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Extended memory refers to memory above the first megabyte of address space in an IBM PC with an 80286 or later processor. Computer data storage, often called storage or memory, refers to Computer components devices and recording media that retain digital A megabyte is a unit of Information or Computer storage equal to either 106 (1000000 Bytes or 220 (1048576 bytes depending on In Computing, an address space defines a range of discrete addresses each of which may correspond to a physical or virtual Memory register, a network host The Intel 286, introduced on February 1, 1982, (originally named 80286, and also called iAPX 286 in the programmer's manual

On x86-based PC's, extended memory is only available with an Intel 80286 processor or higher. See also X86 assembly language The generic term x86 refers to the most commercially successful Instruction set architecture in the history of Personal The Intel 286, introduced on February 1, 1982, (originally named 80286, and also called iAPX 286 in the programmer's manual Only these chips can address more than 1 MB of RAM. The earlier 8086/8088 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, but this does not constitute extended memory as defined in this article.

On a 286 or better PC equipped with more than 640 KB 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. In computing protected mode, also called protected virtual address mode, is an operational mode of X86 -compatible Central processing units (CPU 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 only through EMS, UMB, XMS, or HMA; only applications executing in protected mode can use extended memory directly. Real mode, also called real address mode, is an operating mode of 80286 and later X86 -compatible CPUs. In computing expanded memory (commonly known as EMS memory) is a system of Bank switching introduced around 1984 that provided additional memory to MS-DOS The Upper Memory Area (UMA is a design feature of IBM PC-compatible X86 Computers that was responsible for the 640 KB barrier. The High Memory Area ( HMA) is the RAM area consisting of the first 64 kibibytes (KiB, minus 16 Bytes, of the Extended memory on an In computing protected mode, also called protected virtual address mode, is an operational mode of X86 -compatible Central processing units (CPU In this case, the extended memory is provided by a supervising protected-mode operating system such as Microsoft Windows. 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 Microsoft Windows is a series of Software Operating systems and Graphical user interfaces produced by Microsoft. The processor makes this memory available through the Global Descriptor Table and one or more Local Descriptor Tables (LDTs). 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 The Local Descriptor Table (LDT is a memory table used in the X86 architecture in Protected mode and containing memory Segment descriptors start in 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) on violation. A General Protection Fault ( GPF) in the Intel X86 and AMD X86-64 architectures and other unrelated architectures is a This prevents programs running in protected mode from interfering with each other's memory.

A protected-mode operating system such as Windows can also run real-mode programs and provide expanded memory to them. In computing expanded memory (commonly known as EMS memory) is a system of Bank switching introduced around 1984 that provided additional memory to MS-DOS The DOS Protected Mode Interface is Microsoft's prescribed method for an MS-DOS program to access extended memory under a multitasking environment. In computing the DOS Protected Mode Interface (DPMI is a specification introduced in 1989 which allows a DOS program to run in Protected mode, granting access to MS-DOS (short for M icro' s' oft D isk O perating S ystem is an Operating system commercialized by Microsoft. In computing Multitasking is a method by which multiple tasks also known as processes, share common processing resources such as a CPU.

Extended Memory Specification

Extended Memory Specification or XMS is the specification describing the use of IBM PC extended memory in real mode for storing data (but not for running executable code in it). Real mode, also called real address mode, is an operating mode of 80286 and later X86 -compatible CPUs. Memory is made available by extended memory manager (XMM) software such as HIMEM.SYS. HIMEM is a DOS Device driver which allows DOS programs to store data in Extended memory via the Extended Memory Specification (XMS The XMM functions are accessible through interrupt 2Fh. XMS should not be confused with the somewhat similar EMS (expanded memory specification). In computing expanded memory (commonly known as EMS memory) is a system of Bank switching introduced around 1984 that provided additional memory to MS-DOS

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