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Encyclopedia > Emulator
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DosBox emulates the familiar command line interface of DOS.
DosBox emulates the familiar command line interface of DOS.

An emulator duplicates (provides an emulation of) the functions of one system with a different system, so that the second system behaves like (and appears to be) the first system. This focus on exact reproduction of external behavior is in contrast to simulation, which concerns an abstract model of the system being simulated, often considering internal state. Computer science, or computing science, is the study of the theoretical foundations of information and computation and their implementation and application in computer systems. ... After seeing a Fairlight CMI at a convention in 1979, E-mu founders Scott Wedge and Dave Rossum began working on designing a less expensive sampler. ... The term emulation may have the following meanings. ... Wikipedia does not have an article with this exact name. ... Wiktionary (a portmanteau of wiki and dictionary) is a multilingual, Web-based project to create a free content dictionary, available in over 150 languages. ... Wikipedia does not have an article with this exact name. ... Wiktionary (a portmanteau of wiki and dictionary) is a multilingual, Web-based project to create a free content dictionary, available in over 150 languages. ... Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... DOS Version of Z running in DOSBox in Debian. ... This article or section does not adequately cite its references or sources. ... For other uses, see System (disambiguation). ... This article is about the general term. ... In information processing, a state is the complete set of properties (for example, its energy level, etc. ...


Emulators in computer science

Emulation refers to the ability of a program or device to imitate another program or device. Many printers, for example, are designed to emulate Hewlett-Packard LaserJet printers because so much software is written for HP printers. By emulating an HP printer, a printer can work with any software written for a real HP printer. Emulation "tricks" the software into believing that a device is really some other device. A computer printer, or more commonly a printer, produces a hard copy (permanent human-readable text and/or graphics) of documents stored in electronic form, usually on physical print media such as paper transparencies]]. Many printers are primarily used as computer peripherals, and are attached by a printer cable to... The Hewlett-Packard Company (NYSE: HPQ), commonly known as HP, is a very large, global company headquartered in Palo Alto, California, United States. ... Image:1984 HP Laserjet. ...

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. Read-only memory (usually known by its acronym, ROM) is a class of storage media used in computers and other electronic devices. ... An Altera Stratix II GX FPGA. A field-programmable gate array is a semiconductor device containing programmable logic components called logic blocks, and programmable interconnects. ... Hardware emulation is the process of imitating the behavior of one piece of hardware (typically a system under design) with another piece of hardware, typically a special purpose emulation system. ...

In a theoretical sense, the Church-Turing thesis implies that any operating environment can be emulated within any other. 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. 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. In computability theory the Church-Turing thesis, Churchs thesis, Churchs conjecture or Turings thesis, named after Alonzo Church and Alan Turing, is a hypothesis about the nature of mechanical calculation devices, such as electronic computers. ... Reverse engineering (RE) is the process of taking something (a device, an electrical component, a software program, etc. ...


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. 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 this would need to be emulated. An interpreter is a computer program that executes other programs. ... A system of codes directly understandable by a computers CPU is termed this CPUs native or machine language. ...

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. C-64 redirects here. ... The demoscene is a computer art subculture that specializes itself on producing demos, non-interactive audio-visual presentations, which are run real-time on a computer. ...

In contrast, some other platforms have had very little use of direct hardware addressing. In these cases, a simple compatibility layer may suffice. This translates system calls for the emulated system into system calls for the host system. In software engineering, a compatibility layer allows binaries for an emulated system to run on a 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. 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. In many cases, the simulator is actually produced by the company providing the hardware, which theoretically increases its accuracy. A router, an example of an embedded system. ... “Game console” redirects here. ... This article is about the general term. ... A debugger is a computer program that is used to test and debug other programs. ...

Typically, an emulator is divided into modules that correspond roughly to the emulated computer's subsystems. Most often, an emulator will be composed of the following modules: A module is a software entity that groups a set of (typically cohesive) subprograms and data structures. ...

  • 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.

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). This 68451 MMU could be used with the Motorola 68010 MMU, short for memory management unit or sometimes called paged memory management unit as PMMU, is a class of computer hardware components responsible for handling memory accesses requested by the CPU. Among the functions of such devices are the translation...

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. Features such as bank switching or segmentation may also complicate memory emulation. 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 between the CPU and I/O devices in a computer. ... Read-only memory (usually known by its acronym, ROM) is a class of storage media used in computers and other electronic devices. ... 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 microcomputer systems, to increase the amount of addressable RAM and ROM without extending the address bus. ... On the Intel x86 architecture, a memory segment is the portion of memory which may be addressed by a single index register without changing a 16-bit segment selector. ...

