The thing that made the lightbulb go off in my head w.r.t. mainframes was understanding that mainframe I/O channels are computers. The mainframe had several ded

The thing that made the lightbulb go off in my head w.r.t. mainframes was understanding that mainframe I/O channels are computers. The mainframe had several dedicated computers that each specifically handled I/O to a terminal, printer, disk or tape drive, punchcard reader, etc. Made I/O programming a breeze, as you just had to tell the channel to read or write, specifying a block of memory to use as a buffer, and the channel would DMA out the data to be written, or DMA in read-in data. It also explains the reason why despite having middling CPU power, mainframes had a reputation for stinkloads of I/O bandwidth so they could process everyone's credit card transactions, airline bookings, and that: the mainframe's CPU was involved very little in I/O, that was all handled by the channel processors!