Comment by p_l

But it also meant that release of a new computer often required new OS release, with DEC often patch releases that added just enough code to run the devices included in new computer, because the older versions would at best boot into something unusable.

As for IBM mainframes, the list of devices directly attachable at OS level is quite small, and even then application with appropriate privileges could directly send control words to a channel. That said, things like printers would probably be intermediated by communication controllers translating from channel interface to serial lines.