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How do emulators work and how are they written? (stackoverflow.com)
132 points by alcuadrado on Jan 19, 2011 | hide | past | favorite | 21 comments

Wow, on HN again. While I'm proud of that answer, the real bummer was that I edited it too many times (started with a tiny answer and filled it in from there) and it became Community Wiki. I got all of 4 or 5 upvotes before it went CW, so got 40-50 rep out of it instead of the 5290 I would've gotten otherwise. Glad people have found it useful, though.

From the end of the SO answer: "I'm glad this post has been helpful, and I'm hoping I can get off my arse and finish up my book on the subject by the end of the year/early next year."

Are you still working on the book?

I am, but it's been slow going. Between a new job and a social life, I have little free time and energy to work on it. At this point, it'd be tough to get it done this year.

Somewhat OT, but I never understood why editing your own answer on SO would cause it to go community wiki. If you're improving your own answer, shouldn't you get the rep for it?

I can understand how others' significant edits would cause it to go CW, though.

If it makes you feel any better, StackOverflow caps your rep gains to 300 per day, so assuming that most of those upvotes occurred in bursts when the question was first answered and then at subsequent times when it was linked to from elsewhere, you probably wouldn't have gotten quite that much.

> rep gains to 300 per day,

200 per day, from upvotes only.

http://www.romhacking.net/ is an amazing resource if you want to find documents on the inner workings of older game consoles.

The SO post already links to bsnes, which does an excellent job of balancing readability and accuracy. It's still a little hard to grok the code, but leagues easier than tackling something like SNES9x.

I would also recommend glancing through the vNES source code; it's much simpler than bsnes and is very easy to understand for the most part: http://www.thatsanderskid.com/programming/vnes/index.html

Invariably been linked before, but http://imrannazar.com/GameBoy-Emulation-in-JavaScript:-The-C... also addresses basics of emulator design.

I had the great opportunity in my undergrad college compiler class to write an entire ecosystem to understand better how some of this stuff works. We didn't just write a compiler, we wrote an emulator for a simple, notional architecture. The whole thing was simple enough that we could write the compiler toolkit, the system emulator/VM and run it in a few days.

Then for extra credit we could rewrite the whole thing (or part of it) in another language. And/or get a guaranteed A if we could make the whole shebang self-hosting.

It was a really great way of understanding the basics of a system architecture.

As for the emulator, it was pretty primitive, but basically it loaded the compiled program code into memory, then would read an operation's worth of bytes, look the operation up in some kind of lookup table (or similar) that mapped the emulated system's instructions to local system instructions say, x86, and execute that operation.

Sometimes the mapping resulted in several operations on the host side the emulated system might execute in one.

eg. ADD R1, R2, R3

might add the contents of registers r2 and r3, then put the result into r1. In x86 it would be more like

or some such.

(actually we did it a little higher level then that, but that's the idea).

So an emulator needs to provide all of the various registers and operations and such so that the code can execute, it likely has to have the ability emulate the memory architecture and translate I/O appropriately. Some older systems for example could only output to a printer terminal, or to a simple status display, on a modern system you have to translate calls to those output devices to equivalent ones on your system (like the screen).

More complex system may need to emulate several processors. The SNES and Amiga for example have a handful of chips that not only need to be emulated, but much of the software assumes a particular timing in the interaction of those components that can be very challenging to get right and keeping track of all that and running at reasonable performance can require fast hardware.

These days though, not many emulators use the simple execution mechanism I outlined above. Many of them have sophisticated code profiling and execution caches that can significantly speed up the emulation.

In these terms, emulating a system, or a VM (like Java) or some other runtime environment (like Javascript and the V8 runtime for example) isn't really all that different and there's huge crossover in the theoretical concepts between these areas.

_The Elements of Computing Systems_ (http://www1.idc.ac.il/tecs/) follows a similar approach, building an emulated computer up from NAND gates. Highly recommended!

Great book! I really love this approach to teaching this kind of subject. Seeing the whole stack, top to bottom, really helps tie together so many different threads that are generally left untied during a CS education.

There is a google tech talk on the book which is really good for those curious to find out more.

I find undocumented opcodes most interesting. They were literally holes in the 6502 circuitry. People used them on the C64 to get more speed out of their assembly code. Early C64 emulators couldn't cope with those games/demos. Luckily, they are documented in disk magazines, etc, so they've been implemented now.

Also, the 6502 has been reverse engineered from micrographs, so perfect transistor-level emulation can be done.


byuu is also doing this for the SNES with bsnes - he's even had all of the enhancement chips like the DSP-1~4 and CX4 decapped to achieve cycle-perfect emulation.


Well, the decapped chips have been photographed, but they haven't been made into working diagrams like that 6502 one.

The important thing about the decapped chips is that any on-board ROM has been dumped (electrically, not visually) so that they can be emulated with an opcode interpreter, just like other emulators.

Anyone know which of the listed ways of processor emulation that bsnes uses?

I'd imagine bsnes is somewhere between interpretation and dynamic recompilation.

Static recompilation would be really difficult on systems like the SNES and GBA, which have two processor modes (8bit 6502 emulation/16bit 65816, and 16bit THUMB/32bit ARM respectively), so the width of any given instruction is dependent on the processor state. Dynamic recompilation would be faster than instruction-by-instruction interpretation, but full dynamic recompilation (with subroutine caching and all) would probably get pretty difficult to time (I could be wrong), and cycle accurate timing is a stated goal for bsnes (to the point where byuu had the enhancement chips like the CX4 decapped).

bsnes has taken the low-level route because a decent bit of the software for the SNES is really timing sensitive - even that black circle effect at the end of the level in Super Mario World is done by changing values between scanlines. For systems like N64 on the other hand, where the software is less timing sensitive, high level emulation techniques are more common, for the sake of speed increases.

As best I understand it, bsnes is purely a bytecode interpreter. It does fancy tricks with stack-swizzling to quickly switch between different parts of the code to get the timing right, but it never ever generates instructions for the host CPU architecture.

Actually, I think it's more accurate to say that (at least for the 65816 core), that bsnes is a pipeline interpreter.

Each 65816 opcode takes between 2 and 8 cycles to execute (more for the MVN and MVP opcodes) and there are rules about when interrupts can be asserted -- bsnes correctly emulates these processor details.

Also, I believe the bsnes properly takes into account the read and write states of the memory bus. This is also an intra-opcode level of detail.

AFAIK, some of the other CPU cores (esp. the DSP-n and Supergameboy cores) are straightforward opcode interpreters.

Previous discussing of the same link: http://news.ycombinator.com/item?id=1350343

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