How do emulator/virtual computer work?

How does an emulator work like the android one. And a virtual pc live 开发者_运维知识库VirtualBox?

In one form the software reads the binary in the same way that hardware on the real system would, it fetches instructions decodes them and executes them using variables in a program instead of registers in hardware. Memory and other I/O is similarly emulated/simulated. To be interesting beyond just an instruction set simulator it needs to also simulate hardware, so it may have software that pretends to behave for example like a VGA video card the software run on the emulator ideally cannot tell the Memory/I/O is from simulated hardware, ideally you do enough to fool the software being simulated. Also though you try to honor what those register writes and reads mean by making calls to the operating system you are running on and/or hardware directly (assuming your program of course thinks it is talking to hardware and not an emulator).

The next level up would be a virtual machine. For the case I am describing it is a matching instruction set, so you are say wanting to virtualize an x86 program on an x86 host machine. The long and short of it is the host processor/machine has hardware features that allow you to run the actual instructions of the program being virtualized. so long as the instructions are simple register based or stack or other local memory, once the program ventures out of its memory space the virtualization hardware will interrupt the operating system, the virtual machine software like vmware or virtualbox then examines the Memory or I/O request from the software being virtualized and then determines if that was a video card request or usb device or nic or whatever, and then it emulates the device in question much in the same way that a pure non-virtualized setup would. A virtual machine can often outrun a purely emulated machine because it allows a percentage of the software to run at the full speed of the processor. the downside is you have to have a virtual machine that matches the software being run. An emulator can be far more accurate and portable than a virtual machine at the cost of performance.

The next level up would be something like wine or cygwin where not only are you trying to do something like a virtual machine and run native instructions and trap memory requests but you are going beyond that and trying to trap operating system calls so that you can run a program compiled for one operating system on another operating system, but much faster than a virtual machine. Instead of traping the hardware level register or memory access to a video card, you trap the operating system call for a bitblt or fill or line draw or string draw with a specific font, etc. Then you can translate that operating system request with calls to the native operating system.

At their simplest emulators or virtual computers provide an abstraction layer built on top of the host system (the actual physical system running the emulator) that implements the emulated system's functionality to the code that is to be run.

Emulators and virtual machines simulate hardware like a PC or an android phone. A virtual machine (or virtual pc) looks at an operating system's machine code instructions and runs them on top of your current (host) operating system in a virtual computer.

http://en.wikipedia.org/wiki/Virtual_machine

and

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

Depending on the type or virtualization, a virtual machine is not always an emulator.