Real mode
Encyclopedia
Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs
. Real mode is characterized by a 20 bit segmented memory address space (giving exactly 1 MiB
of addressable memory) and unlimited direct software access to all memory, I/O addresses and peripheral hardware. Real mode provides no support for memory protection, multitasking, or code privilege levels. 80186 CPUs and earlier, back to the original 8086, have only one operational mode, which is equivalent to real mode in later chips. All x86 CPUs in the 80286 series and later start in real mode when reset.
, allowing for (among other things) hardware-level memory protection. Using these new features, however, required a new operating system
that was specifically designed for it. Since a primary design specification
of x86 microprocessors is that they are fully backwards compatible with software written for all x86 chips before them, the 286 chip was made to start in 'real mode' – that is, in a mode which turned off the new memory protection features, so that it could run operating system
s written for the 8086
and the 80186. As of August 2011, even the newest x86 CPUs (including x86-64
CPUs) start in real mode at power-on and can run software written for almost any previous chip (with a few exceptions due to slight instruction set differences).
The PC BIOS which IBM introduced operates in real mode, as do the DOS
operating systems (MS-DOS
, DR-DOS
, etc.). Early versions of Microsoft Windows
ran in real mode, until Windows 386, which ran in protected mode, and the more fully realized Windows 3.0
, which could run in either real or protected mode. Windows 3.0 could actually run in two "flavours" of protected mode: "standard mode", which ran using protected mode, and "386-enhanced mode", which is a virtualized version of standard mode and thus would not run on a 286. Windows 3.1 removed support for real mode, and it was the first mainstream operating environment which required at least an 80286 processor. Almost all modern x86 operating systems (Unix
, Linux
, OS/2
, Windows 95
and later, etc.) switch the CPU into protected mode at startup, but 64-bit operating systems will use this only as another stepping stone to get to long mode
. It is worth noting that the protected mode of the 80286 is considerably more primitive than the improved protected mode introduced with the 80386; the latter is sometimes called 386 protected mode, and is the mode most modern 32-bit x86 operating systems run in.
of memory, whereas the 80186 and earlier would access an address equal to [offset]-10h ( = [offset]-16 decimal), which is at the beginning of the first mebibyte. (Note that on the 80186 and earlier, the first kibibyte
of the address space, starting at address 0, is the permanent, immovable location of the interrupt vector table.) So, the actual amount of memory addressable by the 80286 and later x86 CPUs in real mode is 1 MiB
+ 64 KiB
– 16 B
= 1114096 B.
-based IBM Personal Computer model (machine type) 5150). This mode-switching technique is also the one used by DPMI (under real, not emulated, DOS) and DOS extender
s like DOS/4GW
to allow protected mode programs to run under DOS; the DPMI system or DOS extender switches to real mode to invoke DOS or BIOS calls, then switches back to return to the application program which runs in protected mode. This is probably the reason why until Windows ME it was possible to restart the computer to MS DOS mode from within the operating system. The changing towards the NT kernel resulted in the operating system not needing DOS to boot the computer as well as unable to use it. The need to restart the computer in Real Mode MS DOS declined after Windows 3.1x until it was no longer needed. The only way of currently running DOS applications in Real Mode from within newer versions of Windows is by using emulators such as DOSBox
.
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...
. Real mode is characterized by a 20 bit segmented memory address space (giving exactly 1 MiB
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
of addressable memory) and unlimited direct software access to all memory, I/O addresses and peripheral hardware. Real mode provides no support for memory protection, multitasking, or code privilege levels. 80186 CPUs and earlier, back to the original 8086, have only one operational mode, which is equivalent to real mode in later chips. All x86 CPUs in the 80286 series and later start in real mode when reset.
History
The 286 architecture introduced protected modeProtected mode
In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
, allowing for (among other things) hardware-level memory protection. Using these new features, however, required a new operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
that was specifically designed for it. Since a primary design specification
Design specification
A design specification provides explicit information about the requirements for a product and how the product is to be put together. It is the most traditional kind of specification, having been used historically in public contracting for buildings, highways, and other public works, and represents...
of x86 microprocessors is that they are fully backwards compatible with software written for all x86 chips before them, the 286 chip was made to start in 'real mode' – that is, in a mode which turned off the new memory protection features, so that it could run operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s written for the 8086
Intel 8086
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...
and the 80186. As of August 2011, even the newest x86 CPUs (including x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
CPUs) start in real mode at power-on and can run software written for almost any previous chip (with a few exceptions due to slight instruction set differences).
