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 it). Memory is made available by extended memory manager (XMM) software such as HIMEM.SYS. The XMM functions are accessible through interrupt 2Fh.
XMS version 2.0 allowed for up to 64 MiB of memory, with XMS version 3.0 this increased to 4 GiB. 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 as super extended memory or SXMS.
The extended memory manager is also responsible for managing allocations in the high memory area (HMA) and the upper memory area (UMA; also referred to as upper memory blocks or UMBs). In practice the upper memory area will be provided by the expanded memory manager (EMM), after which DOS will try to allocate them all and manage them itself.
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