> For historical context, a PC compatible is a machine that can run a DOS that is compatible with PC-DOS and that can run applications for the IBM PC running PC DOS.

Historically, a lot more was required in practice. For example, programs that use the BIOS for screen I/O are slow, so most programs wrote directly to video memory. Because of that, video memory had to be laid out identically and had to be located at the same address.

See https://en.wikipedia.org/wiki/IBM_PC_compatible#Non-compatib... for other examples.

True. Also, timing and so on. The thing is "can run applications" was a rather load carrying part of my statement.