Symbolics developed new Lisp Machines and published the operating system under the name ”Genera”. Genera 8.5 is the latest version. Symbolics Genera has been developed from the early 1980s to the early 1990s. In the recent years there were mostly patches developed and very little new functionality.

Symbolics developed Genera based on this foundation of the MIT Lisp machine operating system. It sells the operating system and ”layered software”. Some of the layered software has been integrated into Genera in later releases. Symbolics improved the operating system software from the original MIT Lisp Machine and expanded it. The Genera operating system was only available for Symbolics Lisp Machines and the Open Genera virtual machine.

Symbolics Genera has a large number of features and supported all the versions of various hardware that Symbolics built over its lifetime. Its source code is more than a million lines of code (the number depends on the release and what amount of software is installed). Symbolics Genera was published on tape and CD-ROM. The release of the operating system also provided most of the source code of the operating system and its applications. The user has free access to all parts of the running operating system and can write changes and extensions. The source code of the operating system is divided into ”systems”. These systems bundle sources, binaries and other files. The ”system construction toolkit” (SCT) maintains the dependencies, the components and the versions of all the systems. A system has two numbers: a major and a minor version number. The major version number counts the number of full constructions of a system. The minor version counts the number of patches to that system. A ”patch” is a file that can be loaded to fix problems or provide extensions to a particular version of a system.

Symbolics developed a version of Genera, called Open Genera that included a virtual machine that enables the execution of Genera on DEC Alpha based workstations. Symbolics also developed a new operating system called ”Minima” in Common Lisp (used for embedded applications). Symbolics also developed several extensions and applications for Genera that were sold separately (like the Symbolics ”S-Graphics” suite).

The original Lisp Machine operating system was developed in Lisp Machine Lisp and using the ”Flavors” object-oriented extension to Lisp Machine Lisp. Symbolics provided a successor to Flavors called ”New Flavors”. Later Symbolics also supported Common Lisp and the Common Lisp Object System. Then Symbolics Common Lisp became the default Lisp dialect for writing software with Genera. The software of the operating system was written mostly in Lisp Machine Lisp (called ZetaLisp) and Symbolics Common Lisp. These Lisp dialects are both provided by Genera. Also parts of the software was using either Flavors, New Flavors and Common Lisp Object System. Some of the older parts of the Genera operating system have been rewritten in Symbolics Common Lisp and the Common Lisp Object system. Many parts of the operating systems remained written in ZetaLisp and Flavors (or New Flavors).

Adapted from the Wikipedia article Genera (operating system), under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system. These mechanisms include low-level address space management, thread management, and inter-process communication(I.P.C). As an operating system design approach, microkernels permit typical operating system services, such as device drivers, protocol stacks, file systems and user interface code, to run in user space. If the hardware provides multiple privilege levels, the microkernel is the only software executing at the most privileged level (generally referred to as supervisor or kernel mode).

Microkernels are closely related to exokernels.

They also have much in common with hypervisors,

but the latter make no claim to minimality, and are specialized to supporting virtual machines; indeed, the L4 microkernel frequently finds use in a hypervisor capacity.

The historical term nanokernel has been used to distinguish modern, high-performance microkernels from earlier implementations which still contained many system services. However, nanokernels have all but replaced their microkernel progenitors, and the term has fallen into disuse.

Adapted from the Wikipedia article Microkernel, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Hardware supported rings were among the most revolutionary concepts introduced by the Multics operating system, a highly secure predecessor of today’s UNIX family of operating systems. However, most general-purpose systems use only two rings, even if the hardware they run on provides more CPU modes than that. For example, Windows XP and below only uses two rings, with ring 0 corresponding to kernel mode and ring 3 to user mode.

Many modern CPU architectures (including the popular Intel x86 architecture) include some form of ring protection, although the Windows NT operating system, like Unix, does not fully exploit this feature. Its predecessor, OS/2, did to some extent, as it used three rings: ring 0 for kernel code and device drivers, ring 2 for privileged code (user programs with I/O access permissions), and ring 3 for unprivileged code (nearly all user programs).

There has been a renewed interest in this design structure, with the proliferation of the Xen VMM software, ongoing discussion on monolithic- vs. micro-kernel (particularly in Usenet newsgroups and Web forums), Microsoft’s ”Ring-1” design structure as part of their NGSCB initiative and hypervisors embedded in firmware such as Intel VT-x (formerly Vanderpool).

The original Multics system had eight rings, but many modern systems have fewer. The hardware is aware of the current ring of the executing instruction thread at all times, thanks to special machine registers. In some systems, areas of virtual memory are instead assigned ring numbers in hardware. One example is the Data General Eclipse MV/8000, in which the top three bits of the PC served as the ring register. Thus code executing with the virtual PC set to 0×7200000, for example, would automatically be in ring 7, and calling a subroutine in a different section of memory would automatically cause a ring transfer.

