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6,500 | Yet other operating systems are used almost exclusively in academia, for operating systems education or to do research on operating system concepts. A typical example of a system that fulfills both roles is MINIX, while for example Singularity is used purely for research. Another example is the Oberon System designed at ETH Zürich by Niklaus Wirth, Jürg Gutknecht and a group of students at the former Computer Systems Institute in the 1980s. It was used mainly for research, teaching, and daily work in Wirth's group. | https://en.wikipedia.org/wiki?curid=22194 |
6,501 | Other operating systems have failed to win significant market share, but have introduced innovations that have influenced mainstream operating systems, not least Bell Labs' Plan 9. | https://en.wikipedia.org/wiki?curid=22194 |
6,502 | The components of an operating system all exist in order to make the different parts of a computer work together. All user software needs to go through the operating system in order to use any of the hardware, whether it be as simple as a mouse or keyboard or as complex as an Internet component. | https://en.wikipedia.org/wiki?curid=22194 |
6,503 | With the aid of firmware and device drivers, the kernel provides the most basic level of control over all of the computer's hardware devices. It manages memory access for programs in the RAM, it determines which programs get access to which hardware resources, it sets up or resets the CPU's operating states for optimal operation at all times, and it organizes the data for long-term non-volatile storage with file systems on such media as disks, tapes, flash memory, etc. | https://en.wikipedia.org/wiki?curid=22194 |
6,504 | The operating system provides an interface between an application program and the computer hardware, so that an application program can interact with the hardware only by obeying rules and procedures programmed into the operating system. The operating system is also a set of services which simplify development and execution of application programs. Executing an application program typically involves the creation of a process by the operating system kernel, which assigns memory space and other resources, establishes a priority for the process in multi-tasking systems, loads program binary code into memory, and initiates execution of the application program, which then interacts with the user and with hardware devices. However, in some systems an application can request that the operating system execute another application within the same process, either as a subroutine or in a separate thread, e.g., the LINK and ATTACH facilities of OS/360 and successors.. | https://en.wikipedia.org/wiki?curid=22194 |
6,505 | An interrupt (also known as abort, exception, "fault", signal and "trap") provides an efficient way for most operating systems to react to the environment. Interrupts cause the central processing unit (CPU) to have a control flow change away from the currently running program to an interrupt handler, also known as an interrupt service routine (ISR). An interrupt service routine may cause the central processing unit (CPU) to have a context switch . The details of how a computer processes an interrupt vary from architecture to architecture, and the details of how interrupt service routines behave vary from operating system to operating system. However, several interrupt functions are common. The architecture and operating system must: | https://en.wikipedia.org/wiki?curid=22194 |
6,506 | A software interrupt is a message to a process that an event has occurred. This contrasts with a "hardware interrupt" — which is a message to the central processing unit (CPU) that an event has occurred. Software interrupts are similar to hardware interrupts — there is a change away from the currently running process. Similarly, both hardware and software interrupts execute an interrupt service routine. | https://en.wikipedia.org/wiki?curid=22194 |
6,507 | Software interrupts may be normally occurring events. It is expected that a time slice will occur, so the kernel will have to perform a context switch. A computer program may set a timer to go off after a few seconds in case too much data causes an algorithm to take too long. | https://en.wikipedia.org/wiki?curid=22194 |
6,508 | Software interrupts may be error conditions, such as a malformed machine instruction. However, the most common error conditions are division by zero and accessing an invalid memory address. | https://en.wikipedia.org/wiki?curid=22194 |
6,509 | Users can send messages to the kernel to modify the behavior of a currently running process. For example, in the command-line environment, pressing the "interrupt character" (usually Control-C) might terminate the currently running process. | https://en.wikipedia.org/wiki?curid=22194 |
6,510 | To generate "software interrupts" for x86 CPUs, the INT assembly language instruction is available. The syntax is codice_1, where codice_2 is the offset number (in hexadecimal format) to the interrupt vector table. | https://en.wikipedia.org/wiki?curid=22194 |
