1.
Computing
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Computing is any goal-oriented activity requiring, benefiting from, or creating a mathematical sequence of steps known as an algorithm — e. g. through computers. The field of computing includes computer engineering, software engineering, computer science, information systems, the ACM Computing Curricula 2005 defined computing as follows, In a general way, we can define computing to mean any goal-oriented activity requiring, benefiting from, or creating computers. For example, an information systems specialist will view computing somewhat differently from a software engineer, regardless of the context, doing computing well can be complicated and difficult. Because society needs people to do computing well, we must think of computing not only as a profession, the fundamental question underlying all computing is What can be automated. The term computing is also synonymous with counting and calculating, in earlier times, it was used in reference to the action performed by mechanical computing machines, and before that, to human computers. Computing is intimately tied to the representation of numbers, but long before abstractions like the number arose, there were mathematical concepts to serve the purposes of civilization. These concepts include one-to-one correspondence, comparison to a standard, the earliest known tool for use in computation was the abacus, and it was thought to have been invented in Babylon circa 2400 BC. Its original style of usage was by lines drawn in sand with pebbles, abaci, of a more modern design, are still used as calculation tools today. This was the first known computer and most advanced system of calculation known to date - preceding Greek methods by 2,000 years. The first recorded idea of using electronics for computing was the 1931 paper The Use of Thyratrons for High Speed Automatic Counting of Physical Phenomena by C. E. Wynn-Williams. Claude Shannons 1938 paper A Symbolic Analysis of Relay and Switching Circuits then introduced the idea of using electronics for Boolean algebraic operations, a computer is a machine that manipulates data according to a set of instructions called a computer program. The program has a form that the computer can use directly to execute the instructions. The same program in its source code form, enables a programmer to study. Because the instructions can be carried out in different types of computers, the execution process carries out the instructions in a computer program. Instructions express the computations performed by the computer and they trigger sequences of simple actions on the executing machine. Those actions produce effects according to the semantics of the instructions, computer software or just software, is a collection of computer programs and related data that provides the instructions for telling a computer what to do and how to do it. Software refers to one or more programs and data held in the storage of the computer for some purposes. In other words, software is a set of programs, procedures, algorithms, program software performs the function of the program it implements, either by directly providing instructions to the computer hardware or by serving as input to another piece of software
2.
Computer program
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A computer program is a collection of instructions that performs a specific task when executed by a computer. A computer requires programs to function, and typically executes the programs instructions in a processing unit. A computer program is written by a computer programmer in a programming language. From the program in its form of source code, a compiler can derive machine code—a form consisting of instructions that the computer can directly execute. Alternatively, a program may be executed with the aid of an interpreter. A part of a program that performs a well-defined task is known as an algorithm. A collection of programs, libraries and related data are referred to as software. Computer programs may be categorized along functional lines, such as software or system software. The earliest programmable machines preceded the invention of the digital computer, in 1801, Joseph-Marie Jacquard devised a loom that would weave a pattern by following a series of perforated cards. Patterns could be weaved and repeated by arranging the cards, in 1837, Charles Babbage was inspired by Jacquards loom to attempt to build the Analytical Engine. The names of the components of the device were borrowed from the textile industry. In the textile industry, yarn was brought from the store to be milled, the device would have had a store—memory to hold 1,000 numbers of 40 decimal digits each. Numbers from the store would then have then transferred to the mill. It was programmed using two sets of perforated cards—one to direct the operation and the other for the input variables, however, after more than 17,000 pounds of the British governments money, the thousands of cogged wheels and gears never fully worked together. During a nine-month period in 1842–43, Ada Lovelace translated the memoir of Italian mathematician Luigi Menabrea, the memoir covered the Analytical Engine. The translation contained Note G which completely detailed a method for calculating Bernoulli numbers using the Analytical Engine and this note is recognized by some historians as the worlds first written computer program. In 1936, Alan Turing introduced the Universal Turing machine—a theoretical device that can model every computation that can be performed on a Turing complete computing machine and it is a finite-state machine that has an infinitely long read/write tape. The machine can move the back and forth, changing its contents as it performs an algorithm
3.
Kernel (operating system)
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The kernel is a computer program that is the core of a computers operating system, with complete control over everything in the system. It is the first program loaded on start-up and it handles the rest of start-up as well as input/output requests from software, translating them into data-processing instructions for the central processing unit. It handles memory and peripherals like keyboards, monitors, printers, the critical code of the kernel is usually loaded into a protected area of memory, which prevents it from being overwritten by applications or other, more minor parts of the operating system. The kernel performs its tasks, such as running processes and handling interrupts, in contrast, everything a user does is in user space, writing text in a text editor, running programs in a GUI, etc. This separation prevents user data and kernel data from interfering with other and causing instability. The kernels interface is an abstraction layer. When a process makes requests of the kernel, it is called a system call, Kernel designs differ in how they manage these system calls and resources. A monolithic kernel runs all the operating instructions in the same address space. A microkernel runs most processes in space, for modularity. The kernel takes responsibility for deciding at any time which of the running programs should be allocated to the processor or processors. Random-access memory Random-access memory is used to both program instructions and data. Typically, both need to be present in memory in order for a program to execute, often multiple programs will want access to memory, frequently demanding more memory than the computer has available. The kernel is responsible for deciding which memory each process can use, input/output devices I/O devices include such peripherals as keyboards, mice, disk drives, printers, network adapters, and display devices. The kernel allocates requests from applications to perform I/O to an appropriate device, key aspects necessary in resource management are the definition of an execution domain and the protection mechanism used to mediate the accesses to the resources within a domain. Kernels also usually provide methods for synchronization and communication between processes called inter-process communication, finally, a kernel must provide running programs with a method to make requests to access these facilities. The kernel has full access to the memory and must allow processes to safely access this memory as they require it. Often the first step in doing this is virtual addressing, usually achieved by paging and/or segmentation, virtual addressing allows the kernel to make a given physical address appear to be another address, the virtual address. This allows every program to behave as if it is the one running
4.
Operating system
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An operating system is system software that manages computer hardware and software resources and provides common services for computer programs. All computer programs, excluding firmware, require a system to function. Operating systems are found on many devices that contain a computer – from cellular phones, the dominant desktop operating system is Microsoft Windows with a market share of around 83. 3%. MacOS by Apple Inc. is in place, and the varieties of Linux is in third position. Linux distributions are dominant in the server and supercomputing sectors, other specialized classes of operating systems, such as embedded and real-time systems, exist for many applications. A single-tasking system can run one program at a time. Multi-tasking may be characterized in preemptive and co-operative types, in preemptive multitasking, the operating system slices the CPU time and dedicates a slot to each of the programs. Unix-like operating systems, e. g. Solaris, Linux, cooperative multitasking is achieved by relying on each process to provide time to the other processes in a defined manner. 16-bit versions of Microsoft Windows used cooperative multi-tasking, 32-bit versions of both Windows NT and Win9x, used preemptive multi-tasking. Single-user operating systems have no facilities to distinguish users, but may allow multiple programs to run in tandem, a distributed operating system manages a group of distinct computers and makes them appear to be a single computer. The development of networked computers that could be linked and communicate with each other gave rise to distributed computing, distributed computations are carried out on more than one machine. When computers in a work in cooperation, they form a distributed system. The technique is used both in virtualization and cloud computing management, and is common in large server warehouses, embedded operating systems are designed to be used in embedded computer systems. They are designed to operate on small machines like PDAs with less autonomy and they are able to operate with a limited number of resources. They are very compact and extremely efficient by design, Windows CE and Minix 3 are some examples of embedded operating systems. A real-time operating system is a system that guarantees to process events or data by a specific moment in time. A real-time operating system may be single- or multi-tasking, but when multitasking, early computers were built to perform a series of single tasks, like a calculator. Basic operating system features were developed in the 1950s, such as resident monitor functions that could run different programs in succession to speed up processing
5.
