Os anéis aos quais você está se referindo são discutidos aqui na Wikipedia, intitulado: Ring (segurança do computador) .
Dêumaolhadana
Linux and Windows are two operating systems that use supervisor/user-mode. 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 it back to user space.
Eu também daria uma olhada na página chamada do sistema , que discute um pouco mais a natureza dessas chamadas, prestando atenção especial aos privilégios aqui discutidos:
However, many normal applications obviously need access to these components, so system calls are made available by the operating system to provide well-defined, safe implementations for such operations. The operating system executes at the highest level of privilege, and allows applications to request services via system calls, which are often executed via interrupts; an interrupt automatically puts the CPU into some required privilege level, and then passes control to the kernel, which determines whether the calling program should be granted the requested service. If the service is granted, the kernel executes a specific set of instructions over which the calling program has no direct control, returns the privilege level to that of the calling program, and then returns control to the calling program.
A API do Kernel do Linux também pode esclarecer suas dúvidas sobre o que está no modo de supervisão.