I'm looking to build a VM into a game and was wondering if anyone knew of any really simple VM's (I was thinking RISC/PIC was close to what I wanted) that are usually used for embedded projects such as controlling robots, motors, sensors, etc. My main concern is having to write a compiler/assembler if I roll my own. I'd be nice to use the tools that are already out there or in its simplest form just a C compiler that can compile for it :-p.
I really don't want to re-invent the wheel here but I also need thousands of these running around a virtual world so they have to be as simple and as fast as possible. As one person has already mentioned I also don't care about real world issues such a timing and buses and all that fun stuff. I think their virtual clocks will be limited to somthing quite slow; and eventually I'll probably have to look into native compiling to make them run even faster but for now I'm just putting together prototypes to get a general proof of concept.
As input, I'm planning on distance, light, material and touch sensors mounted around the cylindrical body (16, maybe 32 of them), then simply 2 motors for directional output to control a sort of wheel on each side. essentially the processing won't be too strenuous and the world will be simple enough so that the machine's don't have to throw lots of processing power at simple tasks.
In terms of memory, I'd like them to be able to store enough data to be left alone for a couple of days without intervention for building maps and gathering stats. I don't like 8bit would cut it for processing or memory but 16bit would definitely be a contender. 32 and 64bit would just be pushing it and there's no way they'll have any more than 1mb each of memory - probably closer to 256-512k. (Bill one said 640k would be enough so why can't I!!)
if you want something rooted in the real world, one of the most-used embedded RISC microcontrollers is the PIC family. google gives several emulators, but i don't think the source is available for most.
another possibility is QEMU, which already emulates several ARM varieties.
and, of course, if you're not interested in emulating a real-world device, far easier and better performance would be to roll your own. with only what you need, and not getting into the mess of state flags, overflow bits, limited bus width, RAM timings, etc.