Comment by fulafel
6 days ago
Several operating systems on 286 (eg Xenix, Coherent, OS/2) used its MMU for multitasking and memory protection. See https://en.wikipedia.org/wiki/Intel_80286#Protected_mode
6 days ago
Several operating systems on 286 (eg Xenix, Coherent, OS/2) used its MMU for multitasking and memory protection. See https://en.wikipedia.org/wiki/Intel_80286#Protected_mode
The 286 protected mode did not allow for a 32-bit flat address space and was heavily half-baked in other ways, e.g. no inbuilt way to return the CPU to real mode without a slow and fiddly CPU-reset.
It was architecturally a 16-bit CPU so a flat 32-bit address space would be a non sequitur. If you wanted flat 32-bit addressing, there was a contemporary chip that could do it with virtual memory: Motorola 68010 + the optional external MMU. (Or if you were willing to do some hoops, even a 68000.. see the Sun-1)
Protected mode on the 286 allowed 24-bit addressing, enabling access to 16 MB of memory, but lacked virtual memory and required a reboot to return to real mode. The 386 introduced virtual memory through paging, 32-bit addressing for 4 GB of memory, and virtual 8086 mode for running multiple 8086 programs simultaneously without compromising security.
https://flint.cs.yale.edu/feng/cos/resources/BIOS/procModes....
https://en.wikipedia.org/wiki/Protected_mode
Coherent was the first Unix-like OS I ran, on a 386SX box. I think it was Coherent 4.x.