Writing emulators seems like really hard thing to do.
What is in it for you ? Is it about technical challenge, or something else people from outside might not know?
Around 2004, I learned most of my code didn't run on real hardware, but ran on emulators. I found out why (the main reason: writing to video memory while the screen is drawing), and submitted patches to the SNES emulators of the time to work like real SNES hardware, but they were rejected because too many fan translations already relied on the old mistakes.
No one back then seemed to care about how accurate emulators were, so I set about writing my own SNES emulator with the goal of making it as perfect as possible, and it kind of spiraled out of control from there. Within a few years we were decapping chips to extract coprocessor firmware, I had to emulate the Game Boy for the Super Game Boy cartridge, the GBA CPU for this one Japanese chess game that had an ARM CPU in it, etc.
I guess I just really like the challenge, and never stopped adding more systems over time. The more systems I emulate, the more I already have most or all of the chips used in the system emulated. Eg ColecoVision took me an hour because I already had a Z80 core, a TMS9918 video core, and an SN76489 audio core. I can definitely see how MAME turned into what it is today now.
These days, the emulation is the easy part, and organizing all of this code (around 600,000 lines and counting), and getting all of these really unique pieces of hardware to all work in the same GUI, has become the challenging part. I have this really elaborate tree-view of the emulated systems to support zany things like the Genesis "Tower of Power", a complex scheduler that tracks timeslices in attoseconds, practically a full professional DSP stack for mixing multiple audio streams, Reed-Solomon decoding for CD systems, etc. I'm always learning new stuff, and there's always more to improve. I worry that I won't be able to wrap up higan in my lifetime.
There's not a lot of money in emulation, at most I've been offered $2500 for commercial licenses, but showing my hobby work to my employers landed me my last two jobs in software engineering, the latter of which is in Tokyo, Japan. Emulators literally got me halfway around the world. And I even got to work with the developers on Stephen Hawking's voice machine software at one point.
There's been some downsides, and I had a lot of maturing to do over the years, but on the whole, I wouldn't trade this hobby for anything.
I was curious about it, thus I wanted to read more about it and evidently, the source of Wikipedia is your website, but it seems like you took it offline and Web Archive doesn't have it either.
Are you planning to republish that article?
https://web.archive.org/web/20120307174933/http://byuu.org/a...
The first SNES co-processor chip, the DSP-1, was based on a weird NEC DSP architecture, and if I recall correctly the part-number was figured out from markings on the chip die, and digging through NEC spec-sheets looking for things that approximately matched the chip's known capabilities. The instruction set and encoding was puzzled out by hand.
Luckily, DSP-2, DSP-3, and DSP-4 all used the same NEC DSP core, just with different firmware, so the same emulator could be used for all of them.
The ST010 and ST011 used a slightly different NEC DSP core, so they required a little more work, but after handling the DSP-1 they weren't too difficult.
The ST018 was incredibly daunting to begin with, since its firmware was much larger than all the other co-processors, and there were no identifying marks on the CPU die and no product sheets to dig up. As a last ditch effort, somebody just opened up the firmware in a hex editor and tried to figure out the instruction encoding from first principles... and eventually they said "that looks familiar", and sure enough it turned out to be the most popular CPU architecture on the planet.
Specifically, I think the giveaway was the 4-bit condition codes at the beginning of ARM instructions. The code for always executing an instruction is 0b1110, and seeing almost all 32-bit words start with the same non-zero nibble is rather distinctive.
While creating a complete and shareable emulator is a big undertaking, making a simple one for fun is surprisingly not too bad. You start with the CPU emulation, and there are tons of comprehensive tests, so you just fix those until you’re done.
You should expect to do a lot of reading though, and you’ll become very familiar with how the system you’re emulating works :-)