As I wrote, I created a bytecode which fits the compiler's code generator the most. This keeps the entire code base small, the bytecode VM simple and compilation fast. This is a self-contained solution, the compiler as well as the execution environment is part of the emulator; therefore it has to be compatible with itself only, which opens up solutions for simplicity and effectiveness.

Using real world RISC-V instructions would have introduced a lot of complexity without any real benefit. It's not that you can run the MEG-4 scripts without the API library and emulation environment anyway, so at the end of the day the actual instruction encoding doesn't matter.

BTW, if I were to use an existing instruction encoding, I probably would have chosen WebAssembly instead, because it is a lot easier to generate and lot easier to process than a real CPU's instruction set. There are many wasm VM libraries with nice licensing and easy to integrate interface and virtually no dependencies like https://github.com/kanaka/wac for example. But again, implementing wasm's poorly engineered container format and one of its incompatible ABI would just add complexity and have no real benefit.

All that being said, one could implement a new bytecode format in MEG-4 easily if they really want to. It already supports multiple instruction sets (Lua being a working PoC).