Hard disk drive
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The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a manner, meaning that individual blocks of data can be stored or retrieved in any order. HDDs are a type of storage, retaining stored data even when powered off. Introduced by IBM in 1956, HDDs became the dominant secondary storage device for computers by the early 1960s. Continuously improved, HDDs have maintained this position into the era of servers. More than 200 companies have produced HDDs historically, though after extensive industry consolidation most current units are manufactured by Seagate, Toshiba, as of 2016, HDD production is growing, although unit shipments and sales revenues are declining. While SSDs have higher cost per bit, SSDs are replacing HDDs where speed, power consumption, small size, the primary characteristics of an HDD are its capacity and performance. Capacity is specified in unit prefixes corresponding to powers of 1000, the two most common form factors for modern HDDs are 3. 5-inch, for desktop computers, and 2. 5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as PATA, SATA, Hard disk drives were introduced in 1956, as data storage for an IBM real-time transaction processing computer and were developed for use with general-purpose mainframe and minicomputers. The first IBM drive, the 350 RAMAC in 1956, was approximately the size of two medium-sized refrigerators and stored five million six-bit characters on a stack of 50 disks. In 1962 the IBM350 RAMAC disk storage unit was superseded by the IBM1301 disk storage unit, cylinder-mode read/write operations were supported, and the heads flew about 250 micro-inches above the platter surface. Motion of the head array depended upon a binary system of hydraulic actuators which assured repeatable positioning. The 1301 cabinet was about the size of three home refrigerators placed side by side, storing the equivalent of about 21 million eight-bit bytes, access time was about a quarter of a second. Also in 1962, IBM introduced the model 1311 disk drive, users could buy additional packs and interchange them as needed, much like reels of magnetic tape. Later models of removable pack drives, from IBM and others, became the norm in most computer installations, non-removable HDDs were called fixed disk drives. Some high-performance HDDs were manufactured with one head per track so that no time was lost physically moving the heads to a track, known as fixed-head or head-per-track disk drives they were very expensive and are no longer in production. In 1973, IBM introduced a new type of HDD code-named Winchester and its primary distinguishing feature was that the disk heads were not withdrawn completely from the stack of disk platters when the drive was powered down. Instead, the heads were allowed to land on an area of the disk surface upon spin-down
6.
Process (computing)
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In computing, a process is an instance of a computer program that is being executed. It contains the code and its current activity. Depending on the system, a process may be made up of multiple threads of execution that execute instructions concurrently. A computer program is a collection of instructions, while a process is the actual execution of those instructions. Several processes may be associated with the program, for example. Multitasking is a method to allow processes to share processors. Each CPU executes a task at a time. However, multitasking allows each processor to switch between tasks that are being executed without having to wait for each task to finish. Depending on the system implementation, switches could be performed when tasks perform input/output operations. A common form of multitasking is time-sharing, time-sharing is a method to allow fast response for interactive user applications. In time-sharing systems, context switches are performed rapidly, which makes it seem like multiple processes are being executed simultaneously on the same processor and this seeming execution of multiple processes simultaneously is called concurrency. In general, a system process consists of the following resources. Memory, which includes the code, process-specific data, a call stack. Operating system descriptors of resources that are allocated to the process, such as file descriptors or handles, security attributes, such as the process owner and the process set of permissions. Processor state, such as the content of registers and physical memory addressing, the state is typically stored in computer registers when the process is executing, and in memory otherwise. The operating system holds most of this information about processes in data structures called process control blocks. Any subset of the resources, typically at least the processor state, the operating system keeps its processes separate and allocates the resources they need, so that they are less likely to interfere with each other and cause system failures. The operating system may also provide mechanisms for communication to enable processes to interact in safe
7.
Instruction set architecture
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An ISA includes a specification of the set of opcodes, and the native commands implemented by a particular processor. An instruction set architecture is distinguished from a microarchitecture, which is the set of design techniques used, in a particular processor. Processors with different microarchitectures can share a common instruction set, for example, the Intel Pentium and the AMD Athlon implement nearly identical versions of the x86 instruction set, but have radically different internal designs. The concept of an architecture, distinct from the design of a machine, was developed by Fred Brooks at IBM during the design phase of System/360. Prior to NPL, the companys computer designers had been free to honor cost objectives not only by selecting technologies, the SPREAD compatibility objective, in contrast, postulated a single architecture for a series of five processors spanning a wide range of cost and performance. In addition, these virtual machines execute less frequently used code paths by interpretation, transmeta implemented the x86 instruction set atop VLIW processors in this fashion. A complex instruction set computer has many specialized instructions, some of which may only be used in practical programs. Theoretically important types are the instruction set computer and the one instruction set computer. Another variation is the very long instruction word where the processor receives many instructions encoded and retrieved in one instruction word, machine language is built up from discrete statements or instructions. Examples of operations common to many instruction sets include, Set a register to a constant value. Copy data from a location to a register, or vice versa. Used to store the contents of a register, result of a computation, often called load and store operations. Read and write data from hardware devices, add, subtract, multiply, or divide the values of two registers, placing the result in a register, possibly setting one or more condition codes in a status register. Increment, decrement in some ISAs, saving operand fetch in trivial cases, perform bitwise operations, e. g. taking the conjunction and disjunction of corresponding bits in a pair of registers, taking the negation of each bit in a register. Floating-point instructions for arithmetic on floating-point numbers, branch to another location in the program and execute instructions there. Conditionally branch to another if a certain condition holds. Call another block of code, while saving the location of the instruction as a point to return to. Load/store data to and from a coprocessor, or exchanging with CPU registers, processors may include complex instructions in their instruction set
8.
