6o6 v1.1: Faster 6502-on-6502 virtualization for a C64/Apple II Apple-1 emulator

I'm doing periodic updates on some of my long-term projects, one of them being 6o6, a fully virtualized NMOS 6502 CPU core that runs on a 6502 written in 6502 assembly language. 6o6 implements a completely abstracted memory model and a fully controlled execution environment, but by using the host's ALU and providing a primitive means of instruction fusion it can be faster than a naïve interpreter. This library was something I wrote over two decades earlier for my KIM-1 emulator project for the Commodore 64, and relatively recently I open-sourced and discussed it in detail. It runs on just about any 6502-based system with sufficient memory.

For this update I made some efficiency improvements to addressing modes, trimmed an instruction out of the hot path, provided an option for even more control of the 6502 interrupt flag and implemented a faster lane for direct stores to 6502 zero page (as well as the usual custodial and documentation updates). And, of course, any complex library needs a suite of examples, and of course, any update to a complex library demands new examples to play with too.

So, given that this year is Apple's 50th anniversary (and, as it happens, my own 50th year of existence personally), what better way to show off a 6502-on-6502 virtualization library than with an Apple-1 emulator ... that runs on the Commodore 64 or Apple II? Now yea, verily, this is hardly the first such example and several others have done something of the sort, but I submit that 6o6 makes our take on it here unique, and as a bit of fun we'll discuss the Apple-1's hardware and look at all that prior 8-bit emulator art for comparison (for the C64 and Apple II and even more exotic systems like the SAM Coupé).

Virtualizing the 6502 on a 6502 with 6o6 (and The Incredible KIMplement goes 1.0)

Okay, promises, promises. Here's the first of my bucket list projects I'm completing which I've intermittently worked on for literally two decades. Now that I've finally shaken out more bugs, tuned it up and cleaned it off, it's time to let people play with the source code.

This is the official 1.0 release of the Incredible KIMplement, an emulator of the one kilobyte, 1MHz MOS/Commodore KIM-1 6502-based single board computer. It provides access to the KIM's built-in TTY support (even through your computer's real serial port) and has expanded RAM with 16K of addressing space, all on an unexpanded stock Commodore 64.

It's almost burying the lede to announce that, though, because the real meat in this entry is how the Commodore 64 manages to emulate a very different 6502-based system. That piece is "6o6," for "6502-on-6502," and is a full virtualized software NMOS 6502 CPU that runs on a 6502 CPU — which I've open-sourced too. It has full control of guest code execution, including trapping undocumented and jam opcodes, and completely abstracts all memory access, making it possible to remap addresses, intercept illegal reads or writes, or even run entirely from virtual memory. On top of that, it's complete enough to not only pass a full functional test but also virtualize itself virtualizing itself:

These GIF screencasts are real-time with no tricks. Here a Commodore 64 and Apple IIe are both running a guest "hello world" payload within 6o6 (stage 1), which is nearly instantaneous, then 6o6 running the payload as a payload within another instance of 6o6 (stage 2), which is a little slower, then 6o6 running 6o6 running 6o6 running the payload (stage 3), which is glacial. But all of it works!