Specifications

Computer Architecture and Maintenance (G-Scheme-2014)

Extended memory : Extended memory is above 1MB.

This memory is usually not available to computer

directly. It can be accessed through special software or

program or driver, this memory is available in AT

machines, not in XTs. The limit of extended memory is

16MB for 286 processor and for 386, 486 and higher

processor the extended memory is up to 4GB. System

based on new Pentiums have a limit of 64G of memory.

The XMS (Extended Memory Specification was

developed by Intel, Microsoft , Lotus Development and

AST corp. in 1987 to specify how programs would use

extended memory. This XMS Functions on system

based on 286 and higher processor to use extended

memory and another block of memory which out of the

reach of the DOS. “HIMEM.SYS” is a driver to use

extended memory This driver is loaded through the config.sys file . Extended memory

is generally used by multitasking programs.

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 (655,360 bytes) 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).

Expanded memory : Also known as EMS (Expanded Memory Specification), expanded

memory is a technique for utilizing more than 1MB of main memory in DOS -based

computers. The limit of 1MB is built into the DOS operating system. The upper 384K is

reserved for special purposes, leaving just 640K of conventional memory for programs.

There are several versions of EMS. The original versions, called EMS 3.0 and 3.2, enable

programs to use an additional 8MB of memory, but for data only. An improved version

Prepared By – Prof. Manoj.kavedia (9860174297 – 9324258878 ) (www.kavediasir.yolasite.com)