Just toying around with IBM mainframe emulation using Hercules. To launch zOS, the command command: ipl a80 is entered. IPL means initial program load. But I couldn't figure out the meaning of the parameter a80. What does it mean?
A80 refers to the unit address of the device you are IPLing. In the case of Hercules, it is a reference to a virtual device that contains a volume. This volume contains the RES pack or system residence volume. Effectively the boot disk.
As pointed out, the version of the operating system your attempting to IPL needs to be one that is considered open source to avoid legal issues. At some point MVS SP went from open to closed and this point is referred to as the transition to Object Code Only (OCO). Any version that is OCO requires a license from IBM and other vendors potentially to use it in an ambulatory.
Note, other operating systems like Linux can be used so you need to very the OS license to ensure your compliant legally.
Good luck and welcome to the mainframe!