=== On Unix: ===

when a green thread wants to do a blocking read,

    for(;;)
    {
        read(fd,buf,len)
        if(errno is EINTR)
            continue
        else if(errno is EAGAIN)
        {
            waiting_fds.add(fd,current_thread)
            suspend(current_thread)
            continue
        }
        else
        {
            /* data is now in 'buf' */
            break
        }
    }

then you have an IO thread that runs when no other green threads are runnable,

    for(;;)
    {
    	select(waiting_fds) /* or poll, kqueue, epoll, etc */
        foreach(fd in waiting_fds)
        {
            if(can_read(fd))
                wake_up(waiting_fds.get(fd))
        }
    
        yield()
    }

=== On Windows: ===

when a green thread wants to do a blocking read,

    read(fd,buf,len,&completion)
    waiting_completions.add(completion,current_thread)
    suspend(current_thread)
    /* data is now in buf */

then you have an IO thread that runs when no other green threads are runnable,

    for(;;)
    {
    	completion = GetQueuedCompletionStatus(...)
        wake_up(waiting_completions.get(completion))
        yield()
    }