Operating System Topics

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• system families
• system 360
• application programs
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• handheld computers
• cpu time
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• desktop computers
• user interface
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Operating System, Operating System

The operating system acts as a host for computing applications that are run on the machine. As a host, one of the purposes of an operating system is to handle the details of the operation of the hardware. This relieves application programs from having to manage these details and makes it easier to write applications. Almost all computers (including handheld computers, desktop computers, supercomputers, video game consoles) as well as some robots, domestic appliances (dishwashers, washing machines), and portable media players use an operating system of some type.

For hand-held and desktop computers, the user interface is generally considered part of the operating system. On large multi-user systems like Unix and Unix-like systems, the user interface is generally implemented as an application program that runs outside the operating system. (Whether the user interface should be included as part of the operating system is a point of contention.) Common contemporary operating system families include BSD, Darwin , Linux, SunOS , and Windows NT . While servers generally run Unix or some Unix-like operating system, embedded system markets are split amongst several operating systems.

Application programs running on these operating systems must yield CPU time to the scheduler when they are not using it, either by default, or by calling a function.

The development of the IBM System/360 produced a family of mainframe computers available in widely differing capacities and price points, for which a single operating system OS/360 was planned (rather than developing ad-hoc programs for every individual model). This concept of a single OS spanning an entire product line was crucial for the success of System/360 and, in fact, IBM`s current mainframe operating systems are distant descendants of this original system; applications written for the OS/360 can still be run on modern machines. In the mid-70's, the MVS, the descendant of OS/360 offered the first implementation of using RAM as a transparent cache for disk resident data.

For instance, in OS/360, when a program is started, the operating system keeps track of all of the system resources that are used including storage, locks, data files, and so on. When the process is terminated for any reason, all of these resources are re-claimed by the operating system. An alternative CP-67 system started a whole line of operating systems focused on the concept of virtual machines.

Like many commercial timesharing systems, its interface was an extension of the Dartmouth BASIC operating systems, one of the pioneering efforts in timesharing and programming languages. In the late 1970s, Control Data and the University of Illinois developed the PLATO operating system, which used plasma panel displays and long-distance time sharing networks. Plato was remarkably innovative for its time, featuring real-time chat, and multi-user graphical games.Burroughs Corporation introduced the B5000 in 1961 with the MCP, (Master Control Program) operating system. The B5000 was a stack machine designed to exclusively support high-level languages with no machine language or assembler, and indeed the MCP was the first OS to be written exclusively in a high-level language – ESPOL, a dialect of ALGOL. MCP also introduced many other ground-breaking innovations, such as being the first commercial implementation of virtual memory. During development of the AS400, IBM made an approach to Burroughs to licence MCP to run on the AS400 hardware. This proposal was declined by Burroughs management to protect its existing hardware production. MCP is still in use today in the Unisys ClearPath/MCP line of computers.

One notable early disk-based operating system was CP/M, which was supported on many early microcomputers and was closely imitated in MS-DOS, which became wildly popular as the operating system chosen for the IBM PC (IBM's version of it was called IBM DOS or PC DOS), its successors making Microsoft. In the 80's Apple Computer Inc. (now Apple Inc.) abandoned its popular Apple II series of microcomputers to introduce the Apple Macintosh computer with an innovative Graphical User Interface (GUI) to the Mac OS.

Executing a program involves the creation of a process by the operating system. The kernel creates a process by assigning memory and other resources, establishing a priority for the process (in multi-tasking systems), loading program code into memory, and executing the program. The program then interacts with the user and/or other devices and performs its intended function.

If a program wishes to access hardware for example, it may interrupt the operating system's kernel, which causes control to be passed back to the kernel. The kernel will then process the request.

Similar modes have existed in supercomputers, minicomputers, and mainframes as they are essential to fully supporting UNIX-like multi-user operating systems.

The first few programs to run on the computer, being the BIOS, bootloader and the operating system have unlimited access to hardware - and this is required because, by definition, initializing a protected environment can only be done outside of one. However, when the operating system passes control to another program, it can place the CPU into protected mode.

A user program may leave protected mode only by triggering an interrupt, causing control to be passed back to the kernel. In this way the operating system can maintain exclusive control over things like access to hardware and memory.

If a program fails it may cause memory used by one or more other programs to be affected or overwritten. Malicious programs, or viruses may purposefully alter another program's memory or may affect the operation of the operating system itself. With cooperative memory management it takes only one misbehaved program to crash the system.

These limitations often reflected limitations in the operating systems they were designed for, making it very difficult for an operating system to support more than one file system.

A VFS allows the operating system to provide programs with access to an unlimited number of devices with an infinite variety of file systems installed on them through the use of specific device drivers and file system drivers.

The device driver understands the specific language of the drive and is able to translate that language into a standard language used by the operating system to access all disk drives. On UNIX this is the language of block devices.

A file system driver is used to translate the commands used to access each specific file system into a standard set of commands that the operating system can use to talk to all file systems. Programs can then deal with these file systems on the basis of filenames, and directories/folders, contained within a hierarchical structure. They can create, delete, open, and close files, as well as gather various information about them, including access permissions, size, free space, and creation and modification dates.

Typically this constitutes an interface for communicating with the device, through the specific computer bus or communications subsystem that the hardware is connected to, providing commands to and/or receiving data from the device, and on the other end, the requisite interfaces to the operating system and software applications. It is a specialized hardware-dependent computer program which is also operating system specific that enables another program, typically an operating system or applications software package or computer program running under the operating system kernel, to interact transparently with a hardware device, and usually provides the requisite interrupt handling necessary for any necessary asynchronous time-dependent hardware interfacing needs.

