[gpderetta]

IIRC Segments were used to speed up (para?) virtualization, before built-in support in the ISA.

Segment registers are also used as thread pointer, but that special case is still supported even in 64 bit mode.

[rbanffy]

I was talking about the rings. I remember something that used them.

[gpderetta]

I think NT at some point run drvers and some other subsystems in lesser privileged rings than ring 0. Not sure if this was dropped in later versions.

[rbanffy]

IIRC, NT 4 moved drivers into kernel space for performance. The result was that the system crashed much faster :-P.