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Extended memory
 
 
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In computing, Extended memory refers to memory
Computer storage

Computer data storage, often called storage or memory, refers to computer components, devices, and recording medium that retain digital data used for computing for some interval of time....
 above the first megabyte
Megabyte

Megabyte is a SI prefix-multiple of the unit byte for digital information computer storage or transmission and is equal to 106 bytes....
 of address space
Address space

In computing, an address space defines a range of discrete addresses, each of which may correspond to a physical or virtual memory register, a Node , peripheral device, disk sector or other logical or physical entity....
 in an IBM PC
IBM PC

The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform ....
 with an 80286 or later processor
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
.

86-based PCs, extended memory is only available with an Intel 80286 processor or higher. Only these chips can address more than 1 MB
Megabyte

Megabyte is a SI prefix-multiple of the unit byte for digital information computer storage or transmission and is equal to 106 bytes....
 of RAM
Ram

Ram, ram, or RAM as a non-acronymic wordAs a non-acronymic word Ram, ram, or RAM may refer to:...
. The earlier 8086
Intel 8086

The 8086 is a 16-bit microprocessor chip designed by Intel and introduced on the market in 1978, which gave rise to the x86 architecture. Intel 8088, released in 1979, was essentially the same chip, but with an external 8-bit bus , and is notable as the processor used in the original IBM PC....
/8088
Intel 8088

The Intel 8088 is an Intel x86 microprocessor based on the Intel 8086, with 16-bit registers and an 8-bit external data bus. It can address up to 1 megabyte of random access memory....
 processors can make use of more than 1 MB of RAM, if one employs special hardware to make selectable parts of it appear at addresses below 1 MB (paging
Paging

In computer operating systems that have their main memory divided into page , paging is a transfer of pages between main memory and an auxiliary store, such as hard disk drive....
).

On a 286 or better PC equipped with more than 640 KB
Kilobyte

Kilobyte is a unit of Computer data storage equal to either 1,024 bytes or 1,000 bytes , depending on context.It is abbreviated in a number of ways: KB, kB, K and Kbyte....
 of RAM, the additional memory would generally be re-mapped above the 1 MB boundary, since the IBM PC architecture mandates a 384 KB "hole" in memory between the 640 KB and 1 MB boundaries.






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In computing, Extended memory refers to memory
Computer storage

Computer data storage, often called storage or memory, refers to computer components, devices, and recording medium that retain digital data used for computing for some interval of time....
 above the first megabyte
Megabyte

Megabyte is a SI prefix-multiple of the unit byte for digital information computer storage or transmission and is equal to 106 bytes....
 of address space
Address space

In computing, an address space defines a range of discrete addresses, each of which may correspond to a physical or virtual memory register, a Node , peripheral device, disk sector or other logical or physical entity....
 in an IBM PC
IBM PC

The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform ....
 with an 80286 or later processor
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
.

Overview

On x86-based PCs, extended memory is only available with an Intel 80286 processor or higher. Only these chips can address more than 1 MB
Megabyte

Megabyte is a SI prefix-multiple of the unit byte for digital information computer storage or transmission and is equal to 106 bytes....
 of RAM
Ram

Ram, ram, or RAM as a non-acronymic wordAs a non-acronymic word Ram, ram, or RAM may refer to:...
. The earlier 8086
Intel 8086

The 8086 is a 16-bit microprocessor chip designed by Intel and introduced on the market in 1978, which gave rise to the x86 architecture. Intel 8088, released in 1979, was essentially the same chip, but with an external 8-bit bus , and is notable as the processor used in the original IBM PC....
/8088
Intel 8088

The Intel 8088 is an Intel x86 microprocessor based on the Intel 8086, with 16-bit registers and an 8-bit external data bus. It can address up to 1 megabyte of random access memory....
 processors can make use of more than 1 MB of RAM, if one employs special hardware to make selectable parts of it appear at addresses below 1 MB (paging
Paging

In computer operating systems that have their main memory divided into page , paging is a transfer of pages between main memory and an auxiliary store, such as hard disk drive....
).

On a 286 or better PC equipped with more than 640 KB
Kilobyte

Kilobyte is a unit of Computer data storage equal to either 1,024 bytes or 1,000 bytes , depending on context.It is abbreviated in a number of ways: KB, kB, K and Kbyte....
 of RAM, the additional memory would generally be re-mapped above the 1 MB boundary, since the IBM PC architecture mandates a 384 KB "hole" in memory between the 640 KB and 1 MB boundaries. This way all of the additional memory would be available to programs running in Protected mode
Protected mode

In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units ....
. Even without such remapping, machines with more than 1 MB of RAM would have access to memory above 1 MB.