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 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 is a computer program that executes, or performs, instructions written in a computer programming language. ...

This is made possible by assigning a variable to each register and flag of the simulated CPU. 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. In computer science and mathematics, a variable (IPA pronunciation: ) (sometimes called a pronumeral) is a symbolic representation denoting a quantity or expression. ... In computer architecture, a processor register is a small amount of very fast computer memory used to speed the execution of computer programs by providing quick access to frequently used values—typically, these values are involved in multiple expression evaluations occurring within a small region on the program. ... 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 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. Until not many years ago, emulation in such situations was considered completely impractical by many. 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. ...

What allowed breaking through this restriction were the advances in dynamic recompilation techniques. Simple a priori translation of emulated program code into code runnable on the host architecture is usually impossible because of several reasons: In computer science, dynamic recompilation (sometimes abbreviated to dynarec) is a feature of some emulators and virtual machines, where the system may recompile some part of a program during execution. ...

  • 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.

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. 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, self-modifying code is code that alters its own instructions, whether or not it is on purpose, while it is executing. ... For other uses, see Data (disambiguation). ... In communications, a code is a rule for converting a piece of information (for example, a letter, word, or phrase) into another form or representation, not necessarily of the same type. ... See also Just in time for the business technique In computing, just-in-time compilation (JIT), also known as dynamic translation, is a technique for improving the performance of interpreted programs. ... Look up cache in Wiktionary, the free dictionary. ...

CPU emulators are popular among some gamers. Some computer games are designed for slower computers and will not work properly on newer, faster computers. Some CPU emulators, such as Mo'Slo, can slow down the CPU, or a process of a program, allowing it to work properly.

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 DOS programs, may not even run on faster computers.


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.

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. API and Api redirect here. ...

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 programming is a computer programming technique that provides for the execution of operations in parallel, either within a single computer, or across a number of systems. ...

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)

In computing, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ...

Legal controversy

See article Console emulator — Legal issues

A console emulator is a program that allows a computer to emulate a video game console. ...

See also

This article is about the general term. ... This article lists software that emulates arcade and console game systems, computing platforms and CPUs. ... An arcade emulator is a program that emulates one or more arcade games on a different computer, such as a PC. See the List of emulators for examples of arcade emulators. ... A console emulator is a program that allows a computer to emulate a video game console. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Apple Terminal. ... Game engine recreations are engine interpreters for video games that replace the original engine binary that came with the original game. ... A field-programmable gate array or FPGA is a gate array that can be reprogrammed after it is manufactured, rather than having its programming fixed during the manufacturing &#8212; a programmable logic device. ... In computing, binary translation is the emulation of one instruction set by another through translation of code. ... An in-circuit emulator (ICE) also called on-circuit debugger (OCD) or background debug module (BDM) is a hardware device used to debug the software of an embedded system. ... In computer science, a virtual machine is software that creates a virtualized environment between the computer platform and its operating system, so that the end user can operate software on an abstract machine. ... This article is about Nintendos emulation feature and download service. ... A source port is a source code modification to a computer games engine that allows it to be played on operating systems or computing platforms for which it was not originally created or compatible with. ...


External links

  • HowTo: Writing a Computer Emulator
  • Checklist — Emulator Compatibility lists
  • The History of Emulation — 1800 to 1999: Part 1, Part 2, Part 3, Part 4
  • EmuFAQ — collection of essays surrounding the legality of emulators
  • Mo'Slo — a popular CPU emulator with slow-down capabilities
  • The Oldskool PC — provides information on how to run older games on newer computers
  • EPROM emulator - hardware EPROM emulator for fast, easy program development
  • Wils Stuff - Latest news in the emulation world, including FAQs

  Results from FactBites:
Nintendo DS Emulator - NDS ROMS Emulators (1332 words)
DS emulator is a program you run on your computer that allows you to run other programs meant for a different computer, video game console or another operating system.
A common form of emulation is that of a software emulator, a piece of computer software that allows certain computer programs to run on a platform (computer architecture and/or operating system) other than the one for which they were originally written.
An emulator is used to mimic the behavior of one machine on another.
Emulator - Wikipedia, the free encyclopedia (1565 words)
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; if writing to a specific memory location should influence the screen, for example, this will have to be emulated.
Buses are often not emulated, either for reasons of performance or simplicity, and virtual peripherals communicate directly with the CPU or the memory subsystem.
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).
  More results at FactBites »



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