In order to try to keep this perpetual debate from messing up every new thread in which someone uses the term "emulator", let's have a single endless thread.

If you were active in the field in the 1970's, you might have come across a hardware device called an ICE, or in-circuit emulator. It was an expensive box of digital electronics with an umbilical to a header, which would plug into the microprocessor's socket, replacing it, to allow you to diagnose your design.

These days we very rarely come across ICEs, and the term "emulator" is widely and unambiguously used to refer to software which simulates a microprocessor or a system for the purpose of running or debugging code. Sometimes the term "software emulator" will be used, probably only to forestall commentary that the emulator should properly be called a simulator. Very few people prefer the term "simulator" for this kind of software, and that term is not widely used and does not help communication.

It's relatively common for a hobbyist to decide to write a new emulator, for fun or as a learning experience or to emulate an uncommon processor or platform.

Please, just say "emulator" - no-one will misunderstand you, and only a very few people will be perturbed.

Cheers
Ed

(Sometimes we make use of software which simulates digital designs or analogue circuits, and we might call that software a logic simulator, an HDL simulator, or a circuit simulator. It's general purpose, reading a description at runtime of the circuit to be simulated. It's feasible to write your own, but it's rather uncommon to do so. No confusion arises from this terminology, and there's no debate.)

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I think I take the position that usage trumps definition, but my attempt at definition would be something like:

- The purpose of a simulator is to debug or investigate an implementation. It has an internal structure related to the internal structure of the implementation in question. In this way, it has an external behaviour rather like the implementation in question.

- The purpose of an emulator is to debug or investigate the behaviour of the system surrounding an implementation. The internal structure is whatever is appropriate, such that the external behaviour is rather like the implementation being investigated.

When we emulate a C64 or PET, we replace the real device with some software and we investigate the interactions with a human (or a filesystem)

When we simulate the 6502 with visual6502 or HSPICE, we assemble a data structure like the internal transistors and connections on the chip, and we investigate how they work together to produce the high-level behaviour of a CPU.

Cheers
Ed

My way of thinking: When I wrote my minicomputer emulator I emulated the CPU, but I simulate the operating system. My CPU emulator can execute all (user) programs. But those programs are constantly calling operating system services, and my CPU emulator does not execute the operating system. Instead I simulate the operating system so that it looks to the executing applications that there's actually an OS there. But my simulation only maps *nix files etc. via various tricks and configuration files so that the application is satisfied. When the application thinks it's manipulating directory entries, for example, nothing of that really happens.

There is another person writing an emulator for the same minicomputer though. His emulator is intended to be able to execute the actual operating system too. So no operating system simulation. Peripherals will be simulated though (to start with, at least).

In some ways this way of thinking is the exact opposite of some definitions I have read (also in this forum), where a 'simulator' is supposed to be more 'accurate' than an 'emulator'. In may way of thinking, however, for software I think of emulators as doing the right thing as much as possible, while simulators are just going through the motions to simulate (but not actually do) what it looks like it's doing from the outside. Smoke screens and handwaving.. as my OS simulator.

That doesn't mean that my definition is 'correct', but at least enough other people are thinking the same way to make us able to communicate!

(I imagine this way of thinking about 'simulation' came out of e.g. when someone drafted to military service tried to simulate being insane, ill or blind, but it doesn't hold up to scrutiny)

-Tor

You're right, there is that other meaning of "simulate" and it does lead to an opposite pair of definitions!

Cheers
Ed

According to the English Oxford Dictionary:

simulate: Produce a computer model of
emulate: Reproduce the function or action of (a different computer, software system, etc.)
simultor: A program enabling a computer to execute programs written for a different operating system.

Sadly no emulator in the OED.

Personally I think the difference is a bit fuzzy.

IMHO if you are trying to reproduce function or action against physical hardware then you must have an emulator but if the scope of the reproduction is limited (e.g. just the CPU) or against virtual hardware then you could call it either a simulator or an emulator.

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

I learned it as: you emulate hardware by using other hardware. You simulate hardware by using software running on other hardware. For what it's worth, commercial airline pilots train on simulators, not emulators.

The general basis for this distinction seems to lie in that emulation promotes an exact (ideally one-for-one) behavior, where as simulation is usually a "close enough" activity. To me, at least, the distinction has always been clear.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

A simulator is a tool for logic or program development and allows you to verify a logic design or program, before it is actually put in use in its real environment. So it is never part of the final product. Typical candidates are the simulations inside IDEs or a logic simulator like SPICE. Even a reverse engineering model like visual6502 is a simulator as it only serves the purpose of understanding or learning the concepts applied in the actual CPU. In this case the final product (CPU) already exists.

An emulator is a replacement for the real thing. As such it is or is part of the final product. An emulator can serve the purpose of a simulator if it provides the verification needed for another product.

_________________
6502 sources on GitHub: https://github.com/Klaus2m5

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Time to get hip with the new lingo people.

http://en.wikipedia.org/wiki/Emulator#Emulation_versus_simulation

Emulation is the new simulation (and has been for over 30 years). Time to stop living in the past.

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Oh my god, it is Wikipedia so it must be the truth!

Further down at Functional_simulators
Oh come on! Everything eating binaries is an emulator? That is a bit too simplified for my taste.

_________________
6502 sources on GitHub: https://github.com/Klaus2m5

Language evolves. Words like 'emulate' are adopted into new uses. As Wiki points out in the section I linked:

The last time I worked on a simulator it was modelling the movement of oil and gas in a reservoir. I'd class that as pretty abstract.

Other examples of simulators on Google include ones for flight, weather, goats (http://www.goat-simulator.com/), farming, cameras, railways and whole plethora of things that aren't computers. On the other hand the list of things emulated contains arcade machine, games consoles, mobile phones, terminals. All pretty much programs that pretend to be other computers.

The simple fact is, like it or not, that the word has moved on. It now has a broader interpretation than it once did.

_________________
Andrew Jacobs
6502 & PIC Stuff - http://www.obelisk.me.uk/
Cross-Platform 6502/65C02/65816 Macro Assembler - http://www.obelisk.me.uk/dev65/
Open Source Projects - https://github.com/andrew-jacobs

Quite so!

I agree with you for the above cases. However, there is none of the disputable uses there. How about visual6502?. How about a debugging tool using binaries? I think those are simulators because there is no use for them, if you are not interested in specific aspects of the CPU (program debugging, understanding CPU architecture). They cannot simply run a program for the end user like emulators can.

Emulators need to be able to run at the speed and provide the same interface as the replaced machine. Everything else is a simulator. It is not an emulator just because it can run binaries!

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6502 sources on GitHub: https://github.com/Klaus2m5