Protection ring
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In computer science, hierarchical protection domains, often called protection rings, are mechanisms to protect data and functionality from faults and malicious behaviour. This approach is diametrically opposite to that of capability-based security, computer operating systems provide different levels of access to resources. A protection ring is one of two or more levels or layers of privilege within the architecture of a computer system. This is generally hardware-enforced by some CPU architectures that provide different CPU modes at the hardware or microcode level, rings are arranged in a hierarchy from most privileged to least privileged. On most operating systems, Ring 0 is the level with the most privileges and interacts most directly with the hardware such as the CPU. Special gates between rings are provided to allow an outer ring to access an inner rings resources in a predefined manner, correctly gating access between rings can improve security by preventing programs from one ring or privilege level from misusing resources intended for programs in another. Programs such as web browsers running in higher numbered rings must request access to the network, multiple rings of protection were among the most revolutionary concepts introduced by the Multics operating system, a highly secure predecessor of todays 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, many modern CPU architectures include some form of ring protection, although the Windows NT operating system, like Unix, does not fully utilize this feature. OS/2 did to some extent, using three rings, ring 0 for kernel code and device drivers, ring 2 for privileged code, under DOS, the kernel, drivers and applications typically run on ring 3, whereas 386 memory managers such as EMM386 run at ring 0. In addition to this, DR-DOS EMM3863. xx can optionally run some modules on ring 1 instead, openVMS uses four modes called Kernel, Executive, Supervisor and User. The original Multics system had eight rings, but many systems have fewer. The hardware remains aware of the current ring of the executing instruction thread at all times, 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 program served as the ring register. The hardware severely restricts the ways in which control can be passed from one ring to another, 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, the ARM v7 architecture implements three privilege levels, application, operating system, and hypervisor. Unusually, level 0 is the level, while level 2 is the most-privileged level. Ring protection can be combined with processor modes 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
9.
Address space
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In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity. For software programs to save and retrieve stored data, each unit of data must have an address where it can be located or else the program will be unable to find. The number of spaces available will depend on the underlying address structure. Address spaces are created by combining enough uniquely identified qualifiers to make an address unambiguous, for a persons physical address, the address space would be a combination of locations, such as a neighborhood, town, city, or country. Some elements of a space may be the same– but if any element in the address is different than addresses in said space will reference different entities. An address space usually provides a partitioning to several regions according to the structure it has. In the case of order, as for memory addresses. Some nested domains hierarchy appears in the case of directed ordered tree as for the Domain Name System or a directory structure, another common feature of address spaces are mappings and translations, often forming numerous layers. This usually means that some higher-level address must be translated to lower-level ones in some way, then, for a disk drive connected via Parallel ATA, each of them must be converted to logical cylinder-head-sector address due to the interface historical shortcomings. It is converted back to LBA by the controller and then, finally, to physical cylinder, head. The Domain Name System maps its names to network-specific addresses, which in turn may be mapped to link layer network addresses via Address Resolution Protocol, also, network address translation may occur on the edge of different IP spaces, such as a local area network and the Internet. An iconic example of virtual-to-physical address translation is virtual memory, where different pages of virtual address space map either to page file or to main memory address space. It is possible that several different virtual addresses all refer to one physical address. It is also possible that a virtual address maps to zero, one. Linear address space Name space Virtualization
10.
Frame buffer
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A framebuffer is a portion of RAM containing a bitmap that is used to refresh a video display from a memory buffer containing a complete frame of data. In computing, a buffer is a part of computer memory used by a computer application for the representation of the content to be shown on the computer display. The screen buffer may also be called the buffer, the regeneration buffer. Screen buffers should be distinguished from video memory, to this end, the term off-screen buffer is also used. The information in the buffer typically consists of values for every pixel to be shown on the display. Color values are stored in 1-bit binary, 4-bit palettized, 8-bit palettized, 16-bit high color. An additional alpha channel is used to retain information about pixel transparency. The total amount of memory required for the framebuffer depends on the resolution of the output signal, computer researchers had long discussed the theoretical advantages of a framebuffer, but were unable to produce a machine with sufficient memory at an economically practicable cost. In 1969, A. Michael Noll of Bell Labs implemented a scanned display with a frame buffer, later on, the Bell Labs system was expanded to display an image with a color depth of three bits on a standard color TV monitor. An even earlier scanned display was implemented at the Brookhaven National Laboratory, advances in integrated-circuit memory in the 1970s made it more cost practical to create framebuffers capable of holding a standard video image. In 1972, Richard Shoup developed the SuperPaint system at Xerox PARC and this system had 311,040 bytes of memory and was capable of storing 640 by 480 pixels of data with 8 bits of color depth. The memory was scattered across 16 circuit boards, each loaded with multiple 2-kilobit shift register chips, while workable, this design required that the total framebuffer be implemented as a 307,200 byte shift register that shifted in synchronization with the television output signal. The primary drawback to this scheme was that memory was not random access, rather, a given position could be accessed only when the desired scan-line and pixel time rolled around. This gave the system a maximum latency of 33 ms for writing to the framebuffer, Shoup was also able to use the SuperPaint framebuffer to create an early digital video-capture system. By synchronizing the output signal to the signal, Shoup was able to overwrite each pixel of data as it shifted in. Shoup also experimented with modifying the signal using color tables. These color tables allowed the SuperPaint system to produce a variety of colors outside the range of the limited 8-bit data it contained. This scheme would become commonplace in computer framebuffers
11.
Computer network
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A computer network or data network is a telecommunications network which allows nodes to share resources. In computer networks, networked computing devices exchange data with other using a data link. The connections between nodes are established using either cable media or wireless media, the best-known computer network is the Internet. Network computer devices that originate, route and terminate the data are called network nodes, nodes can include hosts such as personal computers, phones, servers as well as networking hardware. Two such devices can be said to be networked together when one device is able to exchange information with the other device, Computer networks differ in the transmission medium used to carry their signals, communications protocols to organize network traffic, the networks size, topology and organizational intent. In most cases, application-specific communications protocols are layered over other more general communications protocols and this formidable collection of information technology requires skilled network management to keep it all running reliably. The chronology of significant computer-network developments includes, In the late 1950s, in 1960, the commercial airline reservation system semi-automatic business research environment went online with two connected mainframes. Licklider developed a group he called the Intergalactic Computer Network. In 1964, researchers at Dartmouth College developed the Dartmouth Time Sharing System for distributed users of computer systems. The same year, at Massachusetts Institute of Technology, a group supported by General Electric and Bell Labs used a computer to route. Throughout the 1960s, Leonard Kleinrock, Paul Baran, and Donald Davies independently developed network systems that used packets to transfer information between computers over a network, in 1965, Thomas Marill and Lawrence G. Roberts created the first wide area network. This was an precursor to the ARPANET, of which Roberts became program manager. Also in 1965, Western Electric introduced the first widely used telephone switch that implemented true computer control, in 1972, commercial services using X.25 were deployed, and later used as an underlying infrastructure for expanding TCP/IP networks. In July 1976, Robert Metcalfe and David Boggs published their paper Ethernet, Distributed Packet Switching for Local Computer Networks, in 1979, Robert Metcalfe pursued making Ethernet an open standard. In 1976, John Murphy of Datapoint Corporation created ARCNET, a network first used to share storage devices. In 1995, the transmission speed capacity for Ethernet increased from 10 Mbit/s to 100 Mbit/s, by 1998, Ethernet supported transmission speeds of a Gigabit. Subsequently, higher speeds of up to 100 Gbit/s were added, the ability of Ethernet to scale easily is a contributing factor to its continued use. Providing access to information on shared storage devices is an important feature of many networks, a network allows sharing of files, data, and other types of information giving authorized users the ability to access information stored on other computers on the network
12.
