This might seem like a roundabout way to start, but I'd recommend Code by Charles Petzold[0].

It starts out with just wires, switches, and relays, and as the book progresses he goes through the process of building up a simple CPU and RAM one step at a time. The book even walks you through coming up with opcodes and assembly language.

Even if you already know this stuff, I found the book was helpful in developing an intuitive feel for how everything works and fits together.

After reading it, you'd probably have a good mental model of how you'd want to approach writing an emulator.

The Nand to Tetris courses and their accompanying textbook would probably be helpful here too[1][2].

[0] https://www.amazon.com/Code-Language-Computer-Hardware-Softw... [1] https://www.coursera.org/learn/build-a-computer [2] https://www.coursera.org/learn/nand2tetris2

You could try writing a homebrew game for an old system. That's a pretty good way to learn things.

Once you start on the emulator, the first thing you'll do is emulate the CPU, which can be done as a big switch statement inside of a loop. Knowing how to code each instruction is easy when you have an instruction listing handy.

Here's a quick example of how I would write an emulator, with 2 opcodes already implemented (6502 assembly): https://pastebin.com/raw/PhCEqh35

And the instruction listing I used: http://obelisk.me.uk/6502/reference.html#AND

You might want to start with a CHIP-8 emulator. There are a lot of guides on the web, and it isn't a giant time commitment.

I've been looking into writing an emulator for a while now. There are some excellent references for specific systems, but one of the best sources I've found is here:

http://www.codeslinger.co.uk/pages/basics.html

In addition to having source code for several different emulators, they've written a ~150 page document about emulator development. It's easily the best resource I've seen on the subject.

Ditto - I've been wanting to do one on a small ARM Cortex-M core, but it's bewildering to even think about where to start.

It seems like people have put together a lot of good documentation for old systems though, which could at least be a good place to start reading:

https://problemkaputt.de/gbatek.htm

learn assembly, learn how it maps to opcodes. learn microelectronics and learn about cpu, registers, memory layout, cpu flags, etc. it becomes obvious.

i wrote a 6800 emulator just from having this background knowledge and no reference in my college years. I did it because I didn't want to go to the labs early saturday morning to fight over the few 6800 hardware. I could write my code, run, debug, show up just as the lab is ending and hand in my work and go home.

I started with the MOS 6502, the core of the NES, Atari 2600, C64, and a bunch of others. I wrote it in order to build a really simple NES emulator, and I found the chip super approachable and easy to deal with--there just isn't much to it. Documentation is easy to find, too.