The hardware severely restricts the ways in which control can be passed from one ring to another, and also enforces restrictions on the types of memory access that can be performed across rings. Typically there is a special ”gate” or ”call” instruction that transfers control in a secure way towards predefined entry points in lower-level (more trusted) rings; this functions as a supervisor call in many operating systems that use the ring architecture. The hardware restrictions are designed to limit opportunities for accidental or malicious breaches of security. In addition, the most privileged ring may be given special capabilities, (such as real memory addressing that bypasses the virtual-memory hardware).

Ring protection can be combined with processor modes (master/kernel/privileged mode versus slave/user/unprivileged mode) in some systems. Operating systems running on hardware supporting both may use both forms of protection or only one.

Effective use of ring architecture requires close cooperation between hardware and the operating system. Operating systems designed to work on multiple hardware platforms may make only limited use of rings if they are not present on every supported platform. Often the security model is simplified to “kernel” and “user” even if hardware provides finer granularity through rings.

Adapted from the Wikipedia article Ring (computer security), under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The first hypervisor providing full virtualization, IBM’s one-off research CP-40 system, began production use in January 1967, and became the first version of IBM’s CP/CMS operating system. CP-40 ran on a one-off S/360-40 that was customized to support virtualization. Prior to this time, computer hardware had only been virtualized enough to allow multiple user applications to be run (see CTSS and IBM M44/44X). With CP-40, the hardware’s ”supervisor state” was virtualized as well, allowing multiple operating systems to run simultaneously.

Programmers soon re-implemented CP-40 (as CP-67) for the IBM System/360-67, the first production computer-system capable of full virtualization. IBM first shipped this machine in 1966; it included page-translation-table hardware for virtual memory, and other techniques that allowed a full virtualization of all kernel tasks, including I/O and interrupt handling. (Note that its “official” operating system, the ill-fated TSS/360, did not employ full virtualization.) Both CP-40 and CP-67 began production use in 1967. CP/CMS was available to IBM customers from 1968 to 1972, in source code form without support.

CP/CMS formed part of IBM’s attempt to build robust time-sharing systems for its mainframe computers. By running multiple operating systems simultaneously, the hypervisor increased system robustness and stability: Even if one operating system crashed, the others would continue working without interruption. Indeed, this even allowed beta or experimental versions of operating systems – or even of new hardware – to be deployed and debugged, without jeopardizing the stable main production system, and without requiring costly additional development systems.

IBM announced its System/370 series in 1970 without any virtualization features, but it added them to the series in 1972, and virtualization featured in all successor systems. (All modern-day IBM mainframes, such as the zSeries line, retain backwards-compatibility with the 1960s-era IBM S/360 line.) The 1972 announcement also included VM/370, a reimplementation of CP/CMS for the S/370. Unlike CP/CMS, IBM provided support for this version (though it was still distributed in source code form for several releases). VM stands for ”Virtual Machine”, emphasizing that all, and not just some, of the hardware interfaces are virtualized. Both VM and CP/CMS enjoyed early acceptance and rapid development by universities, corporate users, and time-sharing vendors, as well as within IBM. Users played an active role in ongoing development, anticipating trends seen in modern open source projects. However, in a series of disputed and bitter battles, time-sharing lost out to batch processing through IBM political infighting, and VM remained IBM’s “other” mainframe operating system for decades, losing to MVS. It enjoyed a resurgence of popularity and support from 2000 as the z/VM product, for example as the platform for Linux for zSeries.

As mentioned above, the VM control program includes a ”hypervisor-call” handler which intercepts DIAG (“Diagnose”) instructions used within a virtual machine. This provides fast-path non-virtualized execution of file-system access and other operations. (DIAG is a model-dependent privileged instruction, not used in normal programming, and thus is not virtualized. It is therefore available for use as a signal to the “host” operating system.) When first implemented in CP/CMS release 3.1, this use of DIAG provided an operating system interface that was analogous to the System/360 SVC (“supervisor call”) instruction, but that did not require altering or extending the system’s virtualization of SVC.

Adapted from the Wikipedia article Hypervisor, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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microphone array is any number of microphones operating in tandem. There are many applications:

*Systems for extracting voice input from ambient noise (notably telephones, speech recognition systems, hearing aids)

*Surround sound and related technologies

*Locating objects by sound: acoustic source localization, e.g. military use to locate the source(s) of artillery fire. Aircraft location and tracking.

*High fidelity original recordings

Typically, an array is made up of omnidirectional microphones distributed about the perimeter of a space, linked to a computer that records and interprets the results into a coherent form. Arrays may also be formed using numbers of very closely spaced microphones. Given a fixed physical relationship in space between the different individual microphone transducer array elements, simultaneous DSP ( Digital signal processor ) processing of the signals from each of the individual microphone array elements can create one or more “virtual” microphones. Different algorithms permit the creation of virtual microphones with extremely complex virtual polar patterns and even the possibility to steer the individual lobes of the virtual microphones patterns so as to home-in-on, or to reject, particular sources of sound.

There are some implementation of microphone arrays that are really big. At the Massachusetts Institute of Technology, an array of 1020 microphones [http://cag.csail.mit.edu/mic-array/] has been built.