Interrupt
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In system programming, an interrupt is a signal to the processor emitted by hardware or software indicating an event that needs immediate attention. An interrupt alerts the processor to a high-priority condition requiring the interruption of the current code the processor is executing, the processor responds by suspending its current activities, saving its state, and executing a function called an interrupt handler to deal with the event. This interruption is temporary, and, after the interrupt handler finishes, there are two types of interrupts, hardware interrupts and software interrupts. Hardware interrupts are used by devices to communicate that they require attention from the operating system, for example, pressing a key on the keyboard or moving the mouse triggers hardware interrupts that cause the processor to read the keystroke or mouse position. Unlike the software type, hardware interrupts are asynchronous and can occur in the middle of instruction execution, the act of initiating a hardware interrupt is referred to as an interrupt request. A software interrupt is caused either by a condition in the processor itself. The former is called a trap or exception and is used for errors or events occurring during program execution that are exceptional enough that they cannot be handled within the program itself. For example, an exception will be thrown if the processors arithmetic logic unit is commanded to divide a number by zero as this instruction is in error. The operating system will catch this exception, and can choose to abort the instruction, each interrupt has its own interrupt handler. The number of interrupts is limited by the number of interrupt request lines to the processor. Interrupts are a commonly used technique for computer multitasking, especially in real-time computing, such a system is said to be interrupt-driven. Hardware interrupts were introduced as an optimization, eliminating unproductive waiting time in polling loops and they may be implemented in hardware as a distinct system with control lines, or they may be integrated into the memory subsystem. If implemented as part of the controller, interrupts are mapped into the systems memory address space. Interrupts can be categorized into different types, Maskable interrupt. Non-maskable interrupt, an interrupt that lacks an associated bit-mask. NMIs are used for the highest priority tasks such as timers, inter-processor interrupt, a special case of interrupt that is generated by one processor to interrupt another processor in a multiprocessor system. Software interrupt, an interrupt generated within a processor by executing an instruction, software interrupts are often used to implement system calls because they result in a subroutine call with a CPU ring level change. Spurious interrupt, an interrupt that is unwanted
13.
Library (computing)
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In computer science, a library is a collection of non-volatile resources used by computer programs, often to develop software. These may include data, documentation, help data, message templates, pre-written code. In IBMs OS/360 and its successors they are referred to as partitioned data sets, a library is also a collection of implementations of behavior, written in terms of a language, that has a well-defined interface by which the behavior is invoked. For instance, people who want to write a higher level program can use a library to make system calls instead of implementing those system calls over and over again, in addition, the behavior is provided for reuse by multiple independent programs. A program invokes the library-provided behavior via a mechanism of the language, for example, in a simple imperative language such as C, the behavior in a library is invoked by using Cs normal function-call. What distinguishes the call as being to a library, versus being to function in the same program, is the way that the code is organized in the system. This distinction can gain a hierarchical notion when a program grows large, in that case, there may be internal libraries that are reused by independent sub-portions of the large program. The value of a lies in the reuse of the behavior. When a program invokes a library, it gains the behavior implemented inside that library without having to implement that behavior itself, libraries encourage the sharing of code in a modular fashion, and ease the distribution of the code. The behavior implemented by a library can be connected to the program at different program lifecycle phases. If the code of the library is accessed during the build of the invoking program, an alternative is to build the executable of the invoking program and distribute that, independently of the library implementation. The library behavior is connected after the executable has been invoked to be executed, either as part of the process of starting the execution, in this case the library is called a dynamic library. A dynamic library can be loaded and linked when preparing a program for execution, alternatively, in the middle of execution, an application may explicitly request that a module be loaded. Most compiled languages have a library although programmers can also create their own custom libraries. Most modern software systems provide libraries that implement the majority of the system services, such libraries have commoditized the services which a modern application requires. As such, most code used by modern applications is provided in these system libraries, the earliest programming concepts analogous to libraries were intended to separate data definitions from the program implementation. JOVIAL brought the COMPOOL concept to popular attention in 1959, although it adopted the idea from the large-system SAGE software, COBOL also included primitive capabilities for a library system in 1959, but Jean Sammet described them as inadequate library facilities in retrospect. Another major contributor to the library concept came in the form of the subprogram innovation of FORTRAN
14.
Unix-like
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A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-like application is one that behaves like the corresponding Unix command or shell, there is no standard for defining the term, and some difference of opinion is possible as to the degree to which a given operating system or application is Unix-like. The Open Group owns the UNIX trademark and administers the Single UNIX Specification and they do not approve of the construction Unix-like, and consider it a misuse of their trademark. Other parties frequently treat Unix as a genericized trademark, in 2007, Wayne R. Gray sued to dispute the status of UNIX as a trademark, but lost his case, and lost again on appeal, with the court upholding the trademark and its ownership. Unix-like systems started to appear in the late 1970s and early 1980s, many proprietary versions, such as Idris, UNOS, Coherent, and UniFlex, aimed to provide businesses with the functionality available to academic users of UNIX. These largely displaced the proprietary clones, growing incompatibility among these systems led to the creation of interoperability standards, including POSIX and the Single UNIX Specification. Various free, low-cost, and unrestricted substitutes for UNIX emerged in the 1980s and 1990s, including 4. 4BSD, Linux, some of these have in turn been the basis for commercial Unix-like systems, such as BSD/OS and OS X. The various BSD variants are notable in that they are in fact descendants of UNIX, however, the BSD code base has evolved since then, replacing all of the AT&T code. Since the BSD variants are not certified as compliant with the Single UNIX Specification, dennis Ritchie, one of the original creators of Unix, expressed his opinion that Unix-like systems such as Linux are de facto Unix systems. Eric S. Raymond and Rob Landley have suggested there are three kinds of Unix-like systems, Genetic UNIX Those systems with a historical connection to the AT&T codebase. Most commercial UNIX systems fall into this category, so do the BSD systems, which are descendants of work done at the University of California, Berkeley in the late 1970s and early 1980s. Some of these systems have no original AT&T code but can trace their ancestry to AT&T designs. Trademark or branded UNIX These systems—largely commercial in nature—have been determined by the Open Group to meet the Single UNIX Specification and are allowed to carry the UNIX name, many ancient UNIX systems no longer meet this definition. Around 2001, Linux was given the opportunity to get a certification including free help from the POSIX chair Andrew Josey for the price of one dollar. Some non-Unix-like operating systems provide a Unix-like compatibility layer, with degrees of Unix-like functionality. IBM z/OSs UNIX System Services is sufficiently complete to be certified as trademark UNIX, cygwin and MSYS both provide a GNU environment on top of the Microsoft Windows user API, sufficient for most common open source software to be compiled and run. Subsystem for Unix-based Applications provides Unix-like functionality as a Windows NT subsystem, Windows Subsystem for Linux provides a Linux-compatible kernel interface developed by Microsoft and containing no Linux code, with Ubuntu user-mode binaries running on top of it
15.
