Page 408 - DCAP103_Principle of operating system

Unit 14: Case Study of Linux Operating System

Notes
Figure 14.1: Layers in Linux

User
interface
Users
Library
interface Standards utility programs
(shell, editors, compliers etc.)
System
call User
interface mode
Standard library
(open, close, read, write, fork, etc.)

UNIX operating system
(process management, memory management, Kernel mode
the file system, I/O, etc.)

Hardware
(CPU, memory, disks, terminals, etc.)

Programs make system calls by putting the arguments in registers (or sometimes, on the stack),
and issuing trap instructions to switch from user mode to kernel mode. Since there is no way
to write a trap instruction in C, a library is provided, with one procedure per system call.
These procedures are written in assembly language, but can be called from C. Each one first
puts its arguments in the proper place, then executes the trap instruction. Thus to execute the
read system call, a C program can call the read library procedure. As an aside, it is the library
interface, and not the system call interface, that is specified by POSIX. In other words, POSIX
tells which library procedures a conformant system must supply, what their parameters are,
what they must do, and what results they must return. It does not even mention the actual
system calls. In addition to the operating system and system call library, all versions of Linux
supply a large number of standard programs, some of which are specified by the POSIX 1003.2
standard, and some of which differ between Linux versions.

These include the command processor (shell), compilers, editors, text processing programs, and
file manipulation utilities. It is the programs that a user at the keyboard invokes.
Thus we can speak of three different interfaces to Linux—the true system call interface, the
library interface, and the interface formed by the set of standard utility programs. Most personal
computer versions of Linux have replaced this keyboard-oriented user interface with a mouse-
oriented graphical user interface, without changing the operating system itself at all. It is
precisely this flexibility that makes Linux so popular and has allowed it to survive numerous
changes in the underlying technology so well. The GUI for Linux is similar to the first GUIs
developed for UNIX systems in the 1970s, and popularized by Macintosh and later Windows
for PC platforms. The GUI creates a desktop environment, a familiar metaphor with windows,
icons, folders, toolbars, and drag-and-drop capabilities. A full desktop environment contains a
window manager, which controls the placement and appearance of windows, as well as various
applications, and provides a consistent graphical interface. Popular desktop environments for
Linux include GNOME (GNU Network Object Model Environment) and KDE (K Desktop
Environment). GUIs on Linux are supported by the X Windowing System, or commonly X11
or just X, which defines communication and display protocols for manipulating windows on
bitmap displays for UNIX and UNIX-like systems. The X server is the main component which
controls devices such as keyboards, mouse, screen and is responsible for redirecting input to
or accepting output from client programs. The actual GUI environment is typically built on
top of a low-level library, xlib, which contains the functionality to interact with the X server.
The graphical interface extends the basic functionality of X11 by enriching the windows view,
providing buttons, menus, icons, and other options. The X server can be started manually,
from a command line, but is typically started during the boot process by a display manager,

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