DOSBox emulates the familiar command line interface of DOS. DOSBox is an Emulator which simulates a computer running MS-DOS.

An emulator duplicates (provides an emulation of) the functions of one system using a different system, so that the second system behaves like (and appears to be) the first system. System (from Latin systēma, in turn from Greek systēma is a set of interacting or interdependent Entities, real or abstract This focus on exact reproduction of external behavior is in contrast to some other forms of computer simulation, which can concern an abstract model of the system being simulated. A computer simulation, a computer model or a computational model is a Computer program, or network of computers that attempts to simulate an

Emulators in computer science

Emulation refers to the ability of a computer program or electronic device to imitate another program or device. Computer programs (also software programs, or just programs) are instructions for a Computer. Many printers, for example, are designed to emulate Hewlett-Packard LaserJet printers because so much software is written for HP printers. LaserJet is the brand name used by the American computer company Hewlett-Packard (HP for their line of dry electrophotographic (DEP Laser printers By emulating an HP printer, a printer can work with any software written for a real HP printer. Emulation "tricks" the running software into believing that a device is really some other device.

A hardware emulator is an emulator which takes the form of a hardware device. Examples include printer emulators inside the ROM of the printer, and FPGA-based hardware emulators. FPGAs should not be confused with the Flip-chip pin grid array, a form of integrated circuit packaging Hardware emulation is the process of imitating the behavior of one or more pieces of hardware (typically a system under design with another piece of hardware typically a special purpose

In a theoretical sense, the Church-Turing thesis implies that any operating environment can be emulated within any other. However, in practice, it can be quite difficult, particularly when the exact behavior of the system to be emulated is not documented and has to be deduced through reverse engineering. Reverse engineering (RE is the process of discovering the technological principles of a device object or system through analysis of its structure function and operation It also says nothing about timing constraints; if the emulator does not perform as quickly as the original hardware, the emulated software may run much more slowly than it would have on the original hardware, possibly triggering time interrupts to alter performance.

Emulation in preservation

Emulation is a strategy in digital preservation to combat obsolescence. Digital preservation is the management of Digital information over time Emulation focuses on recreating the original computer environment and can be time-consuming and difficult, but valuable because of its ability to maintain a closer connection to the authenticity of the digital object. ^[1]

Emulation addresses the original hardware and software environment of the digital object, and recreates it on a current machine. Typical PC hardware A typical Personal computer consists of a case or chassis in a tower shape (desktop and the following parts Motherboard ^[2] The emulator allows the user to have access to any kind of application or operating system on a current platform, while the software thinks it is in its original environment. Application software is a subclass of Computer software that employs the capabilities of a computer directly and thoroughly to a task that the user wishes to perform An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination In Computing, a platform describes some sort of Hardware architecture or Software framework (including Application frameworks, that allows ^[3] Jeffery Rothenberg, an early proponent of emulation as a digital preservation strategy states, “the ideal approach would provide a single extensible, long-term solution that can be designed once and for all and applied uniformly, automatically, and in synchrony (for example, at every refresh cycle) to all types of documents and media”. Digital preservation is the management of Digital information over time ^[4] He further states that this should not only apply to out of date systems, but also be upwardly mobile to future unknown systems. ^[5] Practically speaking, when a certain application is released in a new version, rather than address compatibility issues and migration for every digital object created in the previous version of that application, we could create one emulator for the application, allowing access to all of the said digital objects. A family of Computer models is said to be compatible if certain Software that runs on one of the models can also be run on all other models of the family Data migration is the process of transferring Data between storage types Formats, or Computer systems Data migration is usually performed programmatically Application software is a subclass of Computer software that employs the capabilities of a computer directly and thoroughly to a task that the user wishes to perform Application software is a subclass of Computer software that employs the capabilities of a computer directly and thoroughly to a task that the user wishes to perform

Advantages

• Emulators maintain the original look, feel, and behavior of the digital object, which is just as important as the digital data itself. ^[6]
• Despite the original cost of developing an emulator, it may prove to be the more cost efficient solution over time. ^[7]
• Reduces labor hours, because rather than continuing an ongoing task of continual data migration for every digital object, once the library of past and present operating systems and application software is established in an emulator, these same technologies are used for every document using those platforms. An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination In Computing, a platform describes some sort of Hardware architecture or Software framework (including Application frameworks, that allows ^[8]
• Many emulators have already been developed and released under GNU General Public License though the open source environment, allowing for wide scale collaboration. Open source is a development methodology which offers practical accessibility to a product's source (goods and knowledge ^[9]