6,511 | To generate "software interrupts" in Unix-like operating systems, the codice_3 system call will send a signal to another process. codice_4 is the process identifier of the receiving process. codice_5 is the signal number (in mnemonic format) to be sent. (The abrasive name of codice_6 was chosen because early implementations only terminated the process.) | https://en.wikipedia.org/wiki?curid=22194 |
6,512 | In Unix-like operating systems, "signals" inform processes of the occurrence of asynchronous events. To communicate asynchronously, interrupts are required. One reason a process needs to asynchronously communicate to another process solves a variation of the classic reader/writer problem. The writer receives a pipe from the shell for its output to be sent to the reader's input stream. The command-line syntax is codice_7. codice_8 will write to the pipe when its computation is ready and then sleep in the wait queue. codice_9 will then be moved to the ready queue and soon will read from its input stream. The kernel will generate "software interrupts" to coordinate the piping. | https://en.wikipedia.org/wiki?curid=22194 |
6,513 | Input/Output (I/O) devices are slower than the CPU. Therefore, it would slow down the computer if the CPU had to wait for each I/O to finish. Instead, a computer may implement interrupts for I/O completion, avoiding the need for polling or busy waiting. | https://en.wikipedia.org/wiki?curid=22194 |
6,514 | Some computers require an interrupt for each character or word, costing a significant amount of CPU time. Direct memory access (DMA) is an architecture feature to allow devices to bypass the CPU and access main memory directly. (Separate from the architecture, a device may perform direct memory access to and from main memory either directly or via a bus.) | https://en.wikipedia.org/wiki?curid=22194 |
6,515 | When a computer user types a key on the keyboard, typically the character appears immediately on the screen. Likewise, when a user moves a mouse, the cursor immediately moves across the screen. Each keystroke and mouse movement generates an "interrupt" called "Interrupt-driven I/O". An interrupt-driven I/O occurs when a process causes an interrupt for every character or word transmitted. | https://en.wikipedia.org/wiki?curid=22194 |
6,516 | Devices such as hard disk drives, solid state drives, and magnetic tape drives can transfer data at a rate high enough that interrupting the CPU for every byte or word transferred, and having the CPU transfer the byte or word between the device and memory, would require too much CPU time. Data is, instead, transferred between the device and memory independently of the CPU by hardware such as a channel or a direct memory access controller; an interrupt is delivered only when all the data is transferred. | https://en.wikipedia.org/wiki?curid=22194 |
6,517 | If a computer program executes a system call to perform a block I/O "write" operation, then the system call might execute the following instructions: | https://en.wikipedia.org/wiki?curid=22194 |
6,518 | While the writing takes place, the operating system will context switch to other processes as normal. When the device finishes writing, the device will "interrupt" the currently running process by "asserting" an interrupt request. The device will also place an integer onto the data bus. Upon accepting the interrupt request, the operating system will: | https://en.wikipedia.org/wiki?curid=22194 |
6,519 | Modern computers support multiple modes of operation. CPUs with this capability offer at least two modes: user mode and supervisor mode. In general terms, supervisor mode operation allows unrestricted access to all machine resources, including all MPU instructions. User mode operation sets limits on instruction use and typically disallows direct access to machine resources. CPUs might have other modes similar to user mode as well, such as the virtual modes in order to emulate older processor types, such as 16-bit processors on a 32-bit one, or 32-bit processors on a 64-bit one. | https://en.wikipedia.org/wiki?curid=22194 |
6,520 | At power-on or reset, the system begins in supervisor mode. Once an operating system kernel has been loaded and started, the boundary between user mode and supervisor mode (also known as kernel mode) can be established. | https://en.wikipedia.org/wiki?curid=22194 |
6,521 | Supervisor mode is used by the kernel for low level tasks that need unrestricted access to hardware, such as controlling how memory is accessed, and communicating with devices such as disk drives and video display devices. User mode, in contrast, is used for almost everything else. Application programs, such as word processors and database managers, operate within user mode, and can only access machine resources by turning control over to the kernel, a process which causes a switch to supervisor mode. Typically, the transfer of control to the kernel is achieved by executing a software interrupt instruction, such as the Motorola 68000 codice_10 instruction. The software interrupt causes the processor to switch from user mode to supervisor mode and begin executing code that allows the kernel to take control. | https://en.wikipedia.org/wiki?curid=22194 |
