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InDOS memory management,extended memory refers tomemory above the firstmegabyte (220 bytes) ofaddress space in anIBM PC or compatible with an80286 or laterprocessor. The term is mainly used under theDOS andWindowsoperating systems. DOS programs, running inreal mode orvirtual x86 mode, cannot directly access this memory, but are able to do so through anapplication programming interface (API) called theExtended Memory Specification (XMS). This API is implemented by adriver (such asHIMEM.SYS) or the operating system kernel, which takes care ofmemory management and copying memory betweenconventional and extended memory, by temporarily switching the processor intoprotected mode. In this context, the term "extended memory" may refer to either the whole of the extended memory or only the portion available through this API.
Extended memory can also be accessed directly by DOS programs running in protected mode usingVCPI orDPMI, two (different and incompatible) methods of using protected mode under DOS.
Extended memory should not be confused withexpanded memory (EMS), an earlier method for expanding the IBM PC's memory capacity beyond 640 kB (655,360 bytes) using anexpansion card withbank switched memory modules. Because of the available support for expanded memory in popular applications, device drivers were developed that emulated expanded memory using extended memory. Later two additional methods were developed allowing direct access to small portions of additional memory above 640 KB from real mode. One of these is referred to as thehigh memory area (HMA), consisting of the first nearly 64 KB of extended memory, and the other is referred to as theupper memory area (UMA; also referred to as upper memory blocks or UMBs), located in the address range between 640 KB and 1 MB which the IBM PC designates for hardware adapters and ROM.
Onx86-based PCs, extended memory is only available with anIntel80286 processor or higher. Only these chips can directly address more than 1 megabyte ofRAM. The earlier8086/8088 processors can make use of more than 1 MB of RAM if one employsspecial hardware to make selectable parts of it appear at addresses below 1 MB.
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 reserves addresses between 640 kB and 1 MB for system ROM and peripherals.
Extended memory is not accessible inreal mode (except for a small portion called thehigh memory area). Only applications executing inprotected mode can use extended memory directly. A supervising protected-modeoperating system such asMicrosoft Windows manages application programs' access to memory. The processor makes this memory available through theGlobal Descriptor Table (GDT) and one or moreLocal Descriptor Tables (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 processorexception (e.g., ageneral protection fault or GPF) on violation. This prevents programs running in protected mode from interfering with each other's memory.[1]
A protected-mode operating system such as Microsoft Windows can also run real-mode programs and provideexpanded memory to them. TheDOS Protected Mode Interface (DPMI) is Microsoft's prescribed method for aDOS program to access extended memory under amultitasking environment.[1]
TheExtended Memory Specification (XMS) is the specification describing the use ofIBM PC extended memory inreal mode for storing data (but not for running executable code in it). Memory is made available byextended memory manager (XMM) software such asHIMEM.SYS. The XMM functions are accessible by direct calls to a variable address that can be found throughsoftware interrupt 2Fh function 4310h.
XMS version 2.0, released in July 1988, allowed for up to 64 MB of memory.[2] With XMS version 3.0 this increased to 4 GB (232 bytes).[3] The difference is a direct result of the sizes of the values used to report the amounts of total and unallocated (free) extended memory in 1 KB (1024-byte) units: XMS 2.0 uses 16-bit unsigned integers, capable of representing a maximum of (65535 * 1 KB) = 64 MB, while XMS 3.0 adds new alternate functions that use 32-bit unsigned integers, capable of representing (4 G * 1 KB) = 4 TB (4 terabytes) but limited by the specification to 4 GB.[2][3] (4 GB is the address range of the 80386 and the 80486, the only 32-bit Intel x86 CPUs that existed when XMS 3.0 was published in 1991.) XMS 3.0 retains the original XMS 2.0 API functions with their original 64 MB limit but adds new "super extended memory" functions that support 4 GB of extended memory (minus the first 1 MB) and can be called only with a 32-bit CPU (since these "super" functions use 32-bit CPU registers to pass values).[3] To differentiate between the possibly different amount of memory that might be available to applications, depending on which version of the specification they were developed to, the latter may be referred to assuper extended memory (SXMS).
The extended memory manager is also responsible for managing allocations in thehigh memory area (HMA) and theupper memory area (UMA; also referred to as upper memory blocks or UMBs). In practice the upper memory area will be provided by theexpanded memory manager (EMM), after which DOS will try to allocate them all and manage them itself.[clarification needed][citation needed]