When people in the gaming community speak of emulation, they are usually referring to the emulation of a hardware console or a computer system. This software allows them to play games on hardware systems which were never engineered to allow it. Imagine, playing Super Mario Kart with your friends through the Internet on your Mac OS X, Xandros, or Microsoft Windows computer. Impossible? Google the ZSNES or SNES9X project for more details.

First of all, there seems to be some confusion over the use of the terms “emulation” and “simulation” on the Internet. It seems many people (myself included) are referring to programs, such as DOSBox or ZSNES, as emulators. Usually, I abhor ignorance (unless it applies to me somehow), but thankfully it’s not simply a misused term stemming from the ignorant, adolescent population on the Internet.

Part of the confusion may come from people in the hardware industry referring to simulators as software that precisely mimics hardware, and emulators as pieces of hardware that are precisely controlled by software. Reread that last sentence since there is a distinct difference in their meaning. For people in the software industry, the terms are a little different. When a programmer attempts to duplicate hardware components through software, it’s often referred to as emulation. Simulation, on the other hand, can be used to mimic the actions of a piece of hardware or software, but the internal workings may be completely different than the original product. It may look and act like an Atari computer, but it’s not really using any of the core components of that machine. Within the realm of software, simulators can also apply to programs which simulate completely different systems such as organic or chemical interactions, economic or disaster forecasts, or the weather.

I’m going to digress for a moment and talk about a simulation used to provoke this global warming hysteria we all are experiencing today. Simulations can be as simple as simulating the periodic movement of a pendulum, or as complex as a weather system. Simulations can also involve complex interactions between other systems, and for these intricate systems to be effective at predicting or postulating outcomes, they must rely entirely on our flawless understanding of how all of these systems work. It’s funny (and sad) to read about simulators which can accurately predict the effect of human pollution on the weather system. These are probably two of the most complex environmental systems attempting to be simulated today. It’s no surprise that even the most sophisticated simulation models used today don’t work, but various environmental groups and political leaders are running with the “evidence” as if it were iron clad proof. The only way they got the simulation to output the desired result was to modify the data going into the system. Now, I’m sure we’re not all scientists here but it should be fairly obvious that manipulating data to validate your own hypothesis isn’t exactly following the scientific method. I don’t want to get off on a tangent here, it’s a good example of simulators being used today and the unexpected complexities that can arise during the development of a simulator.

Most of the time, programmer’s would not attempt to simulate an entire machine, but rather couple the simulation with emulated components. The reason is simple: it’s a lot more work to simulate an entire machine than it is to emulate one. In a simulation, none of the original software for the hardware platform can be used, which means it too must be simulated. Looking at the other side of the coin, we see that pure hardware emulation is also not very practical to implement through software. How does one emulate a hardware bus, for example? The bus is purely an electrical component and has little to do with the purpose of the overall system. Yes, hardware engineers need to implement bus systems, but programmers attempting to emulate the machine can forget about it entirely. Simulation is often used in place of arcane hardware constructs which are part of the electrical system or those pieces which simply cannot be emulated. It acts like a bridge between discrete or unrelated pieces within an emulator so that the emulator can function as a cooperative unit.

When writing an emulator for a video game console, it’s vital to get as much information about the hardware as possible. Doing so will hopefully give you the big picture, and make it easier for you to divide your work into component groups. In a simple system containing only one central processing unit, for example, you will need to emulate the operation of the chip. This includes instruction sets, registers, any available memory caches, and operands, which constitute the general make up for most microprocessors. Advanced microprocessors necessitate advanced emulation, but not all components of a microprocessor need to be emulated in order to maintain accuracy. For example, optimization features used by the chip can essentially be omitted for a perfectly functioning system, unless those pieces modify the data in a peculiar way or they contain a bug which affects the software’s presentation or operation on the original hardware.

Another component group needing particular attention would be the memory subsystem used by the console. The memory architecture used could be as simple as a linear array of addresses or as non-intuitive as paged memory or memory-mapped input/output. The solution chosen for emulating memory should probably favor reading memory over writing memory, since the former occurs a lot more frequently. Some systems, such as the Nintendo Entertainment System, allowed the clever engineers to essentially extend the available memory through the cartridge. They used bank switching within the cartridge to overcome the addressing issue normally encountered when a system attempts to extend the range of available memory than what was originally available.

I could go on and on as there are many other aspects to consider before writing an emulator, such as debugging or functions related to emulation as an application but they are outside the scope of this little write-up.

Categories: Software
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