rob@prism.TMC.COM (01/06/90)
jml@tw-rnd.UUCP writes: >In article <1989Dec29.165724.12683@sj.ate.slb.com> poffen@sj.ate.slb.com >(Russ Poffenberger) writes: >>AT THE SAME TIME. We are NOT talking emulation, but TRUE multiple operating >>systems co-existing. That's how the Sun 386i does their DOS compatibility. >>Just assign a chunk of memory, and away you go. >Your statement would possible be true for any OS constrained as DOS is. More specifically, it's true for any OS running in real mode. The 386/486 basically allow a process running under the direction of a protected mode operating system to be treated by the CPU as a real-mode program. Since in real mode there's no distinction between application and system code (at least from the CPU's viewpoint), DOS runs nicely in this 'virtual-86' mode. It's not quite correct to say 'just assign a chunk of memory'. There's more to it than that, especially if you want to run multiple Virtual-86 processes. The operating system needs to protect the processes from each other by regulating IO access, re-mapping memory, and so on. The 386 contains mechanisms that make this, if not easy, at least possible. >I don't think OS/2 and Unix would cooperate very well running at the same >time. Correct. The 386 can only virtualize the 8086, not the 80286 or itself. Therefore, multiple protected mode operating systems can't easily co-exist. The 486 was rumored to have a 'virtual 286' mode that would permit this, but turned out not to. (Judging from the response to OS/2, few people have any interest in running a _single_ 286 OS, much less several at once.) This brings up a point that's sometimes overlooked in talking about upward compatibility. The Intel line is very upward compatible, probably moreso than the 68000 line - as long as you stay in real mode. To get the most out of newer Intel CPUs requires using protected mode, which is incompatible with real mode. That's why the move to OS/2 is such a headache. The virtual-86 mode of the 386 and 486 makes this transition easier by allowing multitasking of real mode programs under a protected mode OS. Still, moving to 386 protected mode from real mode will require modifications to most programs. Unlike 286 protected mode, however, 386 protected mode offers things that make up for the effort and added complexity.