenlty 10 kms and the temperature at our destination is 24°C". Its benefit is difficult to see, hum...

As a conclusion, the emulator is a real thing intended to work, the simulator is a fake intended to trick the user.

Answer 5

This question is probably best answered by taking a look at historical practice.

In the past, I've seen gaming console emulators on PC for the PlayStation & SEGA.

Simulators are commonplace when referring to software that tries to mimic real life actions, such as driving or flying. Gran Turismo and Microsoft Flight Simulator spring to mind as classic examples of simulators.

As for the linguistic difference, emulation usually refers to the action of copying someone's (or something's) praiseworthy characteristics or behaviors. Emulation is distinct from imitation, in which a person is copied for the purpose of mockery.

The linguistic meaning of the verb 'simulation' is essentially to pretend or mimic someone or something.

Answer 6

A simulation is a system that behaves similar to something else, but is implemented in an entirely different way. It provides the basic behavior of a system but may not necessarily abide by all of the rules of the system being simulated. It is there to give you an idea about how something works.

An emulation is a system that behaves exactly like something else, and abides by all of the rules of the system being emulated. It is effectively a complete replication of another system, right down to being binary compatible with the emulated system's inputs and outputs, but operating in a different environment to the environment of the original emulated system. The rules are fixed, and cannot be changed or the system fails.