[simonblack]

Operating systems didn't have the large memory space which is necessary to do that. The maximum RAM with 16 bits is only 64K. Bank switching chunks of RAM in and out of that 64K can give you a little more, but the compromise is that the bank-switching code takes up RAM too.

In the 8-bit universe the CP/M OS took up somewhere around 6 - 8 KILObytes. My earliest DOS (North Star DOS) took up 0A00 H bytes (a mere 2560 bytes!)

The hardest thing in coming to Unix/Linux from the DOS world was that hardware is out of bounds for direct-access. You have to work with a hardware driver and the OS.