PC compatible memory diagram
Below is an illustration of the four sections of memory. The following specifications are conclusive for all IBM compatible computers.
XMS - Remainder of all Memory above 1 MB except the first 640 KB
HMA - First 64 KB of memory Beyond 1 MB
HMA (high memory area) is the first 64 KB of XMS also requires himem.sys as an external memory manager (loaded in config.sys). Only one program at a time can access HMA commonly used for loading DOS "high," but also used for TSRs.
UMB - 384 KB
CONVENTIONAL (640 KB)
Originally the only memory available to DOS, conventional memory contains devices drivers, conventional variables, the DOS command processor, and TSRs application programs. The conventional memory is static and never changes no matter how much RAM you have on the computer.