Extended Memory

In DOS memory management, extended memory refers to memory above the first megabyte of address space in an IBM PC or compatible with an 80286 or later processor. The term is mainly used under the DOS and Windows operating systems. DOS programs, running in real mode or virtual x86 mode, cannot directly access this memory, but are able to do so through an application programming interface called the eXtended Memory Specification (XMS). This API is implemented by a driver (such as HIMEM.SYS) or the operating system, which takes care of memory management and copying memory between conventional and extended memory, by temporarily switching the processor into protected 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 using VCPI or DPMI, two (different and incompatible) methods of using protected mode under DOS.

Extended memory should not be confused with expanded memory, an earlier method for expanding the IBM PC's memory capacity beyond 640 kb using an expansion card with bank 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 a small portion of extended memory from real mode. These memory areas are referred to as the high memory area (HMA) and the upper memory area (UMA; also referred to as upper memory blocks or UMBs).

Read more about Extended Memory:  Overview, eXtended Memory Specification (XMS)

Other articles related to "extended memory, memory, extended":

Extended Memory - eXtended Memory Specification (XMS)
... The 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 ... Memory is made available by extended memory manager (XMM) software such as HIMEM.SYS ... XMS version 2.0 allowed for up to 64 MiB of memory, with XMS version 3.0 this increased to 4 GiB ...
... HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory via the Extended Memory Specification (XMS). 5.0, HIMEM.SYS was introduced and could be used to load the DOS kernel code into the High Memory Area (HMA) to increase the amount of available conventional memory by specifying DOS=HIGH in CONFIG.SYS ... HIMEM.SYS provides access to the memory beyond the first 1 MB of space and thus is required by Windows 9x/Me in order to load the graphical portions of the operating ...
HP-41 Extension Module - The 82181A Extended Memory Module
... Extended memory modules added more extended memory to the HP-41C ... They required an extended functions module to be present, or the HP-41CX version of the calculator ... Each module added 238 registers of extended memory ...
Operating System RAM Limits - IBM PC and 8086 Addressing Limit
... allow for hardware addressing space in the upper 384 kB (upper memory area (UMA)) of the total addressable memory space of 1024 kB (1 MB) ... buffer space, some third-party utilities could add memory at the top of the 640k conventional memory area, to extend memory up to the base address used by hardware adapters ... Hardware extensions allowed access to more memory than the 8086 CPU could address through paging memory ...
Upper Memory Area - Implementation - Virtual X86 Mode
... Upper memory blocks can be created by mapping extended memory into the upper memory area when running in virtual x86 mode ... This is similar to how expanded memory can be emulated using extended memory so this method of providing upper memory blocks is usually provided by the expanded memory manager (for ... programming interface for managing the upper memory blocks is specified in the eXtended Memory Specification ...

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