A simulator is a computer program that runs on a computer. If it runs the user code without being present in the user's target hardware, it's probably a simulator. I say "probably" only because some hardware emulators will run code without being plugged into the target hardware.

A simulator makes no pretense of excuting in real time. It makes no pretense of reproducing the hardware interactions of the simulated object. It makes limited claim to produce simulated output. A really useful simulator does everything that a hardware emulator does, with the exception of interacting with real hardware. It doesn't matter how slowly of rapidly it runs, aside from allowing the user to go out for lunch while it runs, because all its operations are recorded and can be examined later, it allows the introduction of simulated stimuli from a schedule, and it records the simulator's response to those simulated stimuli.

Some simulators are well-developed and offer features similar to those provided by a monitor running the user code. Others simply offer a raw execution engine interpreting the code and executing it in order to present the effects of each instruction. Usefulness varies a great deal. In most cases, the simulator is capable of showing the logical result of execution of a set of code within a prespecified environment. In the simplest cases, gross assumptions are made about the environment and those can not easily be altered. In most cases, memory contents can be modified, but other hardware simulation is entirely unsupported. In others, the simulator is capable of interpreting an event schedule and processing inputs from that event schedule, producing approriate responses to the stimuli presented. In rare cases, precise timing is calculated in order to provide information about timing in response to scheduled stimuli.

A simulator is software. An emulator is harddware. Emulators interface with the target hardware. They seldom do everything the vendors claim, and, to my experience, NEVER do everything you want. Once you've got 'em, you're stuck.

RE