I have a couple of IBM’s microcomputer-based implementations of their mainframe architecture, plus an IBM XT/370 card set that does not work (it has a bad M-Card).
One of my two working “mainframes” is an IBM PC XT/370. The XT/370 is a PC XT (or, equivalently, a PC with the 256KB mother board and a hard drive), with the PC XT/370 card set and, somewhat optionally, an IBM 3277 emulation adapter.
The PC XT/370 card set comprises two cards: the processor, or P-Card and memory card, or M-Card.
My PC XT/370 did not start out life as an XT/370. Instead it was “cobbled together” (before I acquired it), using an IBM PC/XT with the requisite 256KB of memory on the mother board and the X/370 card set which I acquired separately in 2013. It also has a 3rd-party 20MB Seagate hard disk with its own adapter, and a 3.5″ floppy diskette.
It supports special IBM software, VM/PC that has a somewhat cut-down version of the IBM VM Control Program (CP) and Conversational Monitor System (CMS).
My other “mainframe” is a Pentium II based PC with an IBM P/390E card. This PCI card is capable of acting as an IBM 370 mainframe or as a S/390 mainframe. It is fully capable – it will run the “stock” IBM operating systems.
Ordinarily this card would be installed in an IBM PC Server 330. Not having one of those handy, I used a similar era Pentium II based computer – the same one that I use for capturing floppy disks – but running under OS/2 in this case.