6,522 | In user mode, programs usually have access to a restricted set of processor instructions, and generally cannot execute any instructions that could potentially cause disruption to the system's operation. In supervisor mode, instruction execution restrictions are typically removed, allowing the kernel unrestricted access to all machine resources. | https://en.wikipedia.org/wiki?curid=22194 |
6,523 | The term "user 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 cause a switch to supervisor mode, where the operating system can deal with the illegal operation the program was attempting; for example, by forcibly terminating ("killing") the program. | https://en.wikipedia.org/wiki?curid=22194 |
6,524 | Among other things, a multiprogramming operating system kernel must be responsible for managing all system memory which is currently in use by the programs. This ensures that a program does not interfere with memory already in use by another program. Since programs time share, each program must have independent access to memory. | https://en.wikipedia.org/wiki?curid=22194 |
6,525 | 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 any more, 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,526 | 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,527 | 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 trigger an interrupt, which causes 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 both difficult to assign a meaningful result to such an operation, and because it is usually a sign of a misbehaving program, the kernel generally resorts to terminating the offending program, and reports the error. | https://en.wikipedia.org/wiki?curid=22194 |
6,528 | Windows versions 3.1 through ME had some level of memory protection, but programs could easily circumvent the need to use it. A general protection fault would be produced, indicating a segmentation violation had occurred; however, the system would often crash anyway. | https://en.wikipedia.org/wiki?curid=22194 |
6,529 | The use of virtual memory addressing (such as paging or segmentation) means that the kernel can choose what memory each program may use at any given time, allowing the operating system to use the same memory locations for multiple tasks. | https://en.wikipedia.org/wiki?curid=22194 |
6,530 | 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 is 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,531 | When the kernel detects a page fault it generally adjusts 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,532 | In modern operating systems, memory which is accessed less frequently can be temporarily stored on a 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 used by multiple programs, and what that memory area contains can be swapped or exchanged on demand. | https://en.wikipedia.org/wiki?curid=22194 |
6,533 | "Virtual memory" provides the programmer or the user with the perception that there is a much larger amount of RAM in the computer than is really there. | https://en.wikipedia.org/wiki?curid=22194 |
6,534 | 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,535 | An operating system kernel contains a scheduling program which determines how much time each process spends 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. Later, 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,536 | 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 malicious or malfunctioning program may not only prevent any other programs from using the CPU, but it can hang the entire system if it enters an infinite loop. | https://en.wikipedia.org/wiki?curid=22194 |
6,537 | 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,538 | 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.) | https://en.wikipedia.org/wiki?curid=22194 |
6,539 | On many single user operating systems cooperative multitasking is perfectly adequate, as home computers generally run a small number of well tested programs. AmigaOS is an exception, having preemptive multitasking from its first version. 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). | https://en.wikipedia.org/wiki?curid=22194 |
6,540 | Access to data 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 of the drive's available space. The specific way in which 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,541 | 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,542 | While many simpler operating systems support a limited range of options for accessing storage systems, operating systems like UNIX and Linux support a technology known as a virtual file system or VFS. An operating system such as UNIX supports a wide array of storage devices, regardless of their design or file systems, allowing them 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,543 | A connected storage device, such as a hard drive, is 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,544 | 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,545 | 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. Operating systems tend to recommend using (and so support natively) file systems specifically designed for them; for example, NTFS in Windows and ReiserFS, Reiser4, ext3, ext4 and Btrfs in Linux. However, in practice, third party drivers are usually available to give support for the most widely used file systems in most general-purpose operating systems (for example, NTFS is available in Linux through NTFS-3g, and ext2/3 and ReiserFS are available in Windows through third-party software). | https://en.wikipedia.org/wiki?curid=22194 |
