Page "History of operating systems" Paragraph 10

Early operating systems were very diverse, with each vendor or customer producing one or more operating systems specific to their particular mainframe computer.

Every operating system, even from the same vendor, could have radically different models of commands, operating procedures, and such facilities as debugging aids.

Typically, each time the manufacturer brought out a new machine, there would be a new operating system, and most applications would have to be manually adjusted, recompiled, and retested.