GNU C Library
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The GNU C Library, commonly known as glibc, is the GNU Projects implementation of the C standard library. Despite its name, it now also directly supports C++ and it was started in the early 1990s by the Free Software Foundation for their GNU operating system. Released under the GNU Lesser General Public License, glibc is free software, the Glibc project was initially written mostly by Roland McGrath, working for the Free Software Foundation in the 1980s. In February 1988, FSF described glibc as having completed the functionality required by ANSI C. By 1992, it had the ANSI C-1989 and POSIX. 1-1990 functions implemented, in September 1995 Ulrich Drepper made his first contribution to the glibc project and gradually became over the 1990s the core contributor and maintainer of glibc. Drepper held the position for many years and accumulated until 2012 63% of all commits of the project. In the early 1990s, the developers of the Linux kernel forked glibc and their fork, called Linux libc, was maintained separately for years and released versions 2 through 5. At this point, the Linux kernel developers discontinued their fork, the last used version of Linux libc used the internal name libc. so.5. Following on from this, glibc 2. x on Linux uses the soname libc. so.6, the *. so file name is often abbreviated as libc6 following the normal conventions for libraries. Starting in 2001 the librarys development had been overseen by a committee, with Ulrich Drepper kept as the lead contributor, the steering committee installation was surrounded by a public controversy as it was openly described by Ulrich Drepper as a failed hostile takeover maneuver by RMS. While previously in a CVS repository, in 2009 glibc was migrated to a Git repository on Sourceware, after the change in glibc maintainership Debian and other projects migrated back to the glibc, who before switched to alternatives. Also, since the beginning of 2014, the glibc fork EGLIBC is no longer being developed, for most systems, the version of glibc can be obtained by executing the lib file.2, with all extensions common to XSI compliant systems along with all X/Open UNIX extensions. In addition, glibc also provides extensions that have been deemed useful or necessary while developing GNU, Glibc is used in systems that run many different kernels and different hardware architectures. It officially supports the Hurd and Linux kernels, additionally, there are heavily patched versions that run on the kernels of FreeBSD and NetBSD, as well as a forked-version of OpenSolaris. It is also used and named libroot. so in BeOS, Glibc has been criticized as being bloated and slower than other libraries in the past, e. g. by Linus Torvalds and embedded Linux programmers. For this reason, several alternative C standard libraries have been created which emphasize a smaller footprint, however, many small-device projects use GNU libc over the smaller alternatives because of its application support, standards compliance, and completeness. Examples include Openmoko and Familiar Linux for iPaq handhelds, there are compatibility layers to allow programs written for other ecosystems, e. g. Googles Android and Microsofts Windows, to run on glibc interface offering systems. Libhybris is a compatibility layer for Androids Bionic and Wine can be seen as compatibility layer from Win32 API/ABI to glibc
16.
Windows NT
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Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It is a processor-independent, multiprocessing, multi-user operating system, the first version of Windows NT was Windows NT3.1 and was produced for workstations and server computers. It was intended to complement consumer versions of Windows that were based on MS-DOS, gradually, the Windows NT family was expanded into Microsofts general-purpose operating system product line for all personal computers, deprecating the Windows 9x family. NT was formerly expanded to New Technology but no longer carries any specific meaning, starting with Windows 2000, NT was removed from the product name and is only included in the product version string. NT was the first purely 32-bit version of Windows, whereas its consumer-oriented counterparts, Windows 3. 1x and it is a multi-architecture operating system. Initially, it supported several CPU architectures, including IA-32, MIPS, DEC Alpha, PowerPC, the latest versions support x86 and ARM. This lineage is made clear in Cutlers foreword to Inside Windows NT by Helen Custer and it has been suggested that Dave Cutler intended the initialism WNT as a play on VMS, incrementing each letter by one. However, the project was intended as a follow-on to OS/2 and was referred to as NT OS/2 before receiving the Windows brand. One of the original NT developers, Mark Lucovsky, states that the name was taken from the original target processor—the Intel i860, the letters were dropped from the names of releases from Windows 2000 onwards, though Microsoft described that product as being Built on NT Technology. A main design goal of NT was hardware and software portability, the idea was to have a common code base with a custom Hardware Abstraction Layer for each platform. However, support for MIPS, Alpha, and PowerPC was later dropped in Windows 2000, broad software compatibility was achieved with support for several API personalities, including Windows API, POSIX, and OS/2 APIs – the latter two were phased out starting with Windows XP. Partial MS-DOS compatibility was achieved via an integrated DOS Virtual Machine – although this feature is being phased out in the x86-64 architecture, NT supported per-object access control lists allowing a rich set of security permissions to be applied to systems and services. NT supported Windows network protocols, inheriting the previous OS/2 LAN Manager networking, Windows NT3.1 was the first version of Windows to use 32-bit flat virtual memory addressing on 32-bit processors. Its companion product, Windows 3.1, used segmented addressing, notably, in Windows NT3. x, several I/O driver subsystems, such as video and printing, were user-mode subsystems. In Windows NT4, the video, server, and printer spooler subsystems were moved into kernel mode, NTFS, a journaled, secure file system, was created for NT. Windows NT also allows for other file systems, starting with versions 3.1. Windows NT introduced its own model, the Windows NT driver model. With Windows 2000, the Windows NT driver model was enhanced to become the Windows Driver Model, which was first introduced with Windows 98, but was based on the NT driver model
17.
Assembly language
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Each assembly language is specific to a particular computer architecture. In contrast, most high-level programming languages are generally portable across multiple architectures, Assembly language may also be called symbolic machine code. Assembly language is converted into machine code by a utility program referred to as an assembler. The conversion process is referred to as assembly, or assembling the source code, Assembly time is the computational step where an assembler is run. Assembly language uses a mnemonic to represent each low-level machine instruction or opcode, typically also each architectural register, flag, depending on the architecture, these elements may also be combined for specific instructions or addressing modes using offsets or other data as well as fixed addresses. Many assemblers offer additional mechanisms to facilitate development, to control the assembly process. A macro assembler includes a facility so that assembly language text can be represented by a name. A cross assembler is an assembler that is run on a computer or operating system of a different type from the system on which the code is to run. Cross-assembling facilitates the development of programs for systems that do not have the resources to support software development, a microassembler is a program that helps prepare a microprogram, called firmware, to control the low level operation of a computer. A meta-assembler is a used in some circles for a program that accepts the syntactic and semantic description of an assembly language. An assembler program creates object code by translating combinations of mnemonics and syntax for operations and this representation typically includes an operation code as well as other control bits and data. The assembler also calculates constant expressions and resolves symbolic names for memory locations, the use of symbolic references is a key feature of assemblers, saving tedious calculations and manual address updates after program modifications. Most assemblers also include facilities for performing textual substitution – e. g. to generate common short sequences of instructions as inline. Some assemblers may also be able to some simple types of instruction set-specific optimizations. One concrete example of this may be the ubiquitous x86 assemblers from various vendors, most of them are able to perform jump-instruction replacements in any number of passes, on request. Like early programming languages such as Fortran, Algol, Cobol and Lisp, assemblers have been available since the 1950s, however, assemblers came first as they are far simpler to write than compilers for high-level languages. There may be several assemblers with different syntax for a particular CPU or instruction set architecture, despite different appearances, different syntactic forms generally generate the same numeric machine code, see further below. A single assembler may also have different modes in order to support variations in syntactic forms as well as their exact semantic interpretations, there are two types of assemblers based on how many passes through the source are needed to produce the executable program
18.
