A Journey in NES Emulation - Part I

Writing an NES emulator has been a long time interest of mine, primarily because the subject of software emulation is fascinating. I’ve spent my fair share of time studying how games are developed (from both modern and retro prospectives), but emulation is a completely different beast - an exercise in computer architecture dealing entirely in low-level components including precise hardware specifications and assembly code. Even for something we consider ‘basic’ by today’s standards, the NES classics we all know and love are nothing short of technical masterpieces.

I should address a couple of things up front about this project: first, why Kotlin? Kotlin is a ‘modern’, statically typed language that runs on the JVM. As someone who works regularly with Java, I feel that it’s quickly becoming the preferred language over Java for modern application development and I enjoy working with it (read more about Kotlin here). Writing for the JVM also means that the emulator can be run ‘cross-platform’, and since we’re dealing with NES hardware the performance trade off isn’t a top concern.

Secondly, I haven’t taken on this project without huge support from existing projects in the open source community, namely Andrew Hoffman’s halfnes, which is an existing, largely comprehensive NES emulator written in Java. A lot of my current understanding of certain implementation details have come from this project, although I’m in the process of learning more and tweaking as I go. Another absolutely necessary source of information for this project comes from the nesdev Wiki, which contains both hardware and software documentation for everything NES. Now, on to some of the details concerning the project.

Right out of the gate I was interested in how an emulator could take compiled assembly code and translate that to pixels on a screen. Even having spent some time working through the process, I’m still completely fascinated by the orchestration of it all. Simply put, the compiled source code is represented as nothing more than a series of HEX values - each value signaling a processor directive that, as a whole, make up the instruction set for a given title.

4e45 531a 0800 1100 0000 4e49 2031 2e33
a51f c915 b021 a512 2910 f016 a000 84d1
84bb 84b7 84b8 a5ff 29fc 85ff a91b 851f
...

“The first few lines of compiled source code for ‘Metroid’.”

Since hardware acceleration and other low level bindings aren’t completely necessary to achieve acceptable emulation for the NES on modern hardware, I decided to forgo using a cross-platform library such as SDL in place of a basic JFrame implementation as a first pass rendering solution. This implementation renders a new ‘frame’ as a BufferedImage and displays the contents within the JFrame, although I’ve found that the timing isn’t perfect and is almost entirely based on the performace of the underlying CPU. I plan on discussing how these frames are composed in more detail when I get around to discussing the PPU, or picture processing unit (effectively the ‘graphics processor’ of the NES).

I don’t want to get too far in the weeds right away, but I do want to address one more topic central to console emulation: How is it that the compiled source code addressed above makes its way into a digital file format suited for emulators from the cartridge it originated from? NES cartridge data can be ‘ripped’, or extracted using special hardware/software such as INLretro Dumper-Programmer (formerly named ‘kazzo’). This relatively inexpensive board will allow cartridge owners to back up their cartridges into the digital formats that emulators accept.

This product also supports the ability to flash cartridges, allowing the ability to load your own ROMs onto a playable cartridge for the die-hard enthusiast who wants to see their creations running on actual NES hardware. For those interested in writing games for the NES, the nesdev Wiki is a great starting point, as well as the NES Programming guide hosted on NintendoAge.

Next up I plan on diving into the CPU, and how the NES handles working with the compiled source code mentioned above.

kNES can be found on my GitHub page. This emulator is very much still in its infancy and published primarily to get the ball rolling on documentation and community support.

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