What I meant by the IO is that the aforementioned sparcstation had ethernet card connected to SBus which was capable of directly producing interrupts and doing

What I meant by the IO is that the aforementioned sparcstation had ethernet card connected to SBus which was capable of directly producing interrupts and doing DMA, and all the modems were connected through some multiport serial card/concentrator which tried very hard to offload stuff from the CPU. Which is completely different situation than R-Pi with everything hanging off USB with it's "novel" approach to interrupts.