6,546 | Support for file systems is highly varied among modern operating systems, although there are several common file systems which almost all operating systems include support and drivers for. Operating systems vary on file system support and on the disk formats they may be installed on. Under Windows, each file system is usually limited in application to certain media; for example, CDs must use ISO 9660 or UDF, and as of Windows Vista, NTFS is the only file system which the operating system can be installed on. It is possible to install Linux onto many types of 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...), a USB flash drive, or even contained within a file located on another file system. | https://en.wikipedia.org/wiki?curid=22194 |
6,547 | 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,548 | 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, operating systems essentially dictate how every type of device should be controlled. The function of the device driver is then to translate these operating system 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 ensures that the device appears to operate as usual from the operating system's point of view. | https://en.wikipedia.org/wiki?curid=22194 |
6,549 | Under versions of Windows before Vista and versions of Linux before 2.6, all driver execution was co-operative, meaning that if a driver entered an infinite loop it would freeze the system. More recent revisions of these operating systems incorporate kernel preemption, where the kernel interrupts the driver to give it tasks, and then separates itself from the process until it receives a response from the device driver, or gives it more tasks to do. | https://en.wikipedia.org/wiki?curid=22194 |
6,550 | 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,551 | Client/server networking allows a program on a computer, called a client, to connect via a network to another computer, called a server. Servers 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 IP 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,552 | 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 (SMB) 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,553 | 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,554 | 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 (such as reading files over a network share). Also covered by the concept of requester identity is "authorization"; the particular services and resources accessible by the requester once logged into a system are tied to either the requester's user account or to the variously configured groups of users to which the requester belongs. | https://en.wikipedia.org/wiki?curid=22194 |
6,555 | In addition to the allow or disallow model of security, a system with a high level of security also offers auditing options. These would allow tracking of requests for access to resources (such as, "who has been reading this file?"). Internal security, or security from an already running program is only possible if all possibly harmful requests must be carried out through interrupts to the operating system kernel. If programs can directly access hardware and resources, they cannot be secured. | https://en.wikipedia.org/wiki?curid=22194 |
6,556 | External security involves a request from outside the computer, such as a login at a connected console or some kind of network connection. External requests are often passed through device drivers to the operating system's kernel, where they can be passed onto applications, or carried out directly. 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 trusted operating systems being considered for the processing, storage and retrieval of sensitive or classified information. | https://en.wikipedia.org/wiki?curid=22194 |
6,557 | Network services include offerings such as file sharing, print services, email, web sites, and file transfer protocols (FTP), most of which can have compromised security. At the front line of security are hardware devices known as firewalls or intrusion detection/prevention systems. 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,558 | An alternative strategy, and the only sandbox strategy available in systems that do not meet the Popek and Goldberg virtualization requirements, is where the operating system is not running user programs as native code, but instead either emulates a processor or provides a host for a p-code based system such as Java. | https://en.wikipedia.org/wiki?curid=22194 |
6,559 | Internal security is especially relevant for multi-user systems; it allows each user of the system to have private files that the other users cannot tamper with or read. 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. | https://en.wikipedia.org/wiki?curid=22194 |
