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:
“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)
“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)
“One of the duties which devolve upon women in the present interesting crisis, is to prepare themselves for more extensive usefulness, by making use of those religious and literary privileges and advantages that are within their reach, if they will only stretch out their hands and possess them.”
—Sarah M. Grimke (1792–1873)