I've asked, I've skimmed wikipedia pages, but until you explained the isolation of the main CPU, with controllers for other tasks, I never understood exactly what a mainframe was. So thank you.

Also, thanks for the clear explanation of how to get started with this. I heard about the Open Mainframe project (https://www.openmainframeproject.org) and kinda poked around there, but I think I'll give Hercules a go before Open Mainframe's Zowe.