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In DOS memory administration, prolonged memory refers to memory above the first megabyte (220 bytes) of tackle space in an IBM Laptop or appropriate with an 80286 or later processor. The time period is mainly used below the DOS and Home windows working methods. DOS applications, running in actual mode or digital x86 mode, cannot straight access this memory, however are in a position to take action by means of an application programming interface (API) known as the Extended Memory Specification (XMS). This API is applied by a driver (corresponding to HIMEM.SYS) or the operating system kernel, which takes care of memory administration and copying memory between standard and prolonged memory, by temporarily switching the processor into protected mode. In this context, the time period "extended memory" might check with either the whole of the extended memory or solely the portion accessible via this API. Prolonged memory will also be accessed instantly by DOS programs operating in protected mode using VCPI or DPMI, two (different and incompatible) methods of using protected mode below DOS.

Extended memory shouldn't be confused with expanded memory (EMS), an earlier method for expanding the IBM Laptop's memory capacity past 640 kB (655,360 bytes) using an enlargement card with financial institution switched memory modules. Due to the obtainable assist for expanded memory in widespread purposes, device drivers had been developed that emulated expanded memory utilizing extended memory. Later two extra methods had been developed permitting direct access to small portions of additional memory above 640 KB from actual mode. One of these is referred to as the high memory area (HMA), consisting of the first practically sixty four KB of prolonged memory, and the opposite is referred to as the upper memory space (UMA; additionally known as upper memory blocks or UMBs), situated in the handle vary between 640 KB and 1 MB which the IBM Computer designates for hardware adapters and ROM. On x86-based PCs, extended memory is just obtainable with an Intel 80286 processor or increased, such because the IBM Computer AT.

Only these chips can instantly deal with more than 1 megabyte 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 components of it appear at addresses beneath 1 MB. On a 286 or higher Laptop geared up with more than 640 kB of RAM, the extra memory would generally be re-mapped above the 1 MB boundary, because the IBM Computer architecture reserves addresses between 640 kB and 1 MB for system ROM and peripherals. Prolonged memory is just not accessible in actual mode (aside from a small portion referred to as the excessive memory area). Only applications executing in protected mode can use extended memory straight. A supervising protected-mode operating system equivalent to Microsoft Home windows manages utility applications' access to memory. The processor makes this memory available by the global Descriptor Table (GDT) and one or Memory Wave Routine more Local Descriptor Tables (LDTs).

The memory is "protected" within the sense that memory segments assigned an area descriptor can't be accessed by one other program because that program makes use of a distinct LDT, and memory segments assigned a world descriptor can have their entry rights restricted, inflicting a processor exception (e.g., a normal protection fault or GPF) on violation. This prevents packages working in protected mode from interfering with each other's memory. Prolonged memory went unused at first because no software ran in the 80286's protected mode. By contrast, the industry rapidly adopted 1985's expanded memory normal, which works with all PCs regardless of processor. A protected-mode operating system similar to Microsoft Home windows may run actual-mode applications and supply expanded Memory Wave Routine to them. The DOS Protected Mode Interface (DPMI) is Microsoft's prescribed technique for a DOS program to entry prolonged memory under a multitasking atmosphere. The Prolonged Memory Specification (XMS) is the specification describing using IBM Laptop prolonged memory in real mode for storing data (however not for running executable code in it).

Memory is made accessible by prolonged memory supervisor (XMM) software program akin to HIMEM.SYS. XMS model 2.0, launched 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 (four terabytes) however limited by the specification to 4 GB. 4 GB is the tackle range of the 80386 and the 80486, the only 32-bit Intel x86 CPUs that existed when XMS 3.Zero was printed in 1991.) XMS 3.0 retains the unique XMS 2.0 API capabilities with their authentic 64 MB limit however provides new "tremendous prolonged Memory Wave" functions that help four GB of extended memory (minus the first 1 MB) and could be referred to as only with a 32-bit CPU (since these "tremendous" features use 32-bit CPU registers to go values). To differentiate between the possibly different quantity of memory that is likely to be out there to functions, relying on which model of the specification they had been developed to, the latter could also be referred to as tremendous prolonged memory (SXMS). The prolonged memory supervisor is also accountable for managing allocations within the high memory area (HMA) and the higher memory area (UMA; additionally referred to as higher memory blocks or UMBs). In follow the higher memory area will be provided by the expanded memory supervisor (EMM), after which DOS will try to allocate them all and manage them itself. Mendelson, Edward (1989-12-12). "A Slot Stuffed with RAM". Computer Journal. Vol. 8, no. 21. Ziff Davis Publishing Co. pp.