Decided I wanted to know more about MVS because it is the closest thing I have to learn z/OS which is common today on IBM mainframes. From https://www.ibm.com/it-infrastructure/z/zos … z/OS is a 64-bit operating system for IBM mainframes, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions. [which in turn preceeded OS/390, which in turn preceeded OS/370…which in turn preceeded OS/360…which in turn preceeded the IBM 1401…which in turn preceeded ENIAC…which in turn preceeded the abacus…LOL!]