Microsoft’s Windows Vista computer operating system has built-in support for microphone arrays for increasing the accuracy of its speech recognition feature, letting users connect multiple microphones to a single system, so that the inputs can be combined into a single, higher-quality source.

Adapted from the Wikipedia article Microphone array, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Windows 98 based operating systems (Windows 98, Windows 98 Second Edition, and Windows Me) are able to use both WDM and VxD (Virtual device driver) driver standards. Both drivers models can provide unique and different features for the same hardware. However, usually the newer WDM standard provides more features. For example, if a TV tuner card using a VxD driver is able to capture images at a resolution of 384 x 288 pixels, the same TV Tuner card with the WDM driver model may be able to capture at a resolution of 768 x 576 pixels. This can be attributed to the new Broadcast Driver Architecture model which is part of WDM.

Adapted from the Wikipedia article Windows Driver Model, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Collection of device fingerprints from web clients (browser software) relies on the availability of JavaScript or similar client-side scripting language for the harvesting of a suitably large number of parameters. Two classes of users with limited client-side scripting are those with mobile devices, and those running privacy software.

A separate issue is that a single device may have multiple web clients installed, or even multiple virtual operating systems. As each distinct client and OS has distinct internal parameters, one may change the device fingerprint by simply running a different browser on the same machine.

Adapted from the Wikipedia article Device fingerprint, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The discussion above describes a file as a concept presented to a user or a high-level operating system. However, any file that has any useful purpose, outside of a thought experiment, must have some physical manifestation. That is, a file (an abstract concept) in a real computer system must have a real physical analogue if it is to exist at all.

In physical terms, most computer files are stored on some type of data storage device. For example, there is a ”hard disk”, from which most operating systems run and on which most store their files. Hard disks are the most ubiquitous form of non-volatile storage at the start of the 21st century. Where files contain only temporary information, they may be stored in RAM.

In Unix-like operating systems, many files have no direct association with a physical storage device: /dev/null is a prime example, as are just about all files under /dev, /proc and /sys. These can be accessed as files in user space. They are really virtual files that exist, in reality, as objects within the operating system kernel.

Computer files may be stored on magnetic tape. Files can also be stored on other media in some cases, such as writeable ”compact discs”, ”Digital Versatile Discs”, ”Zip drives,” ”USB flash drives,” etc.

Adapted from the Wikipedia article Computer file, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Technology has improved since the introduction of BBS-operated virtual airlines, allowing a wider variety of tools and resources available to virtual pilots, enhancing realism of flight simulation. Pilots can now fly online using networks such as VATSIM or IVAO. While connected to the network, pilots can see other aircraft, hear and respond to Air Traffic Control and see weather conditions that parallel the real-world weather at their plane’s location. Using these services, most virtual airlines regularly host online events where virtual pilots can participate in group flights with hundreds of other pilots.

While virtual airlines are not real, since the flights only happen inside of a computer, they are considered a serious hobby that has appeal among a very wide age range of participants, with the average age of participants increasing. Some even simulate real-world airlines to the point where flight dispatching and fictional salary are part of the virtual airline’s basic operations, as well as calculating operating costs and the full range of financial data used to manage airlines in the real world. It is common for the virtual airline to offer its members set flight routes to operate, with the offer of receiving awards and promotions as a result. It is also common for pilots to be given custom aircraft files and repaints with customized livery of the imaginary carrier, usually made internally by virtual airline members.

Most virtual airlines have a specific ranking system for their pilots, that tend to involve restricting which airplanes the member is allowed to fly. Pilots complete flights for their airline, using their simulator, either online (using a network such as VATSIM) or offline, and then file a pilot report. A key aspect of these pilot reports is logging the number of hours flown, which directly affects the members promotion to other ranks.

Common elements

There are several elements that are common across many virtual airlines:

*A website as the focal point of the community

*Internet forum where discussion and social interaction can occur

*Customized livery that users can download together with aircraft and install in their flight simulator

*Route schedules for members to fly in their simulators

*Multiplayer events, often on a game network such as VATSIM or IVAO

*An Online database for recording and reporting flights and membership statistics

*Award, rank and recognition systems

*Voice over Internet Protocol (such as TeamSpeak) servers for members to communicate freely by voice

Less common elements include:

*An ACARS server

*Dedicated game server hosting to provide private areas for members to complete flights

Adapted from the Wikipedia article Virtual airline, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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In the context of IT systems and data center management, a “workload” can be broadly defined as “the total requests made by users and applications of a system.” However, it is also possible to break down the entire workload of a given system into sets of self-contained units. Such a self-contained unit constitutes a “workload” in the narrow sense: an integrated stack consisting of application, middleware, database, and operating system devoted to a specific computing task. Typically, a workload is “platform agnostic,” meaning that it can run in physical, virtual or cloud computing environments. Finally, a collection of related workloads which allow end users to complete a specific set of business tasks can be defined as a “business service.”

Adapted from the Wikipedia article Intelligent workload management, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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