Call stack
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In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as a stack, program stack, control stack, run-time stack, or machine stack. Although maintenance of the stack is important for the proper functioning of most software. Many computer instruction sets provide special instructions for manipulating stacks, a call stack is used for several related purposes, but the main reason for having one is to keep track of the point to which each active subroutine should return control when it finishes executing. An active subroutine is one that has been called but is yet to complete execution after which control should be handed back to the point of call, such activations of subroutines may be nested to any level, hence the stack structure. If, for example, a subroutine DrawSquare calls a subroutine DrawLine from four different places, to accomplish this, the address following the call instruction, the return address, is pushed onto the call stack with each call. If a called subroutine calls on yet another subroutine, it will push another return address onto the call stack, if the pushing consumes all of the space allocated for the call stack, an error called a stack overflow occurs, generally causing the program to crash. Adding a subroutines entry to the stack is sometimes called winding, conversely. There is usually exactly one call stack associated with a running program, in high-level programming languages, the specifics of the call stack are usually hidden from the programmer. They are given only to a set of functions. This is an example of abstraction, most assembly languages, on the other hand, require programmers to be involved with manipulating the stack. The actual details of the stack in a language depend upon the compiler, operating system. As noted above, the purpose of a call stack is to store the return addresses. When a subroutine is called, the location of the instruction at which it can later resume needs to be saved somewhere, using a stack to save the return address has important advantages over alternative calling conventions. One is that each task can have its own stack, and thus the subroutine can be reentrant, another benefit is that recursion is automatically supported. When a function calls itself recursively, a return address needs to be stored for each activation of the function so that it can later be used to return from the function activation, stack structures provide this capability automatically. It is often convenient to allocate space for use by simply moving the top of the stack by enough to provide the space. This is very fast when compared to dynamic memory allocation, which uses the heap space, note that each separate activation of a subroutine gets its own separate space in the stack for locals
19.
C (programming language)
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C was originally developed by Dennis Ritchie between 1969 and 1973 at Bell Labs, and used to re-implement the Unix operating system. C has been standardized by the American National Standards Institute since 1989, C is an imperative procedural language. Therefore, C was useful for applications that had formerly been coded in assembly language. Despite its low-level capabilities, the language was designed to encourage cross-platform programming, a standards-compliant and portably written C program can be compiled for a very wide variety of computer platforms and operating systems with few changes to its source code. The language has become available on a wide range of platforms. In C, all code is contained within subroutines, which are called functions. Function parameters are passed by value. Pass-by-reference is simulated in C by explicitly passing pointer values, C program source text is free-format, using the semicolon as a statement terminator and curly braces for grouping blocks of statements. The C language also exhibits the characteristics, There is a small, fixed number of keywords, including a full set of flow of control primitives, for, if/else, while, switch. User-defined names are not distinguished from keywords by any kind of sigil, There are a large number of arithmetical and logical operators, such as +, +=, ++, &, ~, etc. More than one assignment may be performed in a single statement, function return values can be ignored when not needed. Typing is static, but weakly enforced, all data has a type, C has no define keyword, instead, a statement beginning with the name of a type is taken as a declaration. There is no function keyword, instead, a function is indicated by the parentheses of an argument list, user-defined and compound types are possible. Heterogeneous aggregate data types allow related data elements to be accessed and assigned as a unit, array indexing is a secondary notation, defined in terms of pointer arithmetic. Unlike structs, arrays are not first-class objects, they cannot be assigned or compared using single built-in operators, There is no array keyword, in use or definition, instead, square brackets indicate arrays syntactically, for example month. Enumerated types are possible with the enum keyword and they are not tagged, and are freely interconvertible with integers. Strings are not a data type, but are conventionally implemented as null-terminated arrays of characters. Low-level access to memory is possible by converting machine addresses to typed pointers
20.
C++
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C++ is a general-purpose programming language. It has imperative, object-oriented and generic programming features, while also providing facilities for low-level memory manipulation and it was designed with a bias toward system programming and embedded, resource-constrained and large systems, with performance, efficiency and flexibility of use as its design highlights. C++ is a language, with implementations of it available on many platforms and provided by various organizations, including the Free Software Foundation, LLVM, Microsoft, Intel. C++ is standardized by the International Organization for Standardization, with the latest standard version ratified and published by ISO in December 2014 as ISO/IEC14882,2014. The C++ programming language was standardized in 1998 as ISO/IEC14882,1998. The current C++14 standard supersedes these and C++11, with new features, the C++17 standard is due in 2017, with the draft largely implemented by some compilers already, and C++20 is the next planned standard thereafter. Many other programming languages have influenced by C++, including C#, D, Java. In 1979, Bjarne Stroustrup, a Danish computer scientist, began work on C with Classes, the motivation for creating a new language originated from Stroustrups experience in programming for his Ph. D. thesis. When Stroustrup started working in AT&T Bell Labs, he had the problem of analyzing the UNIX kernel with respect to distributed computing, remembering his Ph. D. experience, Stroustrup set out to enhance the C language with Simula-like features. C was chosen because it was general-purpose, fast, portable, as well as C and Simulas influences, other languages also influenced C++, including ALGOL68, Ada, CLU and ML. Initially, Stroustrups C with Classes added features to the C compiler, Cpre, including classes, derived classes, strong typing, inlining, furthermore, it included the development of a standalone compiler for C++, Cfront. In 1985, the first edition of The C++ Programming Language was released, the first commercial implementation of C++ was released in October of the same year. In 1989, C++2.0 was released, followed by the second edition of The C++ Programming Language in 1991. New features in 2.0 included multiple inheritance, abstract classes, static functions, const member functions. In 1990, The Annotated C++ Reference Manual was published and this work became the basis for the future standard. Later feature additions included templates, exceptions, namespaces, new casts, after a minor C++14 update released in December 2014, various new additions are planned for 2017 and 2020. According to Stroustrup, the name signifies the nature of the changes from C. This name is credited to Rick Mascitti and was first used in December 1983, when Mascitti was questioned informally in 1992 about the naming, he indicated that it was given in a tongue-in-cheek spirit
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Application binary interface
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In computer software, an application binary interface is the interface between two program modules, one of which is often a library or operating system, at the level of machine code. ABIs differ from application programming interfaces, which similarly define interfaces between components, but at the source code level. Other ABIs standardize details such as the C++ name mangling, exception propagation, developers writing their own assembly language code may also interface with assembly generated by a compliant compiler. EABIs are designed to optimize for performance within the resources of an embedded system. Therefore, EABIs omit most abstractions that are made between kernel and user code in complex operating systems, the choice of EABI can affect performance. Widely used EABIs include PowerPC, ARM EABI2 and MIPS EABI
22.
