| MAINBUS(4) | Device Drivers Manual (mac68k) | MAINBUS(4) | 
mainbus —
mainbus0 at root
mainbus interface serves as an abstraction used by
  the autoconfiguration system to help find and attach busses (e.g. the NuBus
  expansion bus) or devices (e.g. the math coprocessor) connected to the
  Macintosh main processor bus.
mainbus first appeared in NetBSD
  1.2.
| February 17, 2017 | NetBSD 10.1 |