6,560 | Every computer that is to be operated by an individual requires a user interface. The user interface is usually referred to as a shell and is essential if human interaction is to be supported. The user interface views the directory structure and requests services from the operating system that will acquire data from input hardware devices, such as a keyboard, mouse or credit card reader, and requests operating system services to display prompts, status messages and such on output hardware devices, such as a video monitor or printer. The two most common forms of a user interface have historically been the command-line interface, where computer commands are typed out line-by-line, and the graphical user interface, where a visual environment (most commonly a WIMP) is present. | https://en.wikipedia.org/wiki?curid=22194 |
6,561 | Most of the modern computer systems support graphical user interfaces (GUI), and often include them. In some computer systems, such as the original implementation of the classic Mac OS, the GUI is integrated into the kernel. | https://en.wikipedia.org/wiki?curid=22194 |
6,562 | While technically a graphical user interface is not an operating system service, incorporating support for one into the operating system kernel can allow the GUI to be more responsive by reducing the number of context switches required for the GUI to perform its output functions. 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 macOS 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 the graphics drawing routines of versions between Windows NT 4.0 and Windows Server 2003 exist mostly in kernel space. Windows 9x had very little distinction between the interface and the kernel. | https://en.wikipedia.org/wiki?curid=22194 |
6,563 | Many computer operating systems allow the user to install or create any user interface they desire. The X Window System in conjunction with GNOME or KDE Plasma 5 is a commonly found setup on most Unix and Unix-like (BSD, Linux, Solaris) systems. A number of Windows shell replacements have been released for Microsoft Windows, which offer alternatives to the included Windows shell, but the shell itself cannot be separated from Windows. | https://en.wikipedia.org/wiki?curid=22194 |
6,564 | Numerous Unix-based GUIs have existed over time, most derived from X11. Competition among the various vendors of Unix (HP, IBM, Sun) led to much fragmentation, though an effort to standardize in the 1990s to COSE and CDE failed for various reasons, and were eventually eclipsed by the widespread adoption of GNOME and K Desktop Environment. Prior to free software-based toolkits and desktop environments, Motif was the prevalent toolkit/desktop combination (and was the basis upon which CDE was developed). | https://en.wikipedia.org/wiki?curid=22194 |
6,565 | Graphical user interfaces evolve over time. For example, Windows has modified its user interface almost every time a new major version of Windows is released, and the Mac OS GUI changed dramatically with the introduction of Mac OS X in 1999. | https://en.wikipedia.org/wiki?curid=22194 |
6,566 | A real-time operating system (RTOS) is an operating system intended for applications with fixed deadlines (real-time computing). Such applications include some small embedded systems, automobile engine controllers, industrial robots, spacecraft, industrial control, and some large-scale computing systems. | https://en.wikipedia.org/wiki?curid=22194 |
6,567 | An early example of a large-scale real-time operating system was Transaction Processing Facility developed by American Airlines and IBM for the Sabre Airline Reservations System. | https://en.wikipedia.org/wiki?curid=22194 |
6,568 | Embedded systems that have fixed deadlines use a real-time operating system such as VxWorks, PikeOS, eCos, QNX, MontaVista Linux and RTLinux. Windows CE is a real-time operating system that shares similar APIs to desktop Windows but shares none of desktop Windows' codebase. Symbian OS also has an RTOS kernel (EKA2) starting with version 8.0b. | https://en.wikipedia.org/wiki?curid=22194 |
6,569 | Some embedded systems use operating systems such as Palm OS, BSD, and Linux, although such operating systems do not support real-time computing. | https://en.wikipedia.org/wiki?curid=22194 |
6,570 | A hobby operating system may be classified as one whose code has not been directly derived from an existing operating system, and has few users and active developers. | https://en.wikipedia.org/wiki?curid=22194 |
6,571 | In some cases, hobby development is in support of a "homebrew" computing device, for example, a simple single-board computer powered by a 6502 microprocessor. Or, development may be for an architecture already in widespread use. Operating system development may come from entirely new concepts, or may commence by modeling an existing operating system. In either case, the hobbyist is her/his own developer, or may interact with a small and sometimes unstructured group of individuals who have like interests. | https://en.wikipedia.org/wiki?curid=22194 |
6,572 | Application software is generally written for use on a specific operating system, and sometimes even for specific hardware. When porting the application to run on another OS, the functionality required by that application may be implemented differently by that OS (the names of functions, meaning of arguments, etc.) requiring the application to be adapted, changed, or otherwise maintained. | https://en.wikipedia.org/wiki?curid=22194 |