Exokernel
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Exokernel is an operating system kernel developed by the MIT Parallel and Distributed Operating Systems group, and also a class of similar operating systems. Operating systems generally present hardware resources to applications through high-level abstractions such as file systems, the idea behind exokernels is to force as few abstractions as possible on application developers, enabling them to make as many decisions as possible about hardware abstractions. Implemented applications are called library operating systems, they may request specific memory addresses, disk blocks, the kernel only ensures that the requested resource is free, and the application is allowed to access it. This low-level hardware access allows the programmer to implement custom abstractions and it also allows programmers to choose what level of abstraction they want, high, or low. Traditionally kernel designers have sought to make individual hardware resources invisible to programs by requiring the programs to interact with the hardware via some abstraction model. These models include file systems for storage, virtual address spaces for memory, schedulers for task management. These abstractions of the make it easier to write programs in general. One option is to remove the kernel completely and program directly to the hardware, the program can then link to a support library that implements the abstractions it needs. MIT developed two exokernel-based operating systems, using two kernels, Aegis, a proof of concept with limited support for storage, and XOK, which applied the exokernel concept more thoroughly. The MIT exokernel manages hardware resources as follows, Processor The kernel represents the processor resources as a timeline from which programs can allocate intervals of time, a program can yield the rest of its time slice to another designated program. The kernel notifies programs of processor events, such as interrupts, hardware exceptions, if a program takes a long time to handle an event, the kernel will penalize it on subsequent time slice allocations, in extreme cases the kernel can abort the program. Memory The kernel allocates physical memory pages to programs and controls the translation lookaside buffer, a program can share a page with another program by sending it a capability to access that page. The kernel ensures that programs access only pages for which they have a capability, disk storage The kernel identifies disk blocks to the application program by their physical block address, allowing the application to optimize data placement. When the program initializes its use of the disk, it provides the kernel with a function that the kernel can use to determine which blocks the program controls. The kernel uses this callback to verify that when it allocates a new block, networking The kernel implements a programmable packet filter, which executes programs in a byte code language designed for easy security-checking by the kernel. The available library operating systems for Exokernel include the custom ExOS system, in addition to these, the exokernel team created the Cheetah web server, which uses the kernel directly. The exokernel concept has been around since at least 1994, a concept operating exokernel system is Nemesis, written by University of Cambridge, University of Glasgow, Citrix Systems, and the Swedish Institute of Computer Science. MIT has also built several exokernel based systems, including ExOS, in modern computing the MINIX3 Kernel implements some of the ideas of exokernel, but with the constraint that programs should be subject to reincarnation for the goal of reliability
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Abstraction (software engineering)
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In software engineering and computer science, abstraction is a technique for arranging complexity of computer systems. It works by establishing a level of complexity on which a person interacts with the system, the programmer works with an idealized interface and can add additional levels of functionality that would otherwise be too complex to handle. In addition, a task of sending a message across continents would be extremely complex if the programmer had to start with a piece of fiber optic cable. By using layers of complexity that have created to abstract away the physical cables and network layout, and presenting the programmer with a virtual data channel. Abstraction can apply to control or to data, Control abstraction is the abstraction of actions while data abstraction is that of data structures, Control abstraction involves the use of subroutines and control flow abstractions Data abstraction allows handling pieces of data in meaningful ways. For example, it is the motivation behind the datatype. The notion of an object in object-oriented programming can be viewed as a way to combine abstractions of data, the same abstract definition can be used as a common interface for a family of objects with different implementations and behaviors but which share the same meaning. The inheritance mechanism in object-oriented programming can be used to define a class as the common interface. The recommendation that programmers use abstractions whenever suitable in order to avoid duplication is known as the abstraction principle, the requirement that a programming language provide suitable abstractions is also called the abstraction principle. Computing mostly operates independently of the world, The hardware implements a model of computation that is interchangeable with others. The software is structured in architectures to enable humans to create the enormous systems by concentrating on a few issues at a time and these architectures are made of specific choices of abstractions. Greenspuns Tenth Rule is an aphorism on how such an architecture is both inevitable and complex, a central form of abstraction in computing is language abstraction, new artificial languages are developed to express specific aspects of a system. Computer languages can be processed with a computer, an example of this abstraction process is the generational development of programming languages from the machine language to the assembly language and the high-level language. Each stage can be used as a stone for the next stage. The language abstraction continues for example in scripting languages and domain-specific programming languages, within a programming language, some features let the programmer create new abstractions. These include subroutines, modules, polymorphism, and software components, some other abstractions such as software design patterns and architectural styles remain invisible to a translator and operate only in the design of a system. Some abstractions try to limit the range of concepts a programmer needs to be aware of, some abstractions are designed to interoperate with other abstractions - for example, a programming language may contain a foreign function interface for making calls to the lower-level language. Different programming languages provide different types of abstraction, depending on the applications for the language
24.
DOS/360 and successors
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Disk Operating System/360, also DOS/360, or simply DOS, is a discontinued operating system for IBM mainframes. It was announced by IBM on the last day of 1964, in its time, DOS/360 was the most widely used operating system in the world. TOS/360 was an IBM operating system for the System/360, used in the days around 1965 to support the IBM360 model 30. TOS, as per the Tape in the name, required a tape drive and it shared most of the code base and some manuals with IBMs DOS/360. DOS/360 was the operating system for most small to midsize S/360 installations. The first DOS/VS release was numbered Release 28 to signify an incremental upgrade from DOS/360 and it added virtual memory in support of the new System/370 series hardware. It used a fixed page table which mapped a single space of up to 16 megabytes for all partitions combined. DOS/VS increased the number of partitions from three to five and allowed a system total of fifteen subtasks. DOS/VS was succeeded by DOS/VSE through z/VSE, DOS/VSE was introduced in 1979 as an extended version of DOS/VS to support the new 4300 processors. The 4300 systems included a feature called ECPS, VSE that provided a storage for both the processor and the I/O channels. DOS/VSE provided support for ECPS, VSE, but could run on a System/370 without that feature. VSE was the last free version of DOS, SSX/VSE was an attempt by IBM to simplify purchase and installation of VSE by providing a pre-generated system containing the OS and the most popular products. SSX was released in 1982, and later replaced by VSE/SP, in 1986 IBM released VSE/SP in conjunction with the announcement of the 9370 processors. VSE/SP replaced SSX/VSE and bundled VSE with the most popular VSE program products such as VSE/AF, ACF/VTAM, CICS, VSE/SP supported only 24-bit addresses, despite customer requests to provide an XA version. VSE/ESA was a 31-bit DOS/VSE version, which was released in 1990 with support for up to 384 MB of real storage and it provided up to twelve static partitions and allowed VSE/POWER and ACF/VTAM to be run in private address spaces. It introduced a new feature called dynamic partitions which could allow up to 150 concurrent jobs, version 1 could run in either ESA or 370 mode, with the ESA mode also supporting XA hardware with limitations. Version 2 only supported ESA mode with ESA hardware, IBM released z/VSE3.1 in 2005. This change in naming reflected the new System z branding for IBMs mainframe product line, in particular, it did not support the new 64-bit z/Architecture, running only in 31-bit mode even on 64-bit capable machines
25.