Obstacles

• Intellectual property - Many technology vendors implemented non-standard features during program development in order to establish their niche in the market, while simultaneously implementing ongoing upgrades to remain competitive. Intellectual property ( IP) is a legal field that refers to creations of the mind such as musical literary and artistic works inventions and symbols names While this may have advanced the technology industry and increased vendor’s market share, it has left users lost in a preservation nightmare with little supporting documentation due to the proprietary nature of the hardware and software. Market share, in Strategic management and Marketing, is the percentage or proportion of the total available Market or Market segment that is ^[10]
• Copyright laws are not yet in effect to address saving the documentation and specifications of proprietary software and hardware in an emulator module. Copyright is a legal concept enacted by Governments, giving the creator of an original work of authorship Exclusive rights to control its distribution usually for ^[11]

Emulators in New Media Art

Because of its primary use of digital formats, New Media Art relies heavily on emulation as a preservation strategy. Artists such as Cory Arcangel specialize in resurrecting obsolete technologies in their artwork and recognize the importance of a decentralized and deinstitutionalized process for the preservation of digital culture. Cory Arcangel (born 1978 is a Digital artist who lives and works in Brooklyn

In many cases, the goal of emulation in New Media Art is to preserve a digital medium so that it can be saved indefinitely and reproduced without error, so that there is no reliance on hardware that ages and becomes obsolete. The paradox is that the emulation and the emulator have to be made to work on future computers. ^[12]

Types of emulators

Most emulators just emulate a hardware architecture — if operating system firmware or software is required for the desired software, it must be provided as well (and may itself be emulated). Both the OS and the software will then be interpreted by the emulator, rather than being run by native hardware. In Computer science, an interpreter normally means a Computer program that executes, i Apart from this interpreter for the emulated machine's language, some other hardware (such as input or output devices) must be provided in virtual form as well; for example, if writing to a specific memory location should influence the screen, then this would need to be emulated. Machine code or machine language is a system of instructions and data executed directly by a Computer 's Central processing unit.

While emulation could, if taken to the extreme, go down to the atomic level, basing its output on a simulation of the actual circuitry from a virtual power source, this would be a highly unusual solution. Emulators typically stop at a simulation of the documented hardware specifications and digital logic. Sufficient emulation of some hardware platforms requires extreme accuracy, down to the level of individual clock cycles, undocumented features, unpredictable analog elements, and implementation bugs. This is particularly the case with classic home computers such as the Commodore 64, whose software often depends on highly sophisticated low-level programming tricks invented by game programmers and the demoscene. The demoscene is a Computer art Subculture that specializes in producing demos, which are non-interactive audio-visual presentations that run in

In contrast, some other platforms have had very little use of direct hardware addressing. In these cases, a simple compatibility layer may suffice. A compatibility layer is a term that refers to components that allow for non-native support of components This translates system calls for the emulated system into system calls for the host system.

Developers of software for embedded systems or video game consoles often design their software on especially accurate emulators called simulators before trying it on the real hardware. An embedded system is a special-purpose Computer system designed to perform one or a few dedicated functions often with Real-time computing constraints Simulation is the imitation of some real thing state of affairs or process This is so that software can be produced and tested before the final hardware exists in large quantities, so that it can be tested without taking the time to copy the program to be debugged at a low level without introducing the side effects of a debugger. A debugger is a Computer program that is used to test and Debug other programs In many cases, the simulator is actually produced by the company providing the hardware, which theoretically increases its accuracy.

Structure of an emulator

Typically, an emulator is divided into modules that correspond roughly to the emulated computer's subsystems. Modular programming is a software design technique that increases the extent to which software is composed from separate parts called modules Most often, an emulator will be composed of the following modules:

• a CPU emulator or CPU simulator (the two terms are mostly interchangeable in this case)
• a memory subsystem module
• various I/O devices emulators

Buses are often not emulated, either for reasons of performance or simplicity, and virtual peripherals communicate directly with the CPU or the memory subsystem.