6,573 | Unix was the first operating system not written in assembly language, making it very portable to systems different from its native PDP-11. | https://en.wikipedia.org/wiki?curid=22194 |
6,574 | This cost in supporting operating systems diversity can be avoided by instead writing applications against software platforms such as Java or Qt. These abstractions have already borne the cost of adaptation to specific operating systems and their system libraries. | https://en.wikipedia.org/wiki?curid=22194 |
6,575 | Another approach is for operating system vendors to adopt standards. For example, POSIX and OS abstraction layers provide commonalities that reduce porting costs. | https://en.wikipedia.org/wiki?curid=22194 |
6,576 | In quantum mechanics, Schrödinger's cat is a thought experiment that illustrates a paradox of quantum superposition. In the thought experiment, a hypothetical cat may be considered simultaneously both alive and dead, while it is unobserved in a closed box, as a result of its fate being linked to a random subatomic event that may or may not occur. | https://en.wikipedia.org/wiki?curid=27856 |
6,577 | This thought experiment was devised by physicist Erwin Schrödinger in 1935, in a discussion with Albert Einstein, to illustrate what Schrödinger saw as the problems of the Copenhagen interpretation of quantum mechanics. The scenario is often featured in theoretical discussions of the interpretations of quantum mechanics, particularly in situations involving the measurement problem. | https://en.wikipedia.org/wiki?curid=27856 |
6,578 | Schrödinger intended his thought experiment as a discussion of the EPR article—named after its authors Einstein, Podolsky, and Rosen—in 1935. The EPR article highlighted the counterintuitive nature of quantum superpositions, in which a quantum system such as an atom or photon can exist as a combination of multiple states corresponding to different possible outcomes. | https://en.wikipedia.org/wiki?curid=27856 |
6,579 | The prevailing theory, called the Copenhagen interpretation, says that a quantum system remains in superposition until it interacts with, or is observed by the external world. When this happens, the superposition collapses into one or another of the possible definite states. The EPR experiment shows that a system with multiple particles separated by large distances can be in such a superposition. Schrödinger and Einstein exchanged letters about Einstein's EPR article, in the course of which Einstein pointed out that the state of an unstable keg of gunpowder will, after a while, contain a superposition of both exploded and unexploded states. | https://en.wikipedia.org/wiki?curid=27856 |
6,580 | To further illustrate, Schrödinger described how one could, in principle, create a superposition in a large-scale system by making it dependent on a quantum particle that was in a superposition. He proposed a scenario with a cat in a locked steel chamber, wherein the cat's life or death depended on the state of a radioactive atom, whether it had decayed and emitted radiation or not. According to Schrödinger, the Copenhagen interpretation implies that "the cat remains both alive and dead" until the state has been observed. Schrödinger did not wish to promote the idea of dead-and-live cats as a serious possibility; on the contrary, he intended the example to illustrate the absurdity of the existing view of quantum mechanics. | https://en.wikipedia.org/wiki?curid=27856 |
6,581 | The idea that quantum superpositions of macroscopic states could be possible led to the Many-worlds interpretation of quantum theory. | https://en.wikipedia.org/wiki?curid=27856 |
6,582 | Since Schrödinger's time, various interpretations of the mathematics of quantum mechanics have been advanced by physicists, some of which regard the "alive and dead" cat superposition as quite real, others do not. Intended as a critique of the Copenhagen interpretation (the prevailing orthodoxy in 1935), the Schrödinger's cat thought experiment remains a touchstone for modern interpretations of quantum mechanics and can be used to illustrate and compare their strengths and weaknesses. | https://en.wikipedia.org/wiki?curid=27856 |
6,583 | Schrödinger's famous thought experiment poses the question, ""when" does a quantum system stop existing as a superposition of states and become one or the other?" (More technically, when does the actual quantum state stop being a non-trivial linear combination of states, each of which resembles different classical states, and instead begin to have a unique classical description?) If the cat survives, it remembers only being alive. But explanations of the EPR experiments that are consistent with standard microscopic quantum mechanics require that macroscopic objects, such as cats and notebooks, do not always have unique classical descriptions. The thought experiment illustrates this apparent paradox. Our intuition says that no observer can be in more than one state simultaneously—yet the cat, it seems from the thought experiment, can be in such a condition. Is the cat required to be an observer, or does its existence in a single well-defined classical state require another external observer? Each alternative seemed absurd to Einstein, who was impressed by the ability of the thought experiment to highlight these issues. In a letter to Schrödinger dated 1950, he wrote: | https://en.wikipedia.org/wiki?curid=27856 |
