z/OS is just a silly marketing name for what was previously called OS/390. Which is itself a silly marketing name for what was previously called MVS. Which it itself a silly marketing name for what was previously called by another name I don't remember (OS/VS perhaps ?). IIRC, at one point it was even called OS/370. I believe that the first incarnation of this OS is OS/360, the OS whose creation inspireed Fred Brooks' book "The Mythical Man Month". Ask a mainframe specialist.
Don't forget that IBM is a marketing company as much as a technical company.
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z/OS is just a silly marketing name for what was previously called OS/390. Which is itself a silly marketing name for what was previously called MVS. Which it itself a silly marketing name for what was previously called by another name I don't remember (OS/VS perhaps ?). IIRC, at one point it was even called OS/370. I believe that the first incarnation of this OS is OS/360, the OS whose creation inspireed Fred Brooks' book "The Mythical Man Month". Ask a mainframe specialist.
Don't forget that IBM is a marketing company as much as a technical company.