Memory subsystem

It is possible for the memory subsystem emulation to be reduced to simply an array of elements each sized like an emulated word; however, this model falls very quickly as soon as any location in the computer's logical memory does not match physical memory. Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital

This clearly is the case whenever the emulated hardware allows for advanced memory management (in which case, the MMU logic can be embedded in the memory emulator, made a module of its own, or sometimes integrated into the CPU simulator). A memory management unit ( MMU) sometimes called paged memory management unit ( PMMU) is a Computer hardware component responsible for handling

Even if the emulated computer does not feature an MMU, though, there are usually other factors that break the equivalence between logical and physical memory: many (if not most) architecture offer memory-mapped I/O; even those that do not almost invariably have a block of logical memory mapped to ROM, which means that the memory-array module must be discarded if the read-only nature of ROM is to be emulated. Memory-mapped I/O ( MMIO) and port I/O (also called port-mapped I/O or PMIO) are two complementary methods of performing Input/output Features such as bank switching or segmentation may also complicate memory emulation. Bank switching (also known as "paging" but only loosely related to the ordinary meaning of " Paging " in computing was a technique common in 8-bit In computing memory segmentation is one of the most common ways to achieve Memory protection; another common one is Paging.

As a result, most emulators implement at least two procedures for writing to and reading from logical memory, and it is these procedures' duty to map every access to the correct location of the correct object.

On a base-limit addressing system where memory from address 0 to address ROMSIZE-1 is read-only memory, while the rest is RAM, something along the line of the following procedures would be typical:

void WriteMemory(word Address, word Value) {     word RealAddress;     RealAddress=Address+BaseRegister;     if(RealAddress<LimitRegister) {         if(RealAddress>ROMSIZE) Memory[RealAddress]=Value;     } else {         RaiseInterrupt(INT_SEGFAULT);     } }

word ReadMemory(word Address) {     word RealAddress;     RealAddress=Address+BaseRegister;     if(RealAddress<LimitRegister) {         return Memory[RealAddress];     } else {         RaiseInterrupt(INT_SEGFAULT);         return NULL;     } }

CPU simulator

The CPU simulator is often the most complicated part of an emulator. Many emulators are written using "pre-packaged" CPU simulators, in order to concentrate on good and efficient emulation of a specific machine.

The simplest form of a CPU simulator is an interpreter, which follows the execution flow of the emulated program code and, for every machine code instruction encountered, executes operations on the host processor that are semantically equivalent to the original instructions. In Computer science, an interpreter normally means a Computer program that executes, i

This is made possible by assigning a variable to each register and flag of the simulated CPU. A variable (ˈvɛərɪəbl is an Attribute of a physical or an abstract System which may change its Value while it is under Observation. In Computer architecture, a processor register is a small amount of storage available on the CPU whose contents can be accessed more quickly than storage In Computer programming, flag refers to one or more Bits that are used to store a binary value or Code that has an assigned meaning The logic of the simulated CPU can then more or less be directly translated into software algorithms, creating a software re-implementation that basically mirrors the original hardware implementation.

The following example illustrates how CPU simulation can be accomplished by an interpreter. In this case, interrupts are checked-for before every instruction executed, though this behavior is rare in real emulators for performance reasons.

void Execute(void) {     if(Interrupt!=INT_NONE) {         SuperUser=TRUE;         WriteMemory(++StackPointer, ProgramCounter);         ProgramCounter=InterruptPointer;     }     switch(ReadMemory(ProgramCounter++)) {         /*          * Handling of every valid instruction          * goes here. . .           */         default:         Interrupt=INT_ILLEGAL;     } }

Interpreters are very popular as computer simulators, as they are much simpler to implement than more time-efficient alternative solutions, and their speed is more than adequate for emulating computers of more than roughly a decade ago on modern machines.

However, the speed penalty inherent in interpretation can be a problem when emulating computers whose processor speed is on the same order of magnitude as the host machine. An order of magnitude is the class of scale or magnitude of any amount where each class contains values of a fixed ratio to the class preceding it Until not many years ago, emulation in such situations was considered completely impractical by many.

