Complex sentence: These machines ran some form of DOS (Disk Operating System) and their BIOS provided very poor support for serial communications--so poor that it fell far short of the needs of any non-trivial communications task.
Split into simpler sentences: These machines ran some form of DOS (Disk Operating System) and their BIOS provided very poor support for serial communications. So poor that it fell far short of the needs of any non-trivial communications task.