Extended memory

Extended memory refers to memory above the first megabyte of address space in an IBM PC with an 80286 or later processor.

Extended memory is only available on PC's based on the Intel 80286 or higher processor. Only these chips can access more than 1MB of RAM. On a 286 or better PC equipped with more than 640KB of RAM, the additional memory would generally be re-mapped above the 1MB boundary, making all of it available to programs running in Protected mode. Even without such remapping, machines with more than 1MB of RAM would have access to memory above 1MB.

Extended memory is available in real mode only through EMS, UMB, XMS, or HMA; only applications executing in protected mode can use extended memory directly. In this case, the extended memory is provided by a supervising protected-mode operating system such as Microsoft Windows. The processor makes this memory available through the Global Descriptor Table and one or more Local 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 hardware trap (typically a General Protection Fault) on violation. This prevents programs running in protected mode from interfering with each other's memory.

A protected-mode operating system such as Windows can also run real-mode programs and provide expanded memory to them. The DOS Protected Mode Interface is Microsoft's prescribed method for an MS-DOS program to access extended memory under a multitasking environment.

See also

* Unreal mode
* Conventional memory
* Upper Memory Area
* Extended Memory Specification
* Expanded Memory Specification
* High Memory Area

This article was originally based on material from the Free On-line Dictionary of Computing, used with permission. Update as needed.