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 hard

Answer 1

The distintion between the two terms is a bit fuzzy. Coming from a world where "Emulators" are pieces of hardware that allow you debug embedded systems. And remember products that allowed you to have ICE (In Circuit Emulation) capabilities to debug a PC platform, I find the use of the term "Emulation" to be a somewhat of a misnomer for software that SIMULATES the behaviour of a piece of hardware.

My justification for the current use of the term is Emulation is that it may "augment" the functionality, and only is concerned with a "reasonable" approximation of the behaviour of the system.

ICE: (In Circuit Emulation) A piece of hardware that is plugged into a board in place of the actual processor. It allows you to run the system as if the actual processor was present. Typically these have a variant of the processor on them to actually execute the software with glue logic to allow the user to break executation and single step under hardware control. Some would also provide logging capability. Most modern processors development systems have replace ICE type emulation with JTAG Emulation, where the JTAG just talks to the processor via a special purpose serial link and all execution is perform by the processor mounted on the board.

Software EMULATOR: An 0x86 emulator is only concerned with being able to execute 0x86 assembly language, not providing accurate cycle per cycle behaviourial model of a SPECIFIC 0x86 processor. Bochs is an example of this. QEMU does this, but also allows "virtualization" using special kernel modules.

SIMULATOR: Texas Instruments provides a CYCLE ACCURATE behaviourial model of there processors for software development that is intended to be a accurate SIMULATION of SPECIFIC processor cores behavior for the developers to use prior to having working hardware.

Software EMULATOR augmenting functionality: BLEEM not only allowed you to run Playstation Software, but also allowed the display to be output with higher resolution than the Playstation was able to provide, and also took advantage of more advanced capabilities of GPUs that were avaliable. (i.e. Better blending and smoothing of textures.)

Answer 2

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.

Answer 3

It's a difference in focus. Emulators¹ focus on recreating the behavior of a system, with no regard for how the system functions internally. Simulators² focus on modeling the components of a system. You use an emulator when you care mostly about what a system does, and a simulator when you care about how it does it.

As for their general English meanings, emulation is "the endeavor to equal or to excel another in qualities or actions", while simulation is "to model, replicate, duplicate the behavior, appearance or properties of". Not much difference. Emulation comes from æmulus, "striving, rivaling," and is related to "imitate" and "image," which suggests a surface-lever resemblance. "Simulation" comes from similis "like", as does the word "similar," which perhaps suggests a deeper congruence.

References:

1. Wikipedia: Emulator
2. Wikipedia: Computer Simulation
3. Wiktionary: emulation
4. Wiktionary: simulation
5. Etymology Online: emulation
6. Etymology Online: simulation

Answer 4

Both are models of an object that you have some means of controlling inputs to and observing outputs from.

The key difference is that:

• With an emulator, you want the output exactly match what the object you are emulating would produce.
• With a simulator, you want certain properties of your output to be similar to what the object would produce.

Let me give an example -- suppose you want to do some system testing to see how adding a new sensor (like a thermometer) to a system would affect the system. You know that the thermometer sends a message 8 time a second containing its measurement.

Simulation -- if you do not have the thermometer yet, but you want to test that this message rate will not overload you system, you can simulate the sensor by attaching a unit that sends a random number 8 times a second. You can run any test that does not rely on the actual value the sensor sends.

Emulation -- suppose you have a very expensive thermometer that measures to 0.001 C, and you want to see if you can get by with a cheaper thermometer that only measures to the nearest 0.5 C. You can emulate the cheaper thermometer using an expensive thermometer and then rounding the reading to the nearest 0.5 C and running tests that rely on the temperature values.

Note that simulations can also be used for forecasting or predicting behavior. Finite element analysis simulations are used in many applications, including weather prediction and virtual wind tunnels.

The definitions of the terms:

• emulation -- surpass or exactly match
• simulate -- imitate in appearance or character

Answer 5

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 6

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.