OS

SOURCE:WIKIPEDIA

An operating system (commonly abbreviated OS and O/S) is the software component of a computer system that is responsible for the management and coordination of activities and the sharing of the resources of the computer. The operating system acts as a host for application programs 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 hand-held computers, desktop computers, supercomputers, and even modern video game consoles, use an operating system of some type. Some of the oldest models may however use an embedded OS, that may be contained on a compact disk or other storage device.

Operating systems offer a number of services to application programs and users. Applications access these services through application programming interfaces (APIs) or system calls. By invoking these interfaces, the application can request a service from the operating system, pass parameters, and receive the results of the operation. Users may also interact with the operating system by typing commands or using a graphical user interface (GUI, commonly pronounced “gooey”). For hand-held and desktop computers, the GUI is generally considered part of the operating system. For large multi-user systems, the GUI is generally implemented as an application program that runs outside the operating system.

Common contemporary operating systems include Microsoft Windows, Mac OS X, Linux and Solaris. Microsoft Windows has a significant majority of market share in the desktop and notebook computer markets, while servers generally run on Linux or other Unix-like systems. Embedded device markets are split amongst several operating systems.^[1][2]

An operating system is a collection of technologies which are designed to allow the computer to perform certain functions. These technologies may or may not be present in every operating system, and there are often differences in how they are implemented. However as stated above most modern operating systems are derived from common design ancestors, and are therefore basically similar.

Boot-strapping

In most cases, the operating system is not the first code to run on the computer at startup (boot) time. The initial code executing on the computer is usually loaded from firmware, which is stored in read only memory (ROM). This is sometimes called the BIOS or boot ROM.

The firmware loads and executes code located on a removable disk or hard drive, and contained within the first sector of the drive, referred to as the boot sector. The code stored on the boot sector is called the boot loader, and is responsible for loading the operating system’s kernel from disk and starting it running.

Some simple boot loaders are designed to locate one specific operating system and load it, although many modern ones have the capacity to allow the user to choose from a number of operating systems.

Program execution

An operating system’s most basic function is to support the running of programs by the users. On a multiprogramming operating system, running programs are commonly referred to as processes. Process management refers to the facilities provided by the operating system to support the creation, execution, and destruction of processes, and to facilitate various interactions, and limit others.

The operating system’s kernel in conjunction with underlying hardware must support this functionality.

Executing a program involves the creation of a process by the operating system. The kernel creates a process by setting aside or allocating some memory, loading program code from a disk or another part of memory into the newly allocated space, and starting it running.

Operating system kernels store various information about running processes. This information might include:

• A unique identifier, called a process identifier (PID).
• A list of memory the program is using, or is allowed to access.
• The PID of the program which requested its execution, or the parent process ID (PPID).
• The filename and/or path from which the program was loaded.
• A register file, containing the last values of all CPU registers.
• A program counter, indicating the position in the program.

Interrupts

Interrupts are central to operating systems as they allow the operating system to deal with the unexpected activities of running programs and the world outside the computer. Interrupt-based programming is one of the most basic forms of time-sharing, being directly supported by most CPUs. Interrupts provide a computer with a way of automatically running specific code in response to events. Even very basic computers support hardware interrupts, and allow the programmer to specify code which may be run when that event takes place.

When an interrupt is received, the computer’s hardware automatically suspends whatever program is currently running by pushing the current state on a stack, and its registers and program counter are also saved. This is analogous to placing a bookmark in a book when someone is interrupted by a phone call. This task requires no operating system as such, but only that the interrupt be configured at an earlier time.

In modern operating systems, interrupts are handled by the operating system’s kernel. Interrupts may come from either the computer’s hardware, or from the running program. When a hardware device triggers an interrupt, the operating system’s kernel decides how to deal with this event, generally by running some processing code, or ignoring it. The processing of hardware interrupts is a task that is usually delegated to software called device drivers, which may be either part of the operating system’s kernel, part of another program, or both. Device drivers may then relay information to a running program by various means.