The PC BIOS which IBM introduced operates in real mode, as do the DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
operating systems (MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
, DR-DOS
DR-DOS
DR-DOS is an MS-DOS-compatible operating system for IBM PC-compatible personal computers, originally developed by Gary Kildall's Digital Research and derived from Concurrent PC DOS 6.0, which was an advanced successor of CP/M-86...
, etc.). Early versions of Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
ran in real mode, until Windows 386, which ran in protected mode, and the more fully realized Windows 3.0
Windows 3.0
Windows 3.0, a graphical environment, is the third major release of Microsoft Windows, and was released on 22 May 1990. It became the first widely successful version of Windows and a rival to Apple Macintosh and the Commodore Amiga on the GUI front...
, which could run in either real or protected mode. Windows 3.0 could actually run in two "flavours" of protected mode: "standard mode", which ran using protected mode, and "386-enhanced mode", which is a virtualized version of standard mode and thus would not run on a 286. Windows 3.1 removed support for real mode, and it was the first mainstream operating environment which required at least an 80286 processor. Almost all modern x86 operating systems (Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
, Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
, Windows 95
Windows 95
Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...
and later, etc.) switch the CPU into protected mode at startup, but 64-bit operating systems will use this only as another stepping stone to get to long mode
Long mode
In the x86-64 computer architecture, long mode is the mode where a 64-bit application can access 64-bit instructions and registers...
. It is worth noting that the protected mode of the 80286 is considerably more primitive than the improved protected mode introduced with the 80386; the latter is sometimes called 386 protected mode, and is the mode most modern 32-bit x86 operating systems run in.
Addressing capacity
The 8086, 8088, and 80186 have a 20-bit address bus, but the unusual segmented addressing scheme Intel chose for these processors actually produces effective addresses which can have 21 significant bits. This scheme shifts a 16-bit segment number left four bits (making a 20-bit number with four least-significant zeros) before adding to it a 16-bit address offset; the maximum sum occurs when both the segment and offset are hexadecimal FFFF, yielding FFFF0h + FFFFh = 10FFEFh. (The small "h" on the end of each number denotes hexadecimal notation.) On the 8086, 8088, and 80186, the result of an effective address that overflows 20 bits is that the address "wraps around" to the zero end of the address range, i.e. it is taken modulo 2^20 (2^20 = 1048576 = 100000h). However, the 80286 has 24 address bits and computes effective addresses to 24 bits even in real mode. Therefore, for the segment FFFFh and offset greater than 000Fh, the 80286 would actually make an access into the beginning of the second mebibyteMebibyte
The mebibyte is a multiple of the unit byte for digital information. The binary prefix mebi means 220, therefore 1 mebibyte is . The unit symbol for the mebibyte is MiB. The unit was established by the International Electrotechnical Commission in 2000 and has been accepted for use by all major...
of memory, whereas the 80186 and earlier would access an address equal to [offset]-10h ( = [offset]-16 decimal), which is at the beginning of the first mebibyte. (Note that on the 80186 and earlier, the first kibibyte
Kibibyte
The kibibyte is a multiple of the unit byte for quantities of digital information. The binary prefix kibi means 1024; therefore, 1 kibibyte is . The unit symbol for the kibibyte is KiB. The unit was established by the International Electrotechnical Commission in 1999 and has been accepted for use...
of the address space, starting at address 0, is the permanent, immovable location of the interrupt vector table.) So, the actual amount of memory addressable by the 80286 and later x86 CPUs in real mode is 1 MiB
Mebibyte
The mebibyte is a multiple of the unit byte for digital information. The binary prefix mebi means 220, therefore 1 mebibyte is . The unit symbol for the mebibyte is MiB. The unit was established by the International Electrotechnical Commission in 2000 and has been accepted for use by all major...
+ 64 KiB
Kibibyte
The kibibyte is a multiple of the unit byte for quantities of digital information. The binary prefix kibi means 1024; therefore, 1 kibibyte is . The unit symbol for the kibibyte is KiB. The unit was established by the International Electrotechnical Commission in 1999 and has been accepted for use...
– 16 B
Byte
The byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
= 1114096 B.
