[monopede]

Looking at the instruction set it doesn't even support subroutines. It would only need a way to get and set the current program counter (like ARM) and you could implement subroutine calls using push, pop, and jmp.

[jws]

I think subroutines would go something like:

   PUSH the_address_you_want_to_come_back_to
   JMP  some_other_spot
 the_address_you_want_to_come_back_to:
   BLAH
   BLAH
   ...
 some_other_spot:
   BLAH
   BLAH
   POP some_scratch_address
   JMP some_scratch_address
 

So yes, JSR is two opcodes and RTS is also two opcodes, but it works.

[kragen]

In your first snippet, you seem to be assuming that JMP is direct, and in the second, you seem to be assuming that JMP is indirect. It appears to be direct: https://github.com/GenTiradentes/tinyvm/blob/master/tvm.c#L5...

You could, of course, do it with self-modifying code if that's possible. (I'm not familiar enough with the VM to know if it is.)