Appendix A Extended Vs. Expanded Memory; Some History; Extended (Linear) Memory - Kingston Technology KTM609 Installation Manual

28 Appendix A Extended vs. Expanded Memory
Appendix A Extended vs. Expanded Memory
There are two ways you can define the newly added memory in your PS/2 computer
system. You can define it as Extended memory or Expanded memory, or as a
combination of the two. To understand the difference between the two ways you can
configure memory, we need to take a step back in time.

Some History

When the PC was first designed, everyone thought that 640KB would be more
memory than anyone could possibly use for their programs and data. The IBM PC
originally contained an 8088 microprocessor which could address 1MB of memory,
and the DOS operating system was designed to recognize only that much memory. Of
the 1MB available, 640KB was devoted to running application programs. This
portion of memory is also referred to as conventional or base memory.
The remaining 384KB of addressable memory, between 640KB and 1MB, is referred
to as reserved high memory. This area of memory was devoted to video RAM,
ROM BIOS (the Read Only Memory Basic Input/Output System), and other system
and peripheral functions.
The next stage in the evolution of the PC was the PC AT which contains an 80286
microprocessor capable of addressing 16MB of memory. Current IBM PS/2 Model 50
and Model 60 computers also use this processor. However, DOS retained its 1MB
addressing limitations so it would still be compatible with the older systems.

Extended (Linear) Memory

The original 640K of memory was still available for use by DOS-compatible
application programs, while the memory above 1MB (the Extended memory) could be
used by the processor for print spoolers and RAM disks.
Because the memory addresses continue sequentially from beginning to end, this
memory is known as linear memory. Both conventional and Extended memory are
classified as linear memory.
Kingston Technology Corporation KTM-609/16 Installation Guide - Rev. D1