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Encyclopedia > Unreal mode
x86 processor modes
Mode First supported
Intel 8086
Intel 80286
Intel 80386
  • Unreal mode
Intel 80386
Intel 386SL
AMD Opteron

Unreal mode, also big real mode, huge real mode, or flat real mode, is a variant of real mode (PE=0), in which one or more data segment registers have been loaded with 32-bit addresses and limits. Contrary to its name, it is not a separate addressing mode that the x86 and x86-64 processors can operate in. It is used in the 80386 and later x86 processors. x86 or 80x86 is the generic name of a microprocessor architecture first developed and manufactured by Intel. ... A microprocessor incorporates most or all of the functions of a central processing unit (CPU) on a single integrated circuit (IC). ... Real mode (also called real address mode in Intels manuals) is an operating mode of 80286 and later x86-compatible CPUs. ... Intel Corporation (NASDAQ: INTC, SEHK: 4335), founded in 1968 as Integrated Electronics Corporation, is an American multinational corporation that is best known for designing and manufacturing microprocessors and specialized integrated circuits. ... The 8086 is a 16-bit microprocessor chip designed by Intel in 1978, which gave rise to the x86 architecture. ... Protected mode is an operational mode of x86-compatible CPUs of the 80286 series or later. ... The Intel 80286 is an x86-family 16-bit microprocessor that was introduced by Intel on February 1, 1982. ... In the 80386 and later, Virtual 8086 mode, also called virtual real mode (or VM86), allows the execution of real mode applications that violate the rules under the control of a protected mode operating system. ... The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ... The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ... System Management Mode (SMM) is a mode of Intel x86 microprocessor in which all normal execution (including the operating system) is suspended, and special separate software (usually firmware or a hardware-assisted debugger) is executed in high-privilege mode. ... 386 DX redirects here. ... In the x86-64 CPU architecture Long mode, is the mode where an application (or operating system) can access the 64-bit instructions and registers, while 32-bit programs are executed in a compatibility mode. ... Advanced Micro Devices, Inc. ... The Opteron is AMDs x86 server processor line, and was the first processor to implement the AMD64 instruction set architecture (known generically as x86-64). ... Real mode (also called real address mode in Intels manuals) is an operating mode of 80286 and later x86-compatible CPUs. ... x86 or 80x86 is the generic name of a microprocessor architecture first developed and manufactured by Intel. ... The AMD64 or x86-64 is a 64-bit processor architecture invented by AMD. It is a superset of the x86 architecture, which it natively supports. ... The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ...


Overview

For efficiency reasons, the 80386 and later x86 processors use the base address stored in their internal "descriptor" cache whenever accessing memory, regardless if they are operating in real or protected mode. The "selector", i.e. the 16-bit "segment number" visible to the programmer is used once, while reloading a segment register, to update the various fields of the respective "descriptor", and then is simply disregarded. The Intel 80386 is a microprocessor which was used as the central processing unit (CPU) of many personal computers from 1986 until 1994 and later. ...


Some DOS extenders use this feature to address the high memory. It was used by many computer games in the 1990 to 1995 time frame, since it allowed programmers to use more memory than in real mode, which is restricted to 1 MB (640 KB usable), but still access the DOS operating system, which doesn't work in protected mode. After the introduction of Windows 95 unreal mode quickly fell out of favour as programs using it cannot be run in the DOS prompt of Microsoft Windows; they require a "Restart in MS-DOS mode" in Windows 95 and 98, and cannot be run at all on NT and later Windows systems. For those operating systems, an emulator such as DOSBox is the only way to run programs designed for unreal mode. Unreal mode is still extensively used by BIOS code. Tenberry Softwares DOS/4GW startup banner Developed in the 1980s to cope with the memory limitations of MS-DOS and its derivatives, DOS extenders are programs which enable software to run under the protected mode environment initially introduced with the Intel 80286 processor and later expanded upon with the... The High Memory Area (HMA) is the RAM area consisting of the first 64 kilobytes (KB), minus 16 bytes, of the extended memory on an IBM PC or compatible microcomputer. ... A computer game is a game composed of a computer-controlled virtual universe that players interact with in order to achieve a defined goal or set of goals. ... This article is about the year. ... Year 1995 (MCMXCV) was a common year starting on Sunday. ... This article is about a unit of data. ... Depending on the context in which it is used, the word kilobyte may mean either 1,000 or 1,024 bytes. ... This article is about the family of closely related operating systems for the IBM PC compatible platform. ... Windows 95 is a consumer-oriented graphical user interface-based operating system. ... Windows redirects here. ... Windows 98 (codenamed Memphis) is a graphical operating system released on June 25, 1998 by Microsoft and the successor to Windows 95. ... Windows NT (New Technology) is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. ... This article is about emulators in computer science. ... DOSBox is a program that emulates an Intel x86 computer running MS-DOS. It is intended for running DOS-based IBM PC compatible programs, especially computer games, which may not run properly on newer PCs and may not run at all on non-x86 hardware or operating systems (e. ... For other uses, see Bios. ...


Enabling Unreal Mode

To enable unreal mode without using any undocumented features of the CPU, the program has to enter protected mode, locate a flat descriptor in the GDT or LDT or create such, load some of the segment registers with the respective protected mode "selector", then cancel back to real mode. When jumping back to RM, the processor will continue using the cached descriptors as established in PM - thus allowing access to 4 GB of "extended" memory from inside real mode. The Global Descriptor Table or GDT is a data structure used by Intel x86-family processors starting with the 80286 in order to define the characteristics of the various memory areas used during program execution, for example the base address, the size and access privileges like executability and writability. ... The Local Descriptor Table (LDT) is a memory table used in the x86 architecture in protected mode and containing memory segment descriptors: start in linear memory, size, executability, writability, access privilege, actual presence in memory, etc. ...


A much simpler way to load such information into a selector is with the undocumented LOADALL instruction, which can be used to load arbitrary data into the descriptor cache for each selector. LOADALL is the common name for two different, undocumented machine instructions of Intel 80286 and Intel 80386 processors, which allow to access areas normally outside of the IA-32 API scope, like descriptor cache registers. ...


See also

The correct title of this article is . ...

  Results from FactBites:
 
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Unreal Internet play performance is highly dependent on the bandwidth of your connection, the latency (ping time), and the packet loss.
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Unreal mode - Wikipedia, the free encyclopedia (438 words)
Unlike real mode, protected mode and long mode, unreal mode is not a separate addressing mode the x86 processors or x86-64 processors can operate in.
Rather, unreal mode (also big real mode, huge real mode, flat real mode) is a variant of real mode (PE=0), in which one or more data segment registers have been loaded with 32-bit addresses and limits.
After the introduction of Windows 95 unreal mode quickly fell out of favour as programs using it cannot be run in the DOS prompt of Microsoft Windows; they require a "Restart in MS-DOS mode" in Windows 95 and 98, and cannot be run at all on Windows NT, 2000, Me, and XP systems.
  More results at FactBites »

 
 

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