A program may also trigger an interrupt to the operating system, which are very similar in 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 may then process the request which may contain instructions to be passed onto hardware, or to a device driver. When a program wishes to allocate more memory, launch or communicate with another program, or signal that it no longer needs the CPU, it does so through interrupts.

Protected mode and supervisor mode

Modern CPUs support something called dual mode operation. CPUs with this capability use two modes: protected mode and supervisor mode, which allow certain CPU functions to be controlled and affected only by the operating system kernel. Here, protected mode does not refer specifically to the 80286 (Intel’s x86 16-bit microprocessor) CPU feature, although its protected mode is very similar to it. CPUs might have other modes similar to 80286 protected mode as well, such as the virtual 8086 mode of the 80386 (Intel’s x86 32-bit microprocessor or i386).

However, the term is used here more generally in operating system theory to refer to all modes which limit the capabilities of programs running in that mode, providing things like virtual memory addressing and limiting access to hardware in a manner determined by a program running in supervisor mode. Similar modes have existed in supercomputers, minicomputers, and mainframes as they are essential to fully supporting UNIX-like multi-user operating systems.

When a computer first starts up, it is automatically running in supervisor mode. The first few programs to run on the computer, being the BIOS, bootloader and the operating system have unlimited access to hardware. However when the operating system passes control to another program, it can place the CPU into protected mode.

In protected mode, programs may have access to a more limited set of the CPU’s instructions. 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.

The term “protected mode resource” generally refers to one or more CPU registers, which contain information that the running program isn’t allowed to alter. Attempts to alter these resources generally causes a switch to supervisor mode.

Memory management

Among other things, a multiprogramming operating system kernel must be responsible for managing all system memory which is currently in use by programs. This ensures that a program does not interfere with memory already used by another program. Since programs time share, each program must have independent access to memory.

Cooperative memory management, used by many early operating systems assumes that all programs make voluntary use of the kernel’s memory manager, and do not exceed their allocated memory. This system of memory management is almost never seen anymore, since programs often contain bugs which can cause them to exceed their allocated 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.

Memory protection enables the kernel to limit a process’ access to the computer’s memory. Various methods of memory protection exist, including memory segmentation and paging. All methods require some level of hardware support (such as the 80286 MMU) which doesn’t exist in all computers.

In both segmentation and paging, certain protected mode registers specify to the CPU what memory address it should allow a running program to access. Attempts to access other addresses will trigger an interrupt which will cause the CPU to re-enter supervisor mode, placing the kernel in charge. This is called a segmentation violation or Seg-V for short, and since it is usually a sign of a misbehaving program, the kernel will generally kill the offending program, and report the error.

Windows 3.1-Me had some level of memory protection, but programs could easily circumvent the need to use it. Under Windows 9x all MS-DOS applications ran in supervisor mode, giving them almost unlimited control over the computer. A general protection fault would be produced indicating a segmentation violation had occurred, however the system would often crash anyway.

Virtual memory

The use of virtual memory addressing (such as paging or segmentation) means that the kernel can choose which memory each program may use at any given time, allowing the operating system to use the same memory locations for multiple tasks.

If a program tries to access memory that isn’t in its current range of accessible memory, but nonetheless has been allocated to it, the kernel will be interrupted in the same way as it would if the program were to exceed its allocated memory. (See section on memory management.) Under UNIX this kind of interrupt is referred to as a page fault.

When the kernel detects a page fault it will generally adjust the virtual memory range of the program which triggered it, granting it access to the memory requested. This gives the kernel discretionary power over where a particular application’s memory is stored, or even whether or not it has actually been allocated yet.

In modern operating systems, application memory which is accessed less frequently can be temporarily stored on disk or other media to make that space available for use by other programs. This is called swapping, as an area of memory can be use by multiple programs, and what that memory area contains can be swapped or exchanged on demand.

Multitasking

Multitasking refers to the running of multiple independent computer programs on the same computer, giving the appearance that it is performing the tasks at the same time. Since most computers can do at most one or two things at one time, this is generally done via time sharing, which means that each program uses a share of the computer’s time to execute.

