And that's for a relatively minimal system. Bear in mind that the term "mainframe" comes from an old term for the central processor of a computer - each of the system's primary components (CPU, memory, I/O handler, power regulator, etc) would be built into one or more of those large "frames", of which the processor (made up of a great many individual, interconnected rackmount cards stuffed full of discrete components or, much like HAL 9000, small-scale integrated circuit packages) was of course the "main" example. The singular encompassing the plural, it came to imply any complete computer system built along similar lines, regardless of size...