6,584 | Note that the charge of gunpowder is not mentioned in Schrödinger's setup, which uses a Geiger counter as an amplifier and hydrocyanic poison instead of gunpowder. The gunpowder had been mentioned in Einstein's original suggestion to Schrödinger 15 years before, and Einstein carried it forward to the present discussion. | https://en.wikipedia.org/wiki?curid=27856 |
6,585 | Since Schrödinger's time, other interpretations of quantum mechanics have been proposed that give different answers to the questions posed by Schrödinger's cat of how long superpositions last and when (or "whether") they collapse. | https://en.wikipedia.org/wiki?curid=27856 |
6,586 | A commonly held interpretation of quantum mechanics is the Copenhagen interpretation. In the Copenhagen interpretation, a system stops being a superposition of states and becomes either one or the other when an observation takes place. This thought experiment makes apparent the fact that the nature of measurement, or observation, is not well-defined in this interpretation. The experiment can be interpreted to mean that while the box is closed, the system simultaneously exists in a superposition of the states "decayed nucleus/dead cat" and "undecayed nucleus/living cat", and that only when the box is opened and an observation performed does the wave function collapse into one of the two states. | https://en.wikipedia.org/wiki?curid=27856 |
6,587 | In 1932 John von Neumann described in his book "Mathematical Foundations" a pattern where the radioactive source is observed by a device, which itself is observed by another device, and so on. It makes no difference in the predictions of quantum theory where along this chain of causal effects the superposition collapses. This potentially infinite chain could be broken if the last device is replaced by a conscious observer. This solved the problem because it was claimed that an individual's consciousness cannot be multiple. Neumann asserted that a conscious observer is necessary for collapse to one or the other (e.g. either a live cat or a dead cat) of the terms on the right-hand side of a wave function. This interpretation was later adopted by Eugene Wigner, who then rejected the interpretation in a thought experiment known as Wigner's friend. | https://en.wikipedia.org/wiki?curid=27856 |
6,588 | Wigner supposed that a friend opened the box and observed the cat without telling anyone. From Wigner's conscious perspective, the friend is now part of the wave function and has seen a live cat and seen a dead cat. To a third person's conscious perspective, Wigner himself becomes part of the wave function once Wigner learns the outcome from the friend. This could be extended indefinitely. | https://en.wikipedia.org/wiki?curid=27856 |
6,589 | One of the main scientists associated with the Copenhagen interpretation, Niels Bohr, offered an interpretation that is independent of a subjective observer-induced collapse of the wave function, or of measurement; instead, an "irreversible" or effectively irreversible process causes the decay of quantum coherence which imparts the classical behavior of "observation" or "measurement". Thus, Schrödinger's cat would be either dead or alive long before the box is observed. | https://en.wikipedia.org/wiki?curid=27856 |
6,590 | A resolution of the paradox is that the triggering of the Geiger counter counts as a measurement of the state of the radioactive substance. Because a measurement has already occurred deciding the state of the cat, the subsequent observation by a human records only what has already occurred. Analysis of an actual experiment by Roger Carpenter and A. J. Anderson found that measurement alone (for example by a Geiger counter) is sufficient to collapse a quantum wave function before any human knows of the result. The apparatus indicates one of two colors depending on the outcome. The human observer sees which color is indicated, but they don't consciously know which outcome the color represents. A second human, the one who set up the apparatus, is told of the color and becomes conscious of the outcome, and the box is opened to check if the outcome matches. However, it is disputed whether merely observing the color counts as a conscious observation of the outcome. | https://en.wikipedia.org/wiki?curid=27856 |
6,591 | In 1957, Hugh Everett formulated the many-worlds interpretation of quantum mechanics, which does not single out observation as a special process. In the many-worlds interpretation, both alive and dead states of the cat persist after the box is opened, but are decoherent from each other. In other words, when the box is opened, the observer and the possibly-dead cat split into an observer looking at a box with a dead cat, and an observer looking at a box with a live cat. But since the dead and alive states are decoherent, there is no effective communication or interaction between them. | https://en.wikipedia.org/wiki?curid=27856 |
