All 370 I/O was via IBM’s architected data channels.
Most I/O on subsequent systems likewise was via standard I/O architecture such as USB or FireWire which centralizes all of the real time interactions into device independent kernel like functions.
The special problems of displays present a platform dependent problem for the kernel.
The Luna 88K system devoted a fixed range of real addresses to hold the pit-per-pixel display frame.
Those pages were special kernel page constructs that could be mapped into user space.
Those pages were checkpointed.