What allowed breaking through this restriction were the advances in dynamic recompilation techniques. In Computer science, dynamic recompilation (sometimes abbreviated to dynarec or the Pseudo-acronym DRC) is a feature of some Emulators Simple a priori translation of emulated program code into code runnable on the host architecture is usually impossible because of several reasons:

• code may be modified while in RAM, even if it is modified only by the emulated operating system when loading the code (for example from disk)
• there may not be a way to reliably distinguish data (which should not be translated) from executable code. In Computer science, self-modifying code is code that alters its own instructions, intentionally or otherwise while it is executing. Debt AIDS Trade in Africa (or DATA) is a Multinational non-government organization founded in January 2002 in London by U2 's In Computing, an executable (file causes a computer "to perform indicated tasks according to encoded instructions," as opposed to a file that only contains In Communications a code is a rule for converting a piece of Information (for example a letter, Word, Phrase, or

Various forms of dynamic recompilation, including the popular Just In Time compiler (JIT) technique, try to circumvent these problems by waiting until the processor control flow jumps into a location containing untranslated code, and only then ("just in time") translates a block of the code into host code that can be executed. In Computing, just-in-time compilation ( JIT) also known as dynamic translation, is a technique for improving the runtime performance of a Computer The translated code is kept in a code cache, and the original code is not lost or affected; this way, even data segments can be (meaninglessly) translated by the recompiler, resulting in no more than a waste of translation time. In Computer science, a cache (kæʃ like "cash") is a collection of data duplicating original

Some older games were not designed with the speed of faster computers in mind. A game designed for a 30 MHz PC with a level timer of 300 game seconds might only give the player 30 seconds on a 300 MHz PC. Other programs, such as some DOS programs, may not even run on faster computers.

I/O

Most emulators do not, as mentioned earlier, emulate the main system bus; each I/O device is thus often treated as a special case, and no consistent interface for virtual peripherals is provided. In Personal computers the Front Side Bus ( FSB) is the bus that carries data between the CPU and the northbridge.

This can result in a performance advantage, since each I/O module can be tailored to the characteristics of the emulated device; designs based on a standard, unified I/O API can, however, rival such simpler models, if well thought-out, and they have the additional advantage of "automatically" providing a plug-in service through which third-party virtual devices can be used within the emulator.

A unified I/O API may not necessarily mirror the structure of the real hardware bus: bus design is limited by several electric constraints and a need for hardware concurrency management that can mostly be ignored in a software implementation. Parallel computing is a form of computation in which many instructions are carried out simultaneously operating on the principle that large problems can often

Even in emulators that treat each device as a special case, there is usually a common basic infrastructure for:

• managing interrupts, by means of a procedure that sets flags readable by the CPU simulator whenever an interrupt is raised, allowing the virtual CPU to "poll for (virtual) interrupts"
• writing to and reading from physical memory, by means of two procedures similar to the ones dealing with logical memory (although, contrary to the latter, the former can often be left out, and direct references to the memory array be employed instead)

Legal controversy

See article Console emulator — Legal issues

Emulate or Simulate?

The use of the word "emulate" in the context of software is a modern one. In Computing, an interrupt is an asynchronous signal from hardware indicating the need for attention or a synchronous event in software indicating the need for a change A console emulator is a program that allows a computer or modern console (cross-console emulation to emulate a Video game console. Previous to 1980, "emulate" referred only to hardware emulation. The equivalent word for software was "simulate". Purists continue to insist on this distinction.

See also

• The list of emulators
• The list of computer system emulators
• Computer simulation is the larger field of modeling real-world phenomenon (e. This article lists software that emulates arcade and console game systems computing platforms and CPUs Arcade systems Ace This article lists software and hardware that emulates computing platforms A computer simulation, a computer model or a computational model is a Computer program, or network of computers that attempts to simulate an g. physics and economy) using computers.
• For rewriting a computer program into a different programming language or platform, see Porting. See also Software portability In Computer science, porting is the process of adapting software so that an executable program can be created
• Field-programmable gate arrays (FPGAs)
• Other uses of the term "emulator" in the field of computer science:
  • Console emulator
  • Server emulator
  • Terminal emulator
• Translation:
  • Binary translation

