extended memory
extended memory
(storage)Extended memory is not directly available in real mode, onlythrough EMS, UMB, XMS, or HMA; only applicationsexecuting in protected mode can use extended memorydirectly. In this case, the extended memory is provided by asupervising protected-mode operating system such asMicrosoft Windows. The processor makes this memoryavailable through a system of global descriptor tables andlocal descriptor tables. The memory is "protected" in thesense that memory assigned a local descriptor cannot beaccessed by another program without causing a hardware trap.This prevents programs running in protected mode frominterfering with each other's memory.
A protected-mode operating system such as Windows can alsorun real-mode programs and provide expanded memory tothem. DOS Protected Mode Interface is Microsoft'sprescribed method for an MS-DOS program to access extendedmemory under a multitasking environment.
Having extended memory does not necessarily mean that you havemore than one megabyte of memory since the reserved memoryarea may be partially empty. In fact, if your 386 or higheruses extended memory as expanded memory then that part is notin excess of 1Mb.
See also conventional memory.
extended memory
The name given to memory (RAM) in an Intel PC above 1MB (one megabyte). Starting with the Intel 286, extended memory was used directly by Windows and OS/2 as well as DOS applications that ran with DOS extenders. It was also used under DOS for RAM disks and disk caches. Contrast with "expanded memory" (EMS), which was specialized memory above 1MB. Today, most people never heard of extended or expanded memory, because the 1MB barrier was broken long ago, and thousands of megabytes (MBs) of memory are commonly used. See EMS, XMS and DOS extender. |