Comment by PaulHoule
1 year ago
Wasn't like that all with DEC and I don't think so with IBM mainframes either.
It was common for DEC systems to have custom Unibus cards
https://en.wikipedia.org/wiki/Unibus
as these were really easy to make. They dealt with them by building custom drivers right into the OS when they build an OS image.
Circa 2002 a friend of mine developed custom printer interfaces and drivers for IBM z because IBM's printer interface couldn't support the high rate of printing that New York state needed to satisfy the mandate that any paperwork could be turned around in 2 days or less.
Whatever you say about NY it is impressive how fast you get back your tax returns, driver license, permit to stock triploid grass carp or anything routine like that.
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.