The hardware is quite radically different. It's not the mechanical form factor or bulk electrical and cooling of it, but the silicon. The processor checkpoints

The hardware is quite radically different. It's not the mechanical form factor or bulk electrical and cooling of it, but the silicon. The processor checkpoints state, and if an error is detected it can be rolled back and the checkpoint moved to another physical processor and re-started transparently to software. Stores are tracked all the way down to "RAID memory" (actually called RAIM), whereas in other CPUs they get punted off at the L1 cache. Practically every component and connection is redundant. There are stories of mainframe drawers being hauled to particle accelerators and the beam switched on while they're running. Quite amazing machines. Not to say that has more value than the software (I don't know one way or the other), but the hardware is no gimmick.