[davelnewton]

Then you just need to read the chip docs and initialize "everything" to a reasonable state. There's not much to it beyond that--there's no generic process for doing that.

Loading an app from an SD card is likely harder than you think: it needs to be compiled/assembled correctly, you need to define where it starts, how it's linked, etc.

"Parse over the app executable for validity" is super-broad, and could mean just about anything. If you're trying to validate compiled programs, this will be difficult. If you're trying to validate byte-code programs (or something similar) it's significantly easier (or even unnecessary).

It'd probably be helpful if you were able to define specifically what you're trying to accomplish, and why an existing system doesn't meet your needs. While writing a trivial execution environment is really easy, writing a general-purpose OS is significantly more difficult.