An operating system kernel contains a piece of software called a scheduler which determines how much time each program will spend executing, and in which order execution control should be passed to programs. Control is passed to a process by the kernel, which allows the program access to the CPU and memory. At a later time control is returned to the kernel through some mechanism, so that another program may be allowed to use the CPU. This so-called passing of control between the kernel and applications is called a context switch.

An early model which governed the allocation of time to programs was called cooperative multitasking. In this model, when control is passed to a program by the kernel, it may execute for as long as it wants before explicitly returning control to the kernel. This means that a malfunctioning program may prevent any other programs from using the CPU.

The philosophy governing preemptive multitasking is that of ensuring that all programs are given regular time on the CPU. This implies that all programs must be limited in how much time they are allowed to spend on the CPU without being interrupted. To accomplish this, modern operating system kernels make use of a timed interrupt. A protected mode timer is set by the kernel which triggers a return to supervisor mode after the specified time has elapsed. (See above sections on Interrupts and Dual Mode Operation.)

On many single user operating systems cooperative multitasking is perfectly adequate, as home computers generally run a small number of well tested programs. Windows NT was the first version of Microsoft Windows which enforced preemptive multitasking, but it didn’t reach the home user market until Windows XP, (since Windows NT was targeted at professionals.)

Very early UNIX did not provide preemptive multitasking, as it was not supported on the hardware. As it was designed from minicomputer concepts for multiple users accessing the system via remote terminals, it was quickly migrated to hardware which did allow preemptive multitasking, and was adapted to support these features.

Modern operating systems extend the concepts of application preemption to device drivers and kernel code, so that the operating system has preemptive control over internal run-times as well. Under Windows Vista, the introduction of the Windows Display Driver Model (WDDM) accomplishes this for display drivers, and in Linux, the preemptable kernel model introduced in version 2.6 allows all device drivers and some other parts of kernel code to take advantage of preemptive multi-tasking.

Under Windows and prior to Windows Vista and Linux prior to version 2.6 all driver execution was co-operative, meaning that if a driver entered an infinite loop it would freeze the system.

Disk access and file systems

Access to files stored on disks is a central feature of all operating systems. Computers store data on disks using files, which are structured in specific ways in order to allow for faster access, higher reliability, and to make better use out of the drive’s available space. The specific way files are stored on a disk is called a file system, and enables files to have names and attributes. It also allows them to be stored in a hierarchy of directories or folders arranged in a directory tree.

Early operating systems generally supported a single type of disk drive and only one kind of file system. Early file systems were limited in their capacity, speed, and in the kinds of file names and directory structures they could use. 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.

While many simpler operating systems support a limited range of options for accessing storage systems, more modern operating systems like UNIX and Linux support a technology known as a virtual file system or VFS. A modern operating system like UNIX supports a wide array of storage devices, regardless of their design or file systems to be accessed through a common application programming interface (API). This makes it unnecessary for programs to have any knowledge about the device they are accessing. 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.

A connected storage device such as a hard drive will be accessed through a device driver. 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.

When the kernel has an appropriate device driver in place, it can then access the contents of the disk drive in raw format, which may contain one or more file systems. 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.

Various differences between file systems make supporting all file systems difficult. Allowed characters in file names, case sensitivity, and the presence of various kinds of file attributes makes the implementation of a single interface for every file system a daunting task. While UNIX and Linux systems generally have support for a wide variety of file systems, proprietary operating systems such a Microsoft Windows tend to limit the user to using a single file system for each task. For example the Windows operating system can only be installed on NTFS, and CDs and DVDs can only be recorded using UDF or ISO 9660

Device drivers

A device driver is a specific type of computer software developed to allow interaction with hardware devices. 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.

The key design goal of device drivers is abstraction. 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 for any person.

