How can I generate a random assembly instructions?

I need a way to generate a random 16-bit assembly command in C (doesn't matter if hex or text I can just convert it)... but the thing is that I can't have any non-existent or illegal commands to execute, any way to do that?

Criteria:

1. Totally random command
2. Has to be an executable command
3. Has to be a 16-bit command
4. Has to be able to prodouce it in C

Plese help me.