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:
“Mischief springs from the power which the moneyed interest derives from a paper currency which they are able to control, from the multitude of corporations with exclusive privileges ... which are employed altogether for their benefit.”
—Andrew Jackson (1767–1845)
“Take two kids in competition for their parents’ love and attention. Add to that the envy that one child feels for the accomplishments of the other; the resentment that each child feels for the privileges of the other; the personal frustrations that they don’t dare let out on anyone else but a brother or sister, and it’s not hard to understand why in families across the land, the sibling relationship contains enough emotional dynamite to set off rounds of daily explosions.”
—Adele Faber (20th century)
“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)