Privileges
The design of the microprocessor architecture on practically all modern systems (except some embedded systems) involves a security model (such as the rings model) which specifies multiple privilege levels under which software may be executed; for instance, a program is usually limited to its own address space so that it cannot access or modify other running programs or the operating system itself, and a program is usually prevented from directly manipulating hardware devices (e.g. the frame buffer or network devices).
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.
Read more about this topic: System Call
Famous quotes containing the word privileges:
“We have been weakened in our resistance to the professional anti-Communists because we know in our hearts that our so-called democracy has excluded millions of citizens from a normal life and the normal American privileges of health, housing and education.”
—Agnes E. Meyer (1887–1970)
“Politics ought to be the part-time profession of every citizen who would protect the rights and privileges of free people and who would preserve what is good and fruitful in our national heritage.”
—Dwight D. Eisenhower (1890–1969)
“The Gospel of the army is cunning, as of all other human activities. The wisdom of the snake under the meekness of the sheep is what wins out.
The first Commandment is—never let them get anything on you—
The second: Graft—get privileges others haven’t got—worm yourself into confidence
The Third—seem neat and prosperous—as if you had money in the bank—”
—John Dos Passos (1896–1970)