Every model of hardware (even within the same class of device) is different. Newer models also are released by manufacturers that provide more reliable or better performance and these newer models are often controlled differently. Computers and their operating systems cannot be expected to know how to control every device, both now and in the future. To solve this problem, OSes essentially dictate how every type of device should be controlled. The function of the device driver is then to translate these OS mandated function calls into device specific calls. In theory a new device, which is controlled in a new manner, should function correctly if a suitable driver is available. This new driver will ensure that the device appears to operate as usual from the operating systems' point of view.

Each port number is usually associated with a maximum of one running program, which is responsible for handling requests to that port. A demon, being a user program, can in turn access the local hardware resources of that computer by passing requests to the operating system kernel.

Security of operating systems has long been a concern because of highly sensitive data held on computers, both of a commercial and military nature. The United States Government Department of Defense (DoD) created the Trusted Computer System Evaluation Criteria (TCSEC) which is a standard that sets basic requirements for assessing the effectiveness of security. This became of vital importance to operating system makers, because the TCSEC was used to evaluate, classify and select computer systems being considered for the processing, storage and retrieval of sensitive or classified information.

At the operating system level, there are a number of software firewalls available, as well as intrusion detection/prevention systems. Most modern operating systems include a software firewall, which is enabled by default. A software firewall can be configured to allow or deny network traffic to or from a service or application running on the operating system. Therefore, one can install and be running an insecure service, such as Telnet or FTP, and not have to be threatened by a security breach because the firewall would deny all traffic trying to connect to the service on that port.

Internal security is also vital if auditing is to be of any use, since a program can potentially bypass the operating system, inclusive of bypassing auditing.

However, a lot of the advantages of being a true multi-user operating system were nullified by the fact that, prior to Windows Vista, the first user account created during the setup process was an administrator account, which was also the default for new accounts. Though Windows XP did have limited accounts, the majority of home users did not change to an account type with fewer rights – partially due to the number of programs which unnecessarily required administrator rights – and so most home users ran as administrator all the time.

Modern Solaris based operating systems eclipse the need for volume management through leveraging virtual storage pools in ZFS.

Others have been developed by companies to meet their specific needs, hobbyists, or adapted from UNIX, Microsoft Windows, and other operating systems. Linux has full support for XFS and JFS, along with FAT (the MS-DOS file system), and HFS which is the primary file system for the Macintosh.

ISO 9660 and Universal Disk Format (UDF) are supported which are standard file systems used on CDs, DVDs, and BluRay discs. It is possible to install Linux on the majority of these file systems. Unlike other operating systems, Linux and UNIX allow any file system to be used regardless of the media it is stored in, whether it is a hard drive, a disc (CD,DVD...), an USB key, or even contained within a file located on another file system.

Other operating systems are modular, separating the graphics subsystem from the kernel and the Operating System. In the 1980s UNIX, VMS and many others had operating systems that were built this way. Linux and Mac OS X are also built this way. Modern releases of Microsoft Windows such as Windows Vista implement a graphics subsystem that is mostly in user-space, however versions between Windows NT 4.0 and Windows Server 2003's graphics drawing routines exist mostly in kernel space.

B was replaced by C, and Unix developed into a large, complex family of inter-related operating systems which have been influential in every modern operating system (see History).The Unix-like family is a diverse group of operating systems, with several major sub-categories including System V, BSD, and Linux. The name "UNIX" is a trademark of The Open Group which licenses it for use with any operating system that has been shown to conform to their definitions. "Unix-like" is commonly used to refer to the large set of operating systems which resemble the original Unix.

The POSIX standard can be applied to any operating system, although it was originally created for various Unix variants.

Mac OS X is the world's second most widely used OS after Windows. Mac OS X is the successor to the original Mac OS, which had been Apple's primary operating system since 1984. Unlike its predecessor, Mac OS X is a UNIX operating system built on technology that had been developed at NeXT through the second half of the 1980s and up until Apple purchased the company in early 1997.

Plan 9 was designed from the start to be a networked operating system, and had graphics built-in, unlike Unix, which added these features to the design later. Plan 9 has yet to become as popular as Unix derivatives, but it has an expanding community of developers. It is currently released under the Lucent Public License. Inferno was sold to Vita Nuova Holdings and has been released under a GPL/MIT license.

Some, most notably AmigaOS 4 and RISC OS, continue to be developed as minority platforms for enthusiast communities and specialist applications. OpenVMS formerly from DEC, is still under active development by Hewlett-Packard.There were a number of operating systems for 8 bit computers - Apple's DOS (Disk Operating System) 3.2 & 3.3 for Apple II, ProDOS, UCSD, CP/M - available for various 8 and 16 bit environments.

Singularity is a project at Microsoft Research to develop an operating system with better memory protection based on the .Net managed code model. Systems development follows the same model used by other Software development, which involves maintainers, version control "trees", of revision control software forks, "patches", and specifications. From the AT&T-Berkeley lawsuit the new unencumbered systems were based on 4.4BSD which forked as FreeBSD and NetBSD efforts to replace missing code after the Unix wars. Recent forks include DragonFly BSD and Darwin from BSD Unix.

Source: Wikipedia > Operating System