• Antenna emulator
• In-circuit emulator (ICE)
• Virtual machine
• MAME
• data migration
• backward compatibility
• forward compatibility

References

1. ^ What is emulation?. FPGAs should not be confused with the Flip-chip pin grid array, a form of integrated circuit packaging A console emulator is a program that allows a computer or modern console (cross-console emulation to emulate a Video game console. Server emulator is a term that is used to refer to an internet server that mimics the behavior of another server that is usually more well known A terminal emulator, terminal application, term, or tty for short is a program that emulates a "dumb" video terminal within some other An in-circuit emulator (ICE is a hardware device used to debug the Software of an Embedded system. In Computer science, a virtual machine (VM is a Software implementation of a machine (computer that executes programs like a real machine MAME is an Emulator application designed to recreate the Hardware of Arcade game systems in Software, with the intent of preserving gaming Data migration is the process of transferring Data between storage types Formats, or Computer systems Data migration is usually performed programmatically In Technology, especially Computing (irrespective of platform a product is said to be backward compatible when it is able to take the place of an older product Forward compatibility (sometimes confused with Extensibility) is the ability of a system to gracefully accept Input intended for later versions of itself Koninklijke Bibliotheek. Retrieved on 2007-12-11. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 359 - Honoratus, the first known Prefect of the City of Constantinople, takes office
2. ^ van der Hoeven, Jeffery, Brian Lohman, and Remco Verdegem. “Emulation for Digital Preservation in Practice: The Results. ” The International Journal of Digital Curation 2. 2 (2007): 123-132.
3. ^ Muira, Gregory. “ Pushing the Boundaries of Traditional Heritage Policy: maintaining long-term access to multimedia content. ” IFLA Journal 33 (2007): 323-326.
4. ^ Rothenberg, Jeffrey. “Criteria for an Ideal Solution. ” Avoiding Technological Quicksand: Finding a Viable Technical Foundation for Digital Preservation. Washington, DC: Council on Library and Information Resources, 1998. Council on Library and Information Resources. 2008. 8 Mar. 2008 http://www.clir.org/pubs/reports/rothenberg/contents.html
5. ^ Rothenberg, Jeffrey. “The Emulation Solution. ” Avoiding Technological Quicksand: Finding a Viable Technical Foundation for Digital Preservation. Washington, DC: Council on Library and Information Resources, 1998. Council on Library and Information Resources. 2008. 28 Mar. 2008 http://www.clir.org/pubs/reports/rothenberg/contents.html
6. ^ Muira, Gregory. “ Pushing the Boundaries of Traditional Heritage Policy: maintaining long-term access to multimedia content. ” IFLA Journal 33 (2007): 323-326.
7. ^ Granger, Stewart. Digital Preservation & Emulation: from theory to practice. Proc. of the ichim01 Meeting, vol. 2, 3 -7 Sept. 2001. Milano, Italy. Toronto: Archives and Museum Informatics, University of Toronto, 2001. 28 Mar. 2008 http://www.leeds.ac.uk/cedars/pubconf/papers/ichim01SG.html
8. ^ Muira, Gregory. “ Pushing the Boundaries of Traditional Heritage Policy: maintaining long-term access to multimedia content. ” IFLA Journal 33 (2007): 323-326.
9. ^ van der Hoeven, Jeffery, Brian Lohman, and Remco Verdegem. “Emulation for Digital Preservation in Practice: The Results. ” The International Journal of Digital Curation 2. 2 (2007): 123-132.
10. ^ Granger, Stewart. “Emulation as a Digital Preservation Strategy. ” D-Lib Magazine 6. 19 (2000). 29 Mar 2008 http://www.dlib.org/dlib/october00/granger/10granger.html
11. ^ Rothenberg, Jeffrey. “The Emulation Solution. ” Avoiding Technological Quicksand: Finding a Viable Technical Foundation for Digital Preservation. Washington, DC: Council on Library and Information Resources, 1998. Council on Library and Information Resources. 2008. 28 Mar. 2008
12. ^ Echoes of Art: Emulation as preservation strategy. Retrieved on 2007-12-11. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 359 - Honoratus, the first known Prefect of the City of Constantinople, takes office

• Seeing Double: Emulation in Theory and Practice

External links

• Emulator at the Open Directory Project