It is not necessary to use all four privilege levels.
Often the security model is simplified to "kernel" and "user" even if hardware provides finer granularity through rings.
For other uses, see, ring.
Dobb's Journal, May philip greenman principles of manual medicine 2008.
Today, this high degree of interoperation between the OS and the hardware is not often cost-effective, despite the potential advantages for security and stability.The idea of having two different modes to operate in comes from " with more control comes more responsibility " a program in supervisor mode is trusted never to fail, since a failure may cause the whole computer system to crash.In most existing systems, switching from user mode to kernel mode has an associated high cost in performance.The iopl can be changed using popf(D) and iret(D) only when the current privilege level is Ring.However, it was an exception to the rule.In protected mode and long mode, it shows the I/O privilege level of the current program or task.To perform specialized functions, user mode code must perform a system call into supervisor mode or even to the kernel space where trusted code of the operating system will perform the needed task and return the execution back to the userspace.Supervisor mode is "an execution mode on some processors which enables execution of all instructions, including privileged instructions.16 Potential future uses for the multiple privilege levels supported by the x86 ISA family include containerization and virtual machines.
For example, the reason Windows uses only two levels (ring 0 and ring 3) is that some hardware architectures that were supported in the past (such as PowerPC or mips ) implemented only two privilege levels.
Introduction to OS/2 Presentation Drivers "ARM Architecture.3.4: Privilege levels" foldoc supervisor mode a b Jochen Liedtke.
Operating Systems: Principles and Practice (2nd.).
The hardware restrictions are designed to limit opportunities for accidental or malicious breaches of security.
A Practical Alternative to Hierarchical Integrity Policies.Hypervisor mode edit Recent CPUs from Intel and AMD offer x86 virtualization instructions for a hypervisor to control Ring 0 hardware access.This flag determines whether it would be possible to execute machine code operations such as modifying registers for various descriptor tables, or performing operations such as disabling interrupts.In computer science, hierarchical protection domains, 1 2 often called protection rings, are mechanisms to protect data and functionality from faults (by improving fault tolerance ) and malicious behaviour (by providing computer security ).Operating systems running on hardware supporting both may use both forms of protection or only one.The hardware remains aware of the current ring of the executing instruction thread at all times, with the help of a special machine register.In some systems, areas of virtual memory are instead assigned ring numbers in hardware.See also edit References edit Paul.