Unless you
have a old copy of DOS, what OS can you get for the OLD
MACHINES?
MS-DOS will, of course, work for all.
That should read
MS-DOS will, of course, work for all PCs.
(for some definition of "PC", that usually means 5150 derived clone)
MS-DOS will most certainly NOT work for all OLD MACHINES.
For OLD MACHINES, you will need to locate whatever OS went with it.
CP/M, for example, was for 8080, 8085, Z80, with a few other requirements
such as memory map (TRS80 required relocating memory, or relocating CP/M
to work)
The Epson RC-20 (wrist watch) had a Z80 compatible processor, RAM, ROM,
etc., but nobody ever ported a version of CP/M to it.
I no longer have access to the "ROM-Roader", so I don't think that I can
resurrect mine.
I should see if my Fossil Palm-OS watch still works.