Extended memory is available in real mode
Real mode

Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible Central processing unit. Real mode is characterized by a 20 bit segmented memory address space , direct software access to BIOS routines and peripheral hardware, and no concept of memory protection or computer multitasking at the hardware le...
 only through EMS
Expanded memory

In computing, expanded memory is a system of bank switching introduced around 1984 that provided additional memory to MS-DOS programs that required more than what was available in conventional memory....
, UMB
Upper Memory Area

The Upper Memory Area is a design feature of IBM IBM PC compatible x86 computers that was responsible for the Conventional memory#640 KB barrier....
, XMS, or HMA
High Memory Area

The High Memory Area is the random-access memory area consisting of the first 64 kibibyte, minus 16 bytes, of the extended memory on an IBM PC or IBM PC compatible microcomputer....
; only applications executing in protected mode
Protected mode

In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units ....
 can use extended memory directly. In this case, the extended memory is provided by a supervising protected-mode operating system
Operating system

An operating system is an interface between hardware and applications; it is responsible for the management and coordination of activities and the sharing of the limited resources of the computer....
 such as Microsoft Windows
Microsoft Windows

Microsoft Windows is a series of software operating systems and graphical user interfaces produced by Microsoft. Microsoft first introduced an operating environment named Windows in November 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces ....
. The processor makes this memory available through the Global Descriptor Table
Global Descriptor Table

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....
 and one or more Local Descriptor Table
Local Descriptor Table

The Local Descriptor Table 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....
s (LDTs). The memory is "protected" in the sense that memory segments assigned a local descriptor cannot be accessed by another program because that program uses a different LDT, and memory segments assigned a global descriptor can have their access rights restricted, causing a hardware trap (typically a General Protection Fault
General protection fault

A General Protection Fault in the Intel x86 and AMD x86-64 CPU Architectures, and other unrelated architectures, is a fault which can encompass several cases where protection mechanisms within the processor architecture are violated by any of the programs that are running, whether it be the kernel or user program....
) on violation. This prevents programs running in protected mode from interfering with each other's memory.

A protected-mode operating system such as Microsoft Windows can also run real-mode programs and provide expanded memory
Expanded memory

In computing, expanded memory is a system of bank switching introduced around 1984 that provided additional memory to MS-DOS programs that required more than what was available in conventional memory....
 to them. The DOS Protected Mode Interface
DOS Protected Mode Interface

In computing, the DOS Protected Mode Interface is a specification introduced in 1989 which allows a DOS program to run in protected mode, granting access to many features of the processor not available in real mode....
 is Microsoft's prescribed method for an MS-DOS
MS-DOS

MS-DOS is an operating system commercialized by Microsoft. It was the most commonly used member of the DOS family of operating systems and was the main operating system for personal computers during the 1980s....
 program to access extended memory under a multitasking
Computer multitasking

In computing, multitasking is a method by which multiple tasks, also known as Computer process, share common processing resources such as a Central processing unit....
 environment.

Extended Memory Specification

Extended Memory Specification or XMS is the specification describing the use of IBM PC
IBM PC

The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform ....
 extended memory in real mode
Real mode

Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible Central processing unit. Real mode is characterized by a 20 bit segmented memory address space , direct software access to BIOS routines and peripheral hardware, and no concept of memory protection or computer multitasking at the hardware le...
 for storing data (but not for running executable code in it). Memory is made available by extended memory manager (XMM) software such as HIMEM.SYS
HIMEM

HIMEM is a DOS device driver which allows DOS programs to store data in extended memory via the Extended Memory Specification . This device driver is of particular importance because various versions of Microsoft Windows that ran on top of the DOS operating system required HIMEM.SYS to be loaded to be able to run....
. The XMM functions are accessible through interrupt 2Fh. XMS should not be confused with the somewhat similar EMS (expanded memory
Expanded memory

In computing, expanded memory is a system of bank switching introduced around 1984 that provided additional memory to MS-DOS programs that required more than what was available in conventional memory....
 specification).

See also

  • Unreal mode
    Unreal mode

    Unreal mode, also big real mode, huge real mode, or flat real mode, is a variant of real mode , in which one or more data segment registers have been loaded with 32-bit addresses and limits....
  • Conventional memory
    Conventional memory

    In computing, conventional memory is the first 640 kilobytes of the memory on IBM PC compatible systems....
  • Upper Memory Area
    Upper Memory Area

    The Upper Memory Area is a design feature of IBM IBM PC compatible x86 computers that was responsible for the Conventional memory#640 KB barrier....
     (UMA)
  • Expanded memory
    Expanded memory

    In computing, expanded memory is a system of bank switching introduced around 1984 that provided additional memory to MS-DOS programs that required more than what was available in conventional memory....
     (EMS)
  • High Memory Area
    High Memory Area

    The High Memory Area is the random-access memory area consisting of the first 64 kibibyte, minus 16 bytes, of the extended memory on an IBM PC or IBM PC compatible microcomputer....
     (HMA)