In DOS memory administration, extended memory refers to memory above the first megabyte (220 bytes) of handle space in an IBM Pc or compatible with an 80286 or later processor. The term is mainly used below the DOS and Memory Wave brainwave tool Home windows working programs. DOS packages, operating in real mode or virtual x86 mode, cannot instantly entry this memory, but are able to take action by an utility programming interface (API) referred to as the Extended Memory Specification (XMS). This API is implemented by a driver (resembling HIMEM.SYS) or the working system kernel, which takes care of memory management and copying memory between standard and extended memory, by quickly switching the processor into protected mode. On this context, the time period "prolonged memory" might check with both the whole of the extended memory or solely the portion available by way of this API. Extended memory may also be accessed instantly by DOS packages working in protected mode using VCPI or DPMI, two (totally different and incompatible) methods of utilizing protected mode beneath DOS.

Extended memory shouldn't be confused with expanded memory (EMS), an earlier technique for increasing the IBM Computer's memory capability beyond 640 kB (655,360 bytes) utilizing an growth card with financial institution switched memory modules. Because of the accessible support for expanded memory in popular functions, machine drivers were developed that emulated expanded memory using extended memory. Later two additional strategies had been developed permitting direct access to small portions of extra memory above 640 KB from actual mode. One of these is referred to as the high memory space (HMA), consisting of the primary almost sixty four KB of extended memory, and the opposite is referred to because the upper memory space (UMA; also referred to as higher memory blocks or UMBs), located within the tackle range between 640 KB and 1 MB which the IBM Pc designates for hardware adapters and ROM. On x86-primarily based PCs, extended memory is barely out there with an Intel 80286 processor or larger, such as the IBM Laptop AT.

Only these chips can instantly deal with more than 1 megabyte of RAM. The sooner 8086/8088 processors can make use of more than 1 MB of RAM if one employs particular hardware to make selectable elements of it seem at addresses under 1 MB. On a 286 or better Pc outfitted with greater than 640 kB of RAM, the extra memory would generally be re-mapped above the 1 MB boundary, for the reason that IBM Laptop structure reserves addresses between 640 kB and 1 MB for system ROM and peripherals. Extended memory isn't accessible in actual mode (aside from a small portion referred to as the excessive memory space). Solely purposes executing in protected mode can use extended memory instantly. A supervising protected-mode working system reminiscent of Microsoft Windows manages software packages' entry to memory. The processor makes this memory available through the worldwide Descriptor Desk (GDT) and one or more Local Descriptor Tables (LDTs).

The memory is "protected" in the sense that memory segments assigned an area descriptor cannot be accessed by one other program as a result of that program makes use of a special LDT, and Memory Wave brainwave tool segments assigned a worldwide descriptor can have their access rights restricted, inflicting a processor exception (e.g., a basic protection fault or GPF) on violation. This prevents programs working in protected mode from interfering with each other's memory. Extended memory went unused at first because no software ran within the 80286's protected mode. By distinction, the trade quickly adopted 1985's expanded memory customary, which works with all PCs regardless of processor. A protected-mode working system reminiscent of Microsoft Windows also can run actual-mode packages and provide expanded memory to them. The DOS Protected Mode Interface (DPMI) is Microsoft's prescribed methodology for a DOS program to entry prolonged memory below a multitasking surroundings. The Extended Memory Specification (XMS) is the specification describing using IBM Computer prolonged memory in real mode for storing data (but not for operating executable code in it).

Memory is made obtainable by prolonged memory manager (XMM) software akin to HIMEM.SYS. XMS version 2.0, released in July 1988, allowed for up to sixty four MB of memory. With XMS version 3.0 this elevated to four GB (232 bytes). Four TB (4 terabytes) however limited by the specification to four GB. 4 GB is the address vary of the 80386 and the 80486, the one 32-bit Intel x86 CPUs that existed when XMS 3.Zero was revealed in 1991.) XMS 3.0 retains the original XMS 2.Zero API capabilities with their unique 64 MB restrict however adds new "tremendous extended memory" functions that help 4 GB of prolonged memory (minus the first 1 MB) and will be known as only with a 32-bit CPU (since these "tremendous" features use 32-bit CPU registers to go values). To differentiate between the presumably completely different amount of memory that is likely to be accessible to functions, relying on which model of the specification they had been developed to, the latter could also be referred to as tremendous extended memory (SXMS). The extended memory manager is also answerable for managing allocations in the excessive memory space (HMA) and the upper memory area (UMA; additionally referred to as higher memory blocks or UMBs). In apply the upper memory space shall be offered by the expanded memory supervisor (EMM), after which DOS will attempt to allocate all of them and manage them itself. Mendelson, Edward (1989-12-12). "A Slot Full of RAM". Pc Journal. Vol. 8, no. 21. Ziff Davis Publishing Co. pp.