[nickpsecurity]

GEMSOS and STOP OS built highly-secure systems (for the time) on Intel by using the protection rings and segments. Both only put the security kernel in kernel mode, user apps in user mode, and OS services in middle rings.

Here's architecture for GEMSOS. See design/assurance sections.

http://aesec.com/eval/NCSC-FER-94-008.pdf

Look up SCOMP Final Evaluation Report if you want to see how STOP OS used four rings and had an IOMMU despite that being "invented" recently. ;) The XTS-400 is the Intel version, uses same architecture minus custom hardware, and is still doing its job at hundreds of installations.

Definitely a major performance hit on both GEMSOS and STOP but they were 80's era stuff. Modern separation kernels do most stuff with just user/kernel mode separation with tiny kernels (4-12kloc). LynxSecure claims helps them keep CPU 97% idle with 100,000+ context switches a second. I'd expect old architectures to run even faster with modern techniques.