A20 line
Some programs predating the 80286 were designed to take advantage of the wrap-around (modulo) memory addressing behavior, so the 80286 presented a problem for backward compatibility. Forcing the 21st address line (the actual logic signal wire coming out of the chip) to a logic low, representing a zero, results in a modulo-2^20 effect to match the earlier processors' address arithmetic, but the 80286 has no internal capability to perform this function. When IBM used the 80286 in their IBM Personal Computer AT, they solved this problem by including a software-settable gate to enable or disable (force to zero) the A20 address line, between the A20 pin on the 80286 and the system bus; this is known as Gate-A20 (the A20 gate), and it is still implemented in PC chipsets to this day. Most versions of the HIMEM.SYS extended memory driver for IBM-/MS-DOS famously displayed upon loading a message that they had installed an "A20 handler", a piece of software to control Gate-A20 and coordinate it to the needs of programs. Obviously, in protected mode the A20 line needs to be enabled, or else physical addressing errors will occur, likely leading to a system crash.Switching to real mode
Intel introduced protected mode into the x86 family with the intention that operating systems which used it would run entirely in the new mode and that all programs running under a protected mode operating system would run in protected mode as well. Because of the substantial differences between real mode and even the rather limited 286 protected mode, programs written for real mode cannot run in protected mode without being rewritten. Therefore, with a wide base of existing real mode applications which users depended on, abandoning real mode posed problems for the industry, and programmers sought a way to switch between the modes at will. However, Intel, consistent with their intentions for the processor's usage, provided an easy way to switch into protected mode on the 80286 but no easy way to switch back to real mode. Before the 386 the only way to switch from protected mode back to real mode was to reset the processor; after a reset it always starts up in real mode to be compatible with earlier x86 CPUs back to the 8086. Resetting the processor does not clear the system's RAM, so this, while awkward and inefficient, is actually feasible. From protected mode, the processor's state is saved in memory, then the processor is reset, restarts in real mode, and executes some real mode code to restore the saved state from memory. It can then run other real mode code until the program is ready to switch back to protected mode. The switch to real mode is costly in terms of time, but this technique allows protected mode programs to use services such as BIOS, which runs entirely in real mode (having been designed originally for the 8088Intel 8088
The Intel 8088 microprocessor was a variant of the Intel 8086 and was introduced on July 1, 1979. It had an 8-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and the one megabyte address range were unchanged, however...
-based IBM Personal Computer model (machine type) 5150). This mode-switching technique is also the one used by DPMI (under real, not emulated, DOS) and DOS extender
DOS extender
A DOS extender is a computer software program which enables software to run under a protected mode environment even though the host operating system is only capable of operating in real mode....
s like DOS/4GW
DOS/4GW
DOS/4G is a 32-bit DOS extender developed by Rational Systems . It allows DOS programs to eliminate the 640 KB conventional memory limit by addressing up to 64 MB of extended memory on Intel 80386 and above machines....
to allow protected mode programs to run under DOS; the DPMI system or DOS extender switches to real mode to invoke DOS or BIOS calls, then switches back to return to the application program which runs in protected mode. This is probably the reason why until Windows ME it was possible to restart the computer to MS DOS mode from within the operating system. The changing towards the NT kernel resulted in the operating system not needing DOS to boot the computer as well as unable to use it. The need to restart the computer in Real Mode MS DOS declined after Windows 3.1x until it was no longer needed. The only way of currently running DOS applications in Real Mode from within newer versions of Windows is by using emulators such as DOSBox
DOSBox
DOSBox is emulator software that emulates an IBM PC compatible computer running MS-DOS. It is intended especially for use with old PC games. DOSBox is free software....
.
See also
- 80386
- IA-32IA-32IA-32 , also known as x86-32, i386 or x86, is the CISC instruction-set architecture of Intel's most commercially successful microprocessors, and was first implemented in the Intel 80386 as a 32-bit extension of x86 architecture...
- x86 assembly languageX86 assembly languagex86 assembly language is a family of backward-compatible assembly languages, which provide some level of compatibility all the way back to the Intel 8008. x86 assembly languages are used to produce object code for the x86 class of processors, which includes Intel's Core series and AMD's Phenom and...
- Conventional memoryConventional memoryIn DOS memory management, conventional memory, also called base memory, is the first 640 kilobytes of the memory on IBM PC or compatible systems. It is the read-write memory usable by the operating system and application programs...
- Code Project Real Mode Tutorial