6,592 | When opening the box, the observer becomes entangled with the cat, so "observer states" corresponding to the cat's being alive and dead are formed; each observer state is entangled or linked with the cat so that the "observation of the cat's state" and the "cat's state" correspond with each other. Quantum decoherence ensures that the different outcomes have no interaction with each other. The same mechanism of quantum decoherence is also important for the interpretation in terms of consistent histories. Only the "dead cat" or the "live cat" can be a part of a consistent history in this interpretation. Decoherence is generally considered to prevent simultaneous observation of multiple states. | https://en.wikipedia.org/wiki?curid=27856 |
6,593 | A variant of the Schrödinger's cat experiment, known as the quantum suicide machine, has been proposed by cosmologist Max Tegmark. It examines the Schrödinger's cat experiment from the point of view of the cat, and argues that by using this approach, one may be able to distinguish between the Copenhagen interpretation and many-worlds. | https://en.wikipedia.org/wiki?curid=27856 |
6,594 | The ensemble interpretation states that superpositions are nothing but subensembles of a larger statistical ensemble. The state vector would not apply to individual cat experiments, but only to the statistics of many similarly prepared cat experiments. Proponents of this interpretation state that this makes the Schrödinger's cat paradox a trivial matter, or a non-issue. | https://en.wikipedia.org/wiki?curid=27856 |
6,595 | This interpretation serves to "discard" the idea that a single physical system in quantum mechanics has a mathematical description that corresponds to it in any way. | https://en.wikipedia.org/wiki?curid=27856 |
6,596 | The relational interpretation makes no fundamental distinction between the human experimenter, the cat, or the apparatus, or between animate and inanimate systems; all are quantum systems governed by the same rules of wavefunction evolution, and all may be considered "observers". But the relational interpretation allows that different observers can give different accounts of the same series of events, depending on the information they have about the system. The cat can be considered an observer of the apparatus; meanwhile, the experimenter can be considered another observer of the system in the box (the cat plus the apparatus). Before the box is opened, the cat, by nature of its being alive or dead, has information about the state of the apparatus (the atom has either decayed or not decayed); but the experimenter does not have information about the state of the box contents. In this way, the two observers simultaneously have different accounts of the situation: To the cat, the wavefunction of the apparatus has appeared to "collapse"; to the experimenter, the contents of the box appear to be in superposition. Not until the box is opened, and both observers have the same information about what happened, do both system states appear to "collapse" into the same definite result, a cat that is either alive or dead. | https://en.wikipedia.org/wiki?curid=27856 |
6,597 | In the transactional interpretation the apparatus emits an advanced wave backward in time, which combined with the wave that the source emits forward in time, forms a standing wave. The waves are seen as physically real, and the apparatus is considered an "observer". In the transactional interpretation, the collapse of the wavefunction is "atemporal" and occurs along the whole transaction between the source and the apparatus. The cat is never in superposition. Rather the cat is only in one state at any particular time, regardless of when the human experimenter looks in the box. The transactional interpretation resolves this quantum paradox. | https://en.wikipedia.org/wiki?curid=27856 |
6,598 | On the other hand, the anti-Zeno effect accelerates the changes. For example, if you peek a look into the cat box frequently you may either cause delays to the fateful choice or, conversely, accelerate it. Both the Zeno effect and the anti-Zeno effect are real and known to happen to real atoms. The quantum system being measured must be strongly coupled to the surrounding environment (in this case to the apparatus, the experiment room ... etc.) in order to obtain more accurate information. But while there is no information passed to the outside world, it is considered to be a "quasi-measurement", but as soon as the information about the cat's well-being is passed on to the outside world (by peeking into the box) quasi-measurement turns into measurement. Quasi-measurements, like measurements, cause the Zeno effects. Zeno effects teach us that even without peeking into the box, the death of the cat would have been delayed or accelerated anyway due to its environment. | https://en.wikipedia.org/wiki?curid=27856 |
6,599 | According to objective collapse theories, superpositions are destroyed spontaneously (irrespective of external observation), when some objective physical threshold (of time, mass, temperature, irreversibility, etc.) is reached. Thus, the cat would be expected to have settled into a definite state long before the box is opened. This could loosely be phrased as "the cat observes itself", or "the environment observes the cat". | https://en.wikipedia.org/wiki?curid=27856 |
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