FreeVMS

It was thus said that the Great Bill Pechter once stated: What do you mean by that? The 286 and above have four levels of protection, from ring 0 (all instructions available, all resources visible) to ring 3 (only common registers and instructions available, no (or very limited) resources visible). Most Unix systems for the 386 and above use 0 for kernel, 3 for user and don't bother to use the other two levels. OS/2 may have used the other rings though (it's been over 10 years since I've used OS/2). Then there are the operating modes supported on the 286 and above: real mode (which looks like a very fast 8086) and protected mode. The 386 and above also have a virtual 8086 mode, which allows you to run real mode code under a protected OS---the real mode is virtualized (to an extent, I don't think it's pure vitualizasion like you can get on the IBM 360 line). There is an undocumented mode---real-32 bit (where the CPU is in 32-bit mode, but not running protected) but that's a hack and is probably not recomended. The 386 also has a paging system (in addition to a segmentation system but I'm not going into the details of that) so I don't see why VMS couldn't run on the 386. I do know that the VAX architecture had four address spaces (P0, P1, System, unused) and that can be simulated. -spc (I think you are talking about kernel, executive, supervisor and user, which map quite well to rings 0 to 3 on the 386)