pub fn vm_exec(p: &mut Process, isns: u64) -> Result<u64, (u64, &str)> {

    // FIXME Ugly nested matches can be made into a more functional thing.

    let mut execed = 0;

    while execed < isns {

        let top = p.call_stack.top();
        match top {
            Some(frame) => {
                let ti = frame.get_target_isn();
                match ti {
                    Some(i) => {
                        let ii = i.clone();
                        match ii.execute(p) {
                            Ok(segue) => {
                                execed += 1;
                                match segue {
                                    IsnSegue::Next => {
                                        let mut sf = p.call_stack.pop().unwrap();
                                        sf.next_isn += 1;
                                        p.call_stack.push(sf);
                                    },
                                    IsnSegue::RelJump(diff) => {
                                        let mut sf = p.call_stack.pop().unwrap();
                                        sf.next_isn += diff as u64;
                                        p.call_stack.push(sf)
                                    },
                                    IsnSegue::Return => {
                                        p.call_stack.pop();
                                    },
                                    IsnSegue::Detach => {
                                        let mut sf = p.call_stack.pop().unwrap();
                                        sf.next_isn += 1;
                                        p.call_stack.push(sf);
                                        return Ok(execed + 1);
                                    }
                                }
                            },
                            Err(r) => return Err((execed + 1, r))
                        }
                    },
                    None => {} // Don't push it back onto the stack and don't increment execed.
                }
            },
            None => return Err((execed, "process exited"))
        }

    }

    return Ok(execed);

}