Operating Systems

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Operating system Summary

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Operating Systems

The operating system is software that manages every part of a computer system—all hardware and all other software. To be specific, it controls every file, every device, every section of main memory, every nanosecond of processing time, and every network connection. It controls who can use the system and how. In short, it is the boss—without it, nothing can happen.

When a computer user sends a command by typing it from the keyboard or clicking with the mouse, the operating system must make sure that the command is executed. If it is not executed, the operating system must arrange for the user to receive a message, usually on the monitor, explaining

Platform	Operating System
Microcomputers	Linux, Macintosh OS, MS-DOS, Windows 98, Windows 2000
Minicomputers	Linux, OpenVMS Alpha, UNIX
Mainframe computers	IBM OS/390, IBM OS/400, UNIX
Supercomputers	IRIX, UNICOS
Workstations	HP-UX, Sun Solaris, UNIX
Networks	Novell NetWare, UNIX, Windows NT, Windows 2000
Handheld computers	Microsoft CE, Palm OS

the error. This does not necessarily mean that the operating system executes the command or sends the error message, but it does control the parts of the system that do.

Popular operating systems associated with various platforms.

Every operating system, regardless of its size and complexity, can be represented by a pyramid showing how its five major functions (called managers) work together. The memory manager, the processor manager, the device manager, and the file manager form the pyramid's base; network operating systems add a network manager as well. The user interface —the part of the operating system that communicates with the user—is supported by the other four or five managers.

Responsibilities and Relationships

These virtual managers must do more than perform their individual tasks. They must also be able to work harmoniously with every other manager. For example, they must be able to monitor their resources continuously, enforce the policies that determine who gets what, when, and how much, allocate their resources when it is appropriate, and de-allocate their resources—reclaim them—when appropriate.

The memory manager is in charge of main memory, also known as random access memory (RAM). It checks the validity of each request for memory space and, if it is a legal request, the memory manager allocates a portion that is not already in use. In a multi-user environment, the memory manager sets up a table to keep track of who is using which section of memory. Finally, when the time comes to reclaim the memory, the memory manager de-allocates the memory space. One of the manager's primary responsibilities is to preserve the part of main memory that is occupied by the operating system itself—it cannot allow any part of it to be altered accidentally or intentionally.

The processor manager decides how to allocate the central processing unit (CPU) and keep track of the status of each executable step of every program (called a process or task). For example, the processor manager monitors whether the CPU is executing a process or waiting for a READ or WRITE command to finish execution. Later, when the process is finished, or the maximum amount of time has expired, the processor manager reclaims the processor so it can be allocated again.

The device manager chooses the most efficient way to allocate all of the system's devices, including printers, disk drives, CD-ROMs (compact disc-read only memory), keyboard, monitor, and so forth. The device manager makes these decisions based on a scheduling policy chosen by the system's designers. The device manager allocates a device, starts its operation, and, finally, de-allocates it.

The file manager keeps track of every piece of software in the system, including application programs, data files, directories, etc. The file manager allocates the file by opening it and de-allocates it by closing it. It is based on predetermined access policies to enforce the correct security for each file so that files can be accessed only by individual or group users that have permission to do so. The file manager also controls the amount of flexibility each user is allowed with that file (such as read-only, read-and-write-only, or the authority to create and/or delete records).

Operating systems with networking capability have a fifth element called the network manager, which provides a convenient way for users to share resources. Network resources usually include both hardware (such as CPUs, memory areas, printers, disk drives, modems, and tape drives) and software (such as application programs and data files).

User Interfaces

Most modern operating systems feature a menu-driven graphical user interface (GUI), which is pronounced "gooey," with menus, icons, and task bars. The Macintosh was the first widely used computer with a GUI, which in turn was based on a desktop created by Xerox. Microsoft introduced Windows version 1.0 in 1985, but it was not a real operating system because it acted merely as an interface between the user and the real operating system. Instead, it was called an "environment" that ran only on computers with the MS-DOS operating system. Microsoft's Windows 95, Windows 98, and Windows 2000 are true operating systems.

Operating systems without GUIs (such as early versions of UNIX and Linux) are called command-driven systems. They accept commands that are typed into the system (menus are not available). Command-driven systems are cumbersome for some new users to learn. Therefore, since the late-1990s, most operating systems (including UNIX and Linux) have been converted from command-driven to menu-driven interfaces, which feature GUIs that allow users to click on menus to make the system run.

Adding GUIs was a popular move for anyone who had trouble working with brief or mysterious-looking command lines. For example, the UNIX command to list all subdirectories (but not files) found in the root directory looks cryptic (ls-l / | grep ' [.logicaland] d'). Today, users can achieve a similar result by choosing an option from a menu.

Linux

Linux is an operating system that has been widely adopted in commercial and academic markets around the world. Linux is unique among the most-used operating systems because it is an open-source program, which means the source code is freely available to anyone for improvement.

Programmers from around the world are encouraged to submit improvements to the programming code. If the new code is accepted as a universal improvement to the operating system, it is added to the next version, which is then made available to the computing world for further improvement. This development technique has resulted in a powerful, stable, inexpensive operating system, which is constantly being improved by a variety of people who may never meet in person.

Linux was created in Finland by 21-year-old Linus Torvalds who wanted to build a new operating system for the Intel 80386 microprocessor. Torvalds started with Minix, a miniature version of the well-known UNIX operating system, and rewrote certain parts to add more functionality. The first version of Linux, which was named for Torvalds and UNIX, had much of the basic functionality of the then-popular MS-DOS operating system with UNIX-like power and flexibility. It has been enhanced considerably in the years since.

Although there are similarities between the two systems, Linux is not UNIX. (UNIX is a legal trademark, registered with the federal government. Before a developer can use the term UNIX to describe an operating system, it must demonstrate that it can meet certain certification criteria.)

Policies and Design Considerations

One of the biggest differences among operating systems is the set of policies on which each one is based. These policies, in turn, drive design considerations, which dictate the inner workings of the system, including the following:

Processor time—the amount of uninterrupted time the processor is allocated to a certain job.
Memory space—the amount of memory area that a given job is allowed to monopolize at any time.
Printers—the number of printers that one job is allowed to use.
User access—the number of users who are allowed to log into the system.
File access—the identity of the files that a given user can read, write, modify, or delete.
System resources—the number of system resources that can be allocated to one job before it has to share them.

Before writing an operating system, these issues are examined by system designers who make choices that ideally will optimize the system's day-to-day operations. The goal is to minimize downtime, system crashes, wasted overhead, security breeches, overloaded printers, and other operational problems.

For example, if the designers want to create a simple operating system that would process each job in the order it arrives, without giving a higher priority to any of them, then the team might choose policies that would:

Assign the processor to one job when it is received;
Never interrupt processing once the job begins;
Give it access to all files in the system, just in case they are needed later;
Give the job all available disk space, printers, and network resources.

This would be a fair, unbiased system, but a very inefficient one. For example, if one big job was printing out thousands of pages for a large report, even if the printing required several days, then all other jobs would sit idle as they waited for it to finish. In the meantime, most of the availablememory space, processor time, disk space, and other resources would also sit idle, waiting for the next job to begin.

Therefore, most modern operating systems feature complex formulas, which allow resources to be allocated wisely. Some systems allow multiple tasks, multiple jobs, multiple users, and even multiple processors, to work together so available resources can be shared without causing the system to crash routinely.

No single operating system can be considered the best for every situation because each one is based on policies that favor certain jobs or certain circumstances. That is why one operating system might be chosen to run an architect's computer and another might be preferred to operate a writer's computer.

Ann McIver McHoes