In the earliest days of the PC revolution, people had different, incompatible hardware and software (with the exception of the widespread MS BASIC) even if they sometimes used the same CPUs and audio/video chips. TRS-80, CoCo, MSX, C64, Amiga, Atari ST, ZX Spectrum, Oric, Amstrad CPC, X68000, PC-98, IBM-PC, Apple II, Mac Classic, Macintosh II, Apple II GS, SGI, Sun etc. In fact, hardware and software are a lot more homogenous now than in the beginning: 2 dominant CPU architectures (x86/64 and ARM) and 3 dominant OSs (Windows, MacOS, Linux).