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 :))
Simulation = For analysis and study
Emulation = For usage as a substitute
A simulator is an environment which models but an emulator is one that replicates the usage as on the original device or system.
Simulator mimics the activity of something that it is simulating. It "appears"(a lot can go with this "appears", depending on the context) to be the same as the thing being simulated. For example the flight simulator "appears" to be a real flight to the user, although it does not transport you from one place to another.
Emulator, on the other hand, actually "does" what the thing being emulated does, and in doing so it too "appears to be doing the same thing". An emulator may use different set of protocols for mimicking the thing being emulated, but the result/outcome is always the same as the original object. For example, EMU8086 emulates the 8086 microprocessor on your computer, which obviously is not running on 8086 (= different protocols), but the output it gives is what a real 8086 would give.
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.
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.)
Both terms are something completely different and only intersect very little. To find the right term is actually very easy, just think about following:
A simulation does not do anything for real. You can study it, for example how computer work, but it usually has no outcome other than that. A plane crash in a Flight Simulator causes no real harm. A weather forecast simulation itself does not change the weather.
An emulation does something for real. You can work with an emulated computer like with a physical one and create documents with it. And a plane crash in a Flight Emulator would have an outcome, like people experiencing the real impact including possible physical harm.
Your confusion probably stems from the fact, that "studying the simulation" and "accessing the emulation" often is quite the same thing.
You are not alone with your confusion. The Film "Matrix" speaks of a simulation. However The Matrix is running an emulation, as it has real impact on all members of The Matrix. In contrast the training room has no real impact, so this is a simulation (of The Matrix).
Let's see some examples.
Take a water hose in the garden and let it rain. What's the difference between simulation and emulation here?
When you are simulating rain, people still will blame you for getting wet. Your rain has some real impact on the world, but your simulation hasn't, as the simulation does not fool anybody in that it is real rain.
In contrast, when you are emulating rain, people would blame the weather. This is, your emulated rain really behaves like rain in reality. This rain emulation hence distorts reality, in making people belie in the wrong culprit.
It took me quite some time to understand that. Hence it isn't easy nor obvious which explains all the confusion.
Keep in mind that a simulation can have sideeffects, like the weather forecast is based on simulations, which takes quite some computing power and thus electrical energy, which has an environmental impact.
Hence in the example of "simulated rain", people getting wet just is a sideffect and not part of the simulation. Same is true if you simulate a rainbow with this simulated rain. While the property of "how rainbows work" is part of this simulation, the simulation itself is not providing the rainbow, this just happens due to refraction of the sun on the sideffect of the waterdrops.
While you might think "a simulated computer can have an outcome" this is practically wrong reasoning. If you save files onto a simulated harddrive, these files cannot leave the simulated drive outside of the simulation. You can obtain the files by studying the simulated drive, but this is not part of the simulation itself.
In case the harddrive saves the data such, that the data is actually usable outside of the simulation, you have an emulated harddrive within the simulation to do so.
So an emulation can be part of a simulation and vice versa.
If you simulate a filesystem, you probably, for practicability, will choose to save the files onto your real filesystem as-is (perhaps with some additional meta-information). In that case the simulation seems to create real "value" outside of the simulation: Usable files!
But this is just by coincidence, because your simulated filesystem actually emulates a filesystem as well. You actually emulated the outside filesystem inside your simulation!
A good example of the difference is, when you think of security. A TPM is a specific device to keep it's own keys secure (source of identity) while an HSM is a general device to secure foreign keys (verify identity).
Fun Fact: My fingers constantly type TMP instead of TPM.
If you simulate a TPM this has a huge effect on security, because then you can observe the internal states of the TPM. Which renders all the security void. Even that such a simulation can give you valuable hints of improving the design of a TPM itself, you won't want to expose precious data to the simulated TPM for real.
However if you emulate a TPM you will try to hide these internal states to the outside as good as you can. Such an emulated TPM then can be possibly used to really secure something else better than without it.
With a real TPM you cannot emulate the properties of a real HSM. All you can archive is to simulate an HSM, but this will not have the security properties of a real HSM, so all data which is stored in this simulated HSM will not be protected (they will only be protected within the simulation itself).
In contrast, with a real HSM you can emulate a TPM with all properties of a real TPM. For this the HSM needs to be constructed such, that no information needs to leave the HSM which does not leave a TPM as well.
(Please note that I do not know anything about HSMs or TPMs in particular, so it might be that there are no HSMs out there which are able to provide emulated TPMs.)
If our world is simulated, we are simulations, too. Hence some spectator (let's call her God) can look at us and change the simulation any time. Also we cannot find out if we are simulated or not. As I am pretty sure that I know that I am, I do not think I am simulated, because self-awareness looks like an effect with a real component to me, which contradicts simulation. This also means, our world cannot be a simulation, too, as a simulation can only affect me like the world does, if I am part of the simulation.
But our world still can be emulated (like in the Film "Matrix"), as all I have to "prove the world" is my state of mind and sensory input, which I cannot verify, as I cannot leave myself. If I am not part of the emulation, then there should be a chance to observe discontinuity (like in the film "Matrix"), in case the emulation does not work flawlessly.
This changes when I emulated, too, like running an OS in an emulator. Then I cannot observe such errors, as my state can be reset from within the emulation (call it: Sleep) without observable discontinuation.
However I rather think that the world is a holographic hallucination than something like an emulation. Because if it is emulated, then I am pwned by somebody (call him Rick) who is running the emulation for some purpose, while a hallucination is purely my own thing.
I stop here, because hallucinations lead us to something completely different.
Simulator is something more broader than Emulator and it seems like the duality of this terms is overthought in the posts above.
Emulator
People decided to use a new word emulation in the "computer world" when they started replacing some hardware parts of the existing system in straightforward manner - imitating their behaviour and relying on the computational nature to be sure to not break something and leave everything in the equivalent state. So we have emulated the piece of this! (and the whole still works as before)
Emulator usually used in narrow sense in digital area as replacement and virtualization - presenting in digital form as a piece of software - of something known and existed before (virtual chips, circuit boards, electronic devices). So when the world became more digital and brought the emulator word to the masses, the masses added uncertainty to it (or additional reasons).
Simulator
First of all, I saw many comments about emulators do or replace something real but simulators not.
BUT flight simulator is used for a real thing - it trains pilots, gives them skill up and knowledge and it replaces expensive real planes and saves much of money. And we cannot just say a plane-emulator because we have inner feeling that this is much more than that, so we call it simulator :) Plane simulator could contain emulated radar or transponder that is true.
Contra-statements that simulators are used for analysis and study (and emulators for something real), but that analysis and study not less a real thing than emulated GSM boards (even more in the informational age we live in). Analysis adds a value to the business, cuts costs or points out to profits not less than the replaced (emulated) hardware.
Simulator is similar to modelling of something that we can't obtain for some reason (cost, technology, physical impossibility). It is usually simulated for something new or intangible or complex or not properly known to us like market, weather, combustion, user. So here comes the flight, black hole, stock exchange, simulations.
So finally:
As for words origin
All came from Latin and mean:
Emulation is like Abstruction. It shows what it can do. Example: Car driving emulation.
Simulation is like Encaptulation. It shows how it can do Example: Car engine inner activity.
