[cctalk] Re: MS-DOS

CP/M was effectively limited to 64KiB because it had no traction outside of the 8080/Z80 which had a 64KiB address space. To go beyond that limit on those CPUs involves paging, and some platforms did indeed use paging for RAM disks and to move some OS out of the way to leave more RAM for programs. But as far as programs are concerned, 64KiB is the limit unless they happen to be platform-specific and know how to handle the paging. From what I can tell of a casual peruse of the documentation of CP/M-68K and CP/M-86, they support the full address space of 4GiB and 1MiB respectively. This is kind of obvious on the m68k since why would they artificially limit it, but on x86 it's less obvious because they could have restricted changing the segment registers. CP/M-86 *also* supports an "8080 model" with CS == DS == ES, presumably to ease quick ports of 8080 code through source-to-source translation.