EXTENDED MEMORY (EMS/XMS) (<=4GB)

The EMS/XMS area is the most interesting  aspect of the whole inherited real-mode vs. protected-mode issue. Modern X86 computers (386sx and later) configure that memory as XMS by default. However, in order to support users who had software which required LIM (Lotus-Intel-Microsoft)  EMS drivers were introduced which could convert XMS into EMS. EMS is only required to run software programmed to use it but programming habits are hard to break; only recently are developers leaving the EMS platform. Unless you are running legacy DOS applications EMS is unnecessary. If you need conventional memory though you are obviously running legacy DOS applications.

HIGH MEMORY AREA (HMA) (64k)

The HMA contains the first 64KB of XMS. I'm not sure what it is for except that MS-DOS is loaded there. Perhaps it is reserved for DOS.

UPPER MEMORY AREA (UMA) (384k)

The UMA is the region of memory reserved between 640KB and 1024KB. It contains 24 blocks (the UMBs) of 16KB. Except for the first 64KB,all memory above 1024KB is XMS. This region of memory contains lots of history, most of which you can reallocate for your own use; a significant region is, by default set aside for monochrome video adapters. Also, there are parts of the memory which are now supported by BIOS and are no longer necessary for that purpose.

CONVENTIONAL MEMORY (640k)

When the Intel 8086 processor (the grand-daddy of modern PC hardware) was first developed it accessed memory using a scheme called 'Real Mode'. At the time, real mode addressing was capable of accessing up to 1 MB of RAM, which was considered more than adequate for the foreseeable future. Of the 1 MB, 384KB is subtracted for adapters and other hardware, thus leaving 640KB for your programs, TSRs and other goodies. Luckily, as the Intel 80X86 architecture developed, memory accessing schemes improved. With the 80286 CPU, a 24-bit memory address mode (Potected 16) permitted up to 16MB RAM. With the 80386 and later CPUs, a 32-bit address mode became available,and Protected-32 mode can handle up to 4GB RAM. What does this mean to you? It turns out that DOS runs (by default) only in Real Mode. Special extensions must be loaded for DOS to access Protected Mode (anything over 640KB) memory.