Free electronic library

This design was similar to that of DEC??™s VAX, a popular architecture
at the time of the design of the 80386. VMS, the native operating system for the
VAX, made use of all four rings, and so it was believed that this would aid porting
of VMS, and would be useful to designers of new operating systems. Since then,
2.2. Restricting Operations with Privilege Rings 29
VMS, which is now sold by HP, has been ported to the Alpha and Itanium (both
of which only support two rings), but not x86.
The concept of multiple hardware-enforced privilege layers originated in MULTICS,
which proposed a system of eight rings. The UNIX system was designed
to run on simpler systems (the DEC PDP-11) and so only required two: one for
the kernel and one for everyone else.
Most operating systems written for IA32, including Windows NT and most
UNIX-like systems, only use two. One exception to this was OS/2, which ran
device drivers in a lower privilege ring than the rest of the kernel. Another was
Novell Netware, which, in later versions, loaded modules in lower privileged rings.


Pages:
69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93