Thursday, June 25, 2009

Computer Memory

Expanded Memory, in computer science, on IBM PCs and compatible computers, a type of physical memory, up to 8 megabytes (MB), that can be added to machines based on the Intel 8086/8088 microprocessor or to machines with 80286/80386/80486 microprocessors running MS-DOS in real (8086-emulation) mode. The use of expanded memory is defined by the Expanded Memory Specification (EMS). Because it represents memory that is not normally accessible to programs running under MS-DOS, expanded memory requires an interface called the Expanded Memory Manager (EMM), which maps pages (blocks) of bytes from expanded memory onto reserved areas called “page frames” in an accessible memory area. Only EMS-compatible software can make use of expanded memory.

Extended Memory, in computer science, system memory beyond 1 megabyte (MB) in computers based on the Intel 80286/386/486 microprocessors. This memory is accessible only when the processor is operating in protected mode or in virtual real mode on the 386/486. Extended memory is not typically available to MS-DOS programs. However, it is available to OS/2 programs and can be made available to MS-DOS programs by the use of software that temporarily places the processor into protected mode or by the use of 386/486 features to remap portions of extended memory into conventional memory based on the EMS conventions.