Okay. In order to fully understand why some things work and some don't, you need to have an idea on how videogames work and how emulation works.
All games we typically play are run off of computers. The games themselves are simply sets of instructions for the computer chips to run. Because these arcade games are built specifically for one function, the people who developed them wrote the code specifically for those computer chips. They don't need to write it so that it can run on 8-million different machines like modern PC Games do. They know EXACTLY what system the games will run on so they write the code in that "language".
When you emulate something, you are running the code designed for one piece of hardware on a completely different piece of hardware. As a result, this code needs to be "translated" into something that our computers can use. The CPU in your computer is not able to run the code that is written for the CPUs and other chips on an arcade board. An emulator is custom written code that will translate the code from those arcade chips (ROMs) into instructions that your computer can use.
A good analogy is reading a book that is written in Japanese. If you understand Japanese you are able to read that book quite quickly and without any issues at al. If you don't read Japanese, you have to have that book translated into the language you know, and some languages are easier to translate into than others. Therefore, it will take you longer to read that book than someone who understands Japanese. In terms of emulation, our computers are the people who don't know Japanese, and the arcade games are the readers who do know Japanese. The game code itself (ROMs) is the book.
So why are we able to run some games really fast and others really slow? It all depends on how fast the arcade games' chips ran, and how many they had. If a game has a lot of processors, even if they run relatively slowly the emulation will be quite slow. If a game has a small number of chips, but one or more run really fast (See NFL Blitz) then it will result in a slow emulation. The emulator has to translate ALL of the code into something the computer runs so there is a lot of emulation overhead. This is why the games that you think should run pretty fast because they are only 100 MHz chips take quite a lot of CPU power. Remember, the emulator has to emulate every single input, output, video signal, audio signal, etc. etc. that existes on a game board.
As emulators progress, the accuracy of their emulation tends to increase. Initially, a game might run faster because instead of emulating a particular process, they use a hack which doesn't accurately function like the real board does, but generates the same response. While this is great for those who want to play, it's not great for those who actually want to document how the game works. In addition, the hacks might not always work properly and it will result in behavior that is unexpected or unwanted.
With Donkey Kong, for a long time the sound in that game was accomlished via a hack. Instead of doing the proper emulation, they just went and used recorded sound samples and had the emulator call those samples when a sound was needed. Since they didn't have to translate that sound generation, the game ran very fast. Donkey Kong, as well as a lot of other games, used special circuitry and electronics to develop the sounds. Due to hardware limitations and various other factors, the actual sound wasn't emulated until just recently. Now, they have figured out how to use code compatible with modern computers to recreate the sounds that were generated by the electronics and circuitry in those original boards. This meant that a lot more chips and processes had to be "translated" so the overall speed of the emulation dropped significantly while the actual accuracy of the emulation went up tremendously. A lot of people whined and cried about this because they wanted to just play the game, but the end result is that the way the game itself ran is now properly documented and people years from now would be able to recreate that game's hardware if needed.
In the end, you can always use an older version of MAME and sacrifice accuracy for speed, or you can upgrade the hardware you have and sacrifice speed for better accuracy. Whenever you build a cab, this is something that must be kept in mind.
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Topic: Why are some games choppy? Even very basic older games. (Read 2747 times)