Z/OS
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Z/OS is a 64-bit operating system for IBM mainframes, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions, like OS/390, z/OS combines a number of formerly separate, related products, some of which are still optional. Z/OS offers the attributes of modern operating systems but also much of the functionality originating in the 1960s. Z/OS was first introduced in October 2000, as a result, z/OS hosts a broad range of commercial and open source software. Z/OS can communicate directly via TCP/IP, including IPv6, and includes standard HTTP servers along with common services such as FTP, NFS. Another central design philosophy is support for high quality of service, even within a single operating system instance. This capability inherently supports multi-tenancy within an operating system image. However, modern IBM mainframes also offer two levels of virtualization, LPARs and z/VM. These new functions within the hardware, z/OS, and z/VM — and Linux, many of them utilize the WebSphere Application Server for z/OS middleware. From its inception z/OS has supported tri-modal addressing, IBM support for z/OS1.5 ended on March 31,2007. Now z/OS is only supported on z/Architecture mainframes and only runs in 64-bit mode, z/Architecture hardware always starts running in 31-bit mode, but current z/OS releases quickly switch to 64-bit mode and will not run on hardware that does not support 64-bit mode. However, increasing numbers of products and applications, such as DB2 Version 8 and above, now require. IBM markets z/OS as its operating system, suited for continuous. U. S. standard commercial z/OS pricing starts at about $125 per month, including support, z/OS introduced Variable Workload License Charges and Entry Workload License Charges which are sub-capacity billing options. VWLC and EWLC customers only pay for peak monthly z/OS usage, VWLC and EWLC are also available for most IBM software products running on z/OS, and their peaks are separately calculated but can never exceed the z/OS peak. To be eligible for sub-capacity licensing, a customer must be running in 64-bit mode, must have completely eliminated OS/390 from the system. Sub-capacity billing substantially reduces software charges for most IBM mainframe customers, advanced Workload License Charges is the successor to VWLC on mainframe models starting with the zEnterprise 196, and EAWLC is an option on zEnterprise 114 models. AWLC and EAWLC offer further sub-capacity discounts, within each address space, z/OS typically permits the placement of only data, not code, above the 2 GB bar
26.
Macro (computer science)
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A macro in computer science is a rule or pattern that specifies how a certain input sequence should be mapped to a replacement output sequence according to a defined procedure. The mapping process that instantiates a macro use into a sequence is known as macro expansion. A facility for writing macros may be provided as part of an application or as a part of a programming language. In the former case, macros are used to make using the application less repetitive. In the latter case, they are a tool allows a programmer to enable code reuse or even to design domain-specific languages. Macros are used to make a sequence of computing instructions available to the programmer as a single statement, making the programming task less tedious. The term derives from macro instruction, and such expansions were originally used in generating assembly language code, keyboard macros and mouse macros allow short sequences of keystrokes and mouse actions to transform into other, usually more time-consuming, sequences of keystrokes and mouse actions. In this way, frequently used or repetitive sequences of keystrokes, separate programs for creating these macros are called macro recorders. These programs were based on the TSR mode of operation and applied to all keyboard input, keyboard macros have in more recent times come to life as a method of exploiting the economy of massively multiplayer online role-playing games. By tirelessly performing a boring, repetitive, but low risk action and this effect is even larger when a macro-using player operates multiple accounts simultaneously, or operates the accounts for a large amount of time each day. As this money is generated without human intervention, it can upset the economy of the game. For this reason, use of macros is a violation of the TOS or EULA of most MMORPGs, keyboard and mouse macros that are created using an applications built-in macro features are sometimes called application macros. They are created by carrying out the once and letting the application record the actions. An underlying macro programming language, most commonly a scripting language, the programmers text editor, Emacs, follows this idea to a conclusion. In effect, most of the editor is made of macros, Emacs was originally devised as a set of macros in the editing language TECO, it was later ported to dialects of Lisp. Another programmers text editor, Vim, also has full implementation of macros and it can record into a register what a person types on the keyboard and it can be replayed or edited just like VBA macros for Microsoft Office. Vim also has a language called Vimscript to create macros. Visual Basic for Applications is a programming language included in Microsoft Office up to Office 2013, however, its function has evolved from and replaced the macro languages that were originally included in some of these applications
27.
Unix
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Among these is Apples macOS, which is the Unix version with the largest installed base as of 2014. Many Unix-like operating systems have arisen over the years, of which Linux is the most popular, Unix was originally meant to be a convenient platform for programmers developing software to be run on it and on other systems, rather than for non-programmer users. The system grew larger as the system started spreading in academic circles, as users added their own tools to the system. Unix was designed to be portable, multi-tasking and multi-user in a time-sharing configuration and these concepts are collectively known as the Unix philosophy. By the early 1980s users began seeing Unix as a universal operating system. Under Unix, the system consists of many utilities along with the master control program. To mediate such access, the kernel has special rights, reflected in the division between user space and kernel space, the microkernel concept was introduced in an effort to reverse the trend towards larger kernels and return to a system in which most tasks were completed by smaller utilities. In an era when a standard computer consisted of a disk for storage and a data terminal for input and output. However, modern systems include networking and other new devices, as graphical user interfaces developed, the file model proved inadequate to the task of handling asynchronous events such as those generated by a mouse. In the 1980s, non-blocking I/O and the set of inter-process communication mechanisms were augmented with Unix domain sockets, shared memory, message queues, and semaphores. In microkernel implementations, functions such as network protocols could be moved out of the kernel, Multics introduced many innovations, but had many problems. Frustrated by the size and complexity of Multics but not by the aims and their last researchers to leave Multics, Ken Thompson, Dennis Ritchie, M. D. McIlroy, and J. F. Ossanna, decided to redo the work on a much smaller scale. The name Unics, a pun on Multics, was suggested for the project in 1970. Peter H. Salus credits Peter Neumann with the pun, while Brian Kernighan claims the coining for himself, in 1972, Unix was rewritten in the C programming language. Bell Labs produced several versions of Unix that are referred to as Research Unix. In 1975, the first source license for UNIX was sold to faculty at the University of Illinois Department of Computer Science, UIUC graduate student Greg Chesson was instrumental in negotiating the terms of this license. During the late 1970s and early 1980s, the influence of Unix in academic circles led to adoption of Unix by commercial startups, including Sequent, HP-UX, Solaris, AIX. In the late 1980s, AT&T Unix System Laboratories and Sun Microsystems developed System V Release 4, in the 1990s, Unix-like systems grew in popularity as Linux and BSD distributions were developed through collaboration by a worldwide network of programmers