Networking

Currently most operating systems support a variety of networking protocols, hardware, and applications for using them. This means that computers running dissimilar operating systems can participate in a common network for sharing resources such as computing, files, printers, and scanners using either wired or wireless connections. Networks can essentially allow a computer’s operating system to access the resources of a remote computer to support the same functions as it could if those resources were connected directly to the local computer. This includes everything from simple communication, to using networked file systems or even sharing another computer’s graphics or sound hardware. Some network services allow the resources of a computer to be accessed transparently, such as SSH which allows networked users direct access to a computer’s command line interface.

Client/server networking involves a program on a computer somewhere which connects via a network to another computer, called a server. Servers, usually running UNIX or Linux, offer (or host) various services to other network computers and users. These services are usually provided through ports or numbered access points beyond the server’s network address. Each port number is usually associated with a maximum of one running program, which is responsible for handling requests to that port. A daemon, being a user program, can in turn access the local hardware resources of that computer by passing requests to the operating system kernel.

Many operating systems support one or more vendor-specific or open networking protocols as well, for example, SNA on IBM systems, DECnet on systems from Digital Equipment Corporation, and Microsoft-specific protocols on Windows. Specific protocols for specific tasks may also be supported such as NFS for file access. Protocols like ESound, or esd can be easily extended over the network to provide sound from local applications, on a remote system’s sound hardware.

Security

A computer being secure depends on a number of technologies working properly. A modern operating system provides access to a number of resources, which are available to software running on the system, and to external devices like networks via the kernel.

The operating system must be capable of distinguishing between requests which should be allowed to be processed, and others which should not be processed. While some systems may simply distinguish between “privileged” and “non-privileged”, systems commonly have a form of requester identity, such as a user name. To establish identity there may be a process of authentication. Often a username must be quoted, and each username may have a password. Other methods of authentication, such as magnetic cards or biometric data, might be used instead. In some cases, especially connections from the network, resources may be accessed with no authentication at all.

The first computers did not have operating systems. By the early 1960s, commercial computer vendors were supplying quite extensive tools for streamlining the development, scheduling, and execution of jobs on batch processing systems. Examples were produced by UNIVAC and Control Data Corporation, amongst others.

MS-DOS provided many operating system like features, such as disk access. However many DOS programs bypassed it entirely and ran directly on hardware.

The operating systems originally deployed on mainframes, and, much later, the original microcomputer operating systems, only supported one program at a time, requiring only a very basic scheduler. Each program was in complete control of the machine while it was running. Multitasking (timesharing) first came to mainframes in the 1960s.

In 1969-70, UNIX first appeared on the PDP-7 and later the PDP-11. It soon became capable of providing cross-platform time sharing using preemptive multitasking, advanced memory management, memory protection, and a host of other advanced features. UNIX soon gained popularity as an operating system for mainframes and minicomputers alike.

IBM microcomputers, including the IBM PC and the IBM PC XT could run Microsoft Xenix, a UNIX-like operating system from the early 1980s. Xenix was heavily marketed by Microsoft as a multi-user alternative to its single user MS-DOS operating system. The CPUs of these personal computer, could not facilitate kernel memory protection or provide dual mode operation, so Microsoft Xenix relied on cooperative multitasking and had no protected memory.

The 80286-based IBM PC AT was the first computer technically capable of using dual mode operation, and providing memory protection.

Classic Mac OS, and Microsoft Windows 1.0-3.11 supported only cooperative multitasking (Windows 95, 98, & ME supported preemptive multitasking only when running 32 bit applications, but ran legacy 16 bit applications using cooperative multitasking), and were very limited in their abilities to take advantage of protected memory. 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.

Windows NT’s underlying operating system kernel which was a designed by essentially the same team as Digital Equipment Corporation’s VMS, a UNIX-like operating system which provided protected mode operation for all user programs, kernel memory protection, preemptive multi-tasking, virtual file system support, and a host of other features.

Classic AmigaOS and Windows 1.0-Me did not properly track resources allocated by processes at runtime. If a process had to be terminated, the resources might not be freed up for new programs until the machine was restarted.